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2013 Annual Workshop Proceedings

CGSE Annual Workshop Bunker Bay, 10 to 11 Dec 2013

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Content Workshop Program .............................................................................................................................. 5

Keynotes .............................................................................................................................................. 9

8th Terzaghi Oration: Protecting society from landslides – the role of the geotechnical engineer ................................................................................................................................................... 10

2nd Mcclelland Lecture: Analytical contributions to offshore geotechnical engineering ............. 11

Presentation Session 1: Industry Impacts ........................................................................................... 12

Offshore shallow foundation optimisation – industry impact ..................................................... 13

Pipeline stability on mobile seabeds ........................................................................................... 14

Performance improvement of rail tracks using geosynthetic and resilient inclusions: industry impact ........................................................................................................................................ 15

Suction caisson installation and extraction in Angola Clay ......................................................... 16

Presentation Session 2: Ballina Soft Soil Testing Facility ..................................................................... 17

Progress at the Ballina field test facility ...................................................................................... 18

Contributions of University of Wollongong for Ballina field test facility ..................................... 20

Characterization of smear zone due to installation of vertical drains ......................................... 21

Numerical simulation of CPT cone penetration in sand .............................................................. 22

Presentation Session 3: Multiphysics Modelling and Moving Boundary Problems ............................. 23

Modelling cementing minefill ..................................................................................................... 24

Application of material point method to geotechnical problems ............................................... 25

Assessment of post‐compacted soil using a non‐destructive appraoch ...................................... 26

Large deformation analysis of geomechanics problems by high‐order elements ....................... 27

A new critical state mohr‐coulomb soil model for large deformation anaylsis of sand............... 28

Considerations on the design of plate anchors ........................................................................... 29

Presentation Session 4: Georisk ......................................................................................................... 31

Recent advances in centrifuge modelling of offshore foundations ............................................. 32

Probabilistic analysis of dry soil mix columns ............................................................................. 33

Failure mechanism and bearing capacity of footings buried at various depths in spatially random soils ............................................................................................................................................ 34

Determination of material properties by inverse analysis .......................................................... 35

Predicting the settlement of shallow foundations on sand ........................................................ 36

3 Minute Postgraduate Student Thesis Presentations: Day 1 ............................................................. 37


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Quantitative risk assessment of rainfall‐induced landslides ....................................................... 38

Combined vertical, horizontal and moment capacity of an hybrid foudnation system ............... 40

Piezoball testing at the ballina soft soil research site ................................................................. 41

Dynamic simulation of submarine landslides and its consequences on pipelines with material point method ............................................................................................................................. 43

Design of working platforms for tracked plant ........................................................................... 44

Effects of installation on the capacity of helical anchors in clay ................................................. 45

Numerical analysis of dynamic compaction of soils .................................................................... 46

Long‐term performance of suction embedded plate anchors in permanent mooring systems .. 47

Estimating the punch‐through potential for spudcan penetrating sand overlying clay .............. 48

Numerical and experimental study of particle migration in soils ................................................ 49

Efficient sequential and parallel iterative solvers for large‐scale geotechanical problems ......... 50

Investigation of ploughing process in sand with application to offshore geotechnics ................ 51

The numerical simulation of contact problems using a third medium ........................................ 52

Pipeline on‐bottom stability and thermal expansion management: the interaction of sediment transport, geotechnics and structural behaviour ........................................................................ 53

Rate effects on the uplift capacity of skirted foundations on clay .............................................. 54

3 Minute Postgraduate Student Thesis Presentations: Day 2 ............................................................. 55

Simplified riser‐soil interaction model for fatigue design of steel catenary risers ...................... 56

Interpretation of cone penetration data in layered clays ........................................................... 57

Numerical modelling of submarine landslide and its impact to offshore infrastructure using the material point method (MPM) ................................................................................................... 58

Sliding behaviour of a mobile foundation ................................................................................... 60

Mesh optimisation methods for solving large deformation geotechnical problems ................... 61

The hydrodynamics of a recirculating (O‐tube) flume ................................................................ 62

The variation of nc for a sphere penetrating soft soil .................................................................. 63

Improvement of soil stability along rail corridors through native vegetation – bio engineering . 65

Creep model capturing vacuum preloading ................................................................................ 66

Soft soil improvement using vacuum preloading and vertical drains .......................................... 67

Artificial neural network development for stress analysis of steel catenary risers in touchdown zone ............................................................................................................................................ 68

Finite element algorithms for dynamic analysis of saturated porous media .............................. 70

Micromechanically‐inspired stone column behaviour ................................................................ 71

Sampling disturbance of an intermediate soil ............................................................................ 72

Particle finite element analysis of the granular column collapse ................................................ 73


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Numerical modelling of spudcan and cone penetration in multi‐layer soils ............................... 74

Abstracts Not Presented .................................................................................................................... 75

Consolidation of soft soil stabilized by vertical drains under cyclic loadings ............................... 76

The role of compression behaviour in constitutive modelling of soft soils ................................. 77

Estimation of spudcan penetration resistance in layered soils from field penetrometer data and quantification onf punch‐through risk ........................................................................................ 79

A detail study of fluid‐soil‐riser interaction at touch down zone ................................................ 80

Group Workshop Sessions .................................................................................................................. 81

Session 1: Industry Impacts ........................................................................................................ 82

Session 2: Field Testing – Methods & Interpretation .................................................................. 83

Session 3: Soft Soil Constitutive Models & Laboratory Testing ................................................... 85

Session 4: Georisk ....................................................................................................................... 87


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Workshop Program

PRE‐WORKSHOP (MONDAY 9 DEC 2013) 2.00 pm – 5.00 pm Bus departs from UWA to Bunker Bay (meeting point at Fairway Entrance 3)7.00 pm Welcome BBQ buffet dinner

DAY 1 (TUESDAY 10 DEC 2013) 7.00 am – 8.30 am Breakfast at the Moon Restaurant 8.30 am – 8.40 am Welcome – Mark Cassidy 8.40 am – 9.00 am Opening: Achievements & Future Direction of CGSE – Scott Sloan 9.00 am – 9.30 am Keynote 1: Terzaghi Oration ‐ Protecting society from landslides – the role of

the geotechnical engineer (Suzanne Lacasse) 9.30 am – 10.30 am

Presentation Session 1: Industry Impacts (Chair: Harry Poulos) ‐ Opening by Chair (5 mins) ‐ Presentations (8 mins presentation + 3 mins Q & A)

o Offshore shallow foundation optimisation ‐ industry impact (Susan Gourvenec) o Pipeline stability on mobile seabeds (Scott Draper) o Performance improvement of rail tracks using geosynthetic and resilient inclusions ‐

industry impact (Sanjay Nimbalkar) o Suction caisson installation and extraction in Angola clay (Christophe Gaudin)

‐ Closing discussion (10 mins) 10.30 am – 11.00 am Morning tea 11.00 am – 12.15 pm 3‐minute thesis presentation (Chair: Susan Gourvenec)

‐ Presentations by PhD students (see Page 7 for more detail)12.15 pm – 1.30 pm Lunch at the Moon Restaurant 1.30 pm – 2.30 pm Presentation Session 2: Ballina Soft Soil Testing Facility (Chair: Richard Kelly)

‐ Opening by Chair (5 mins) ‐ Presentations (8 mins presentation + 3 mins Q & A)

o Progress at the Ballina field test facility (Richard Kelly) o Contributions of University of Wollongong for Ballina field test facility (Buddhima

Indraratna) o Characterization of smear zone due to installation of vertical drains (Cholochat

Rujikiatkamjorn) o Numerical simulation of CPT cone penetration in sand (George Kouretzis)

‐ Closing discussion (10 mins) 2.30 pm – 3.30 pm Group Workshop (2 parallel sessions)

‐ Opening by session leaders (updates and future visions) ‐ Parallel group discussion with themes of:

o Session 1 : Industry Impacts (Lead: Marc Senders & Christophe Gaudin) o Session 2 : Field testing – methods and interpretation (Lead: Richard Kelly &

Buddhima Indraratna) 3.30 pm – 4.00 pm Afternoon tea 4.00 pm – 5.00 pm/ 4.00 pm – 6.00 pm

Networking/Physical modeling at beach Advisory board meeting

6.30 pm Workshop dinner at the Eagle Bay Brewery (bus departs from the resort at 6.30 pm)


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DAY 2 (WEDNESDAY 11 DEC 2013) 7.00 am ‐ 8.30 am Breakfast at the Moon Restaurant8.30 am – 9.00 am Keynote 2: McClelland lecture – Analytical contributions to offshore

geotechnical engineering (Mark Randolph) 9.00 am – 10.30 am Presentation Session 3: Multiphysics Modelling & Moving Boundary

Problems (Chair: Kristian Krabbenhoft) ‐ Opening by Chair (5 mins) ‐ Presentations (8 mins presentation + 3 mins Q & A)

o Modelling cementing minefill (David Muir Wood) o Application of material point method to geotechnical problems (Wojtek Solowski) o Assessment of post‐compacted soil using a non‐destructive approach (Ana Heitor) o Large deformation analysis of geomechanics problems by high‐order elements

(Majidreza Nazem) o A new critical state Mohr‐Coulomb soil model for large deformation analysis of sand

(Yuxia Hu) o Considerations on the design of plate anchors (Yinghui Tian)

‐ Closing discussion (10 mins)10.30 am – 11.00 am Morning tea 11.00 am – 12.15 pm 3‐minute thesis presentation (Chair: Scott Draper)

‐ Presentations by PhD students (see Page 8 for more detail)12.15 pm – 1.30 pm Lunch at the Moon Restaurant1.30 pm – 2.45 pm

Presentation Session 4: Georisk (Chair: Mark Cassidy)‐ Opening by Chair (5 mins) ‐ Presentations (8 mins presentation + 3 mins Q & A)

o Recent advances in centrifuge modelling for offshore foundations (Mark Cassidy) o Probabilistic analysis of dry soil mix columns (Jinsong Huang) o Failure mechanism and bearing capacity of footings buried at various depths in

spatially random soils (Lisa Li) o Determination of material properties by inverse analysis (James Hambleton) o Predicting the settlement of shallow foundations on sand (James Doherty)

‐ Closing discussion (10 mins)2.45 pm – 3.45 pm Group Workshop (2 parallel sessions)

‐ Opening by session leaders (updates and future visions) ‐ Parallel group discussion with themes of:

o Session 1 : Soft Soil Constitutive Models and Laboratory Testing (Lead: David Muir Wood & Antonio Carraro)

o Session 2 : Georisk (Lead: Mark Cassidy & Jinsong Huang)3.45 pm – 4.15 pm Afternoon tea4.15 pm – 5.00 pm

Closing session‐ Concluding remarks/presentations from each theme leader (5 mins each) ‐ Overall concluding remarks – Scott Sloan ‐ 3MT prize presentation – Mark Cassidy ‐ Closing – Mark Cassidy

POST WORKSHOP (THURSDAY 12 DEC 2013) 9.00 am Bus departs from Bunker Bay to Perth

POST WORKSHOP (FRIDAY 13 DEC 2013) 9.00 am – 10.30 am CGSE management committee meeting 10.30 am – 12.30 pm Workshop on moving boundary (Chair: John Carter)12.30 pm – 1.30 pm Lunch at the UWA UniClub


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3‐Minute Thesis Presentation Schedule

Day 1 10 Dec 2013 (11.00 am to 12.15 pm)

No. Last Name First Name Presentation Title

1 Ali Abid Quantitative risk assessment of rainfall‐induced landslides

2 Cheng Steven Combined vertical, horizontal and moment capacity of an hybrid foundation system

3 Colreavy Cathal Piezoball testing at the ballina soft soil research site

4 Dong Youkou Dynamics simulation of submarine landslides and its consequences on pipelines with Material Point Method

5 Eshkevari Seyed Nima Salimi Design of working platforms for tracked plant

6 Ghasemi Todeshkjoei

Ahmad Effects of installation on the capacity of helical anchors in clay

7 Ghorbani Javad Numerical analysis of dynamic compaction of soils

8 Han Chao Long‐term performance of suction embedded plate anchors in permanent mooring systems

Q & A session 1 (15 minutes) 9 Hu Pan Estimating the punch‐through potential for spudcan

penetrating sand overlying clay 10 Karambakhsh Pooya Numerical and experimental study of particle migration in soils

11 Kardani Omid Efficient sequential and parallel iterative solvers for large‐scale geotechanical problems

12 Kashizadeh Elaheh Investigation of ploughing process in sand with application to off‐shore geotechnics

13 Khishvand Mohammad The numerical simulation of contact problems using a third medium

14 Leckie Simon Pipeline on‐bottom stability and thermal expansion management: The interaction of sediment transport, geotechnics and structural behaviour

15 Li Xiaojun Rate effects on the uplift capacity of skirted foundations on clay

Q & A session 2 (15 minutes)


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3‐Minute Thesis Presentation Schedule

Day 2 11 Dec 2013 (11.00 am to 12.15 pm)

No. Last Name First Name Presentation Title

1 Liu Jerry Simplified riser‐soil interaction model for fatigue design of steel catenary risers

2 Ma Mark Interpretation of cone penetration data in layered clays

3 Ma Jiajie Numerical modelling of submarine landslide and its impact to offshore infrastructure using the material point method (MPM)

4 Michael Cocjin Sliding behaviour of a mobile foundation

5 Moavenian Mohammad Mesh optimisation methods for solving large deformation geotechnical problems

6 Mohr Henning The hydrodynamics of a recirculating (o‐tube) flume

7 Morton John The variation of Nc for a sphere penetrating soft soil

8 Muditha Pallewattha Improvement of soil stability along rail corridors through native vegetation ‐ bio engineering

Q & A session 1 (15 minutes) 9 Pankaj Boral Creep model capturing vacuum preloading

10 Perera Darshana Soft soil improvement using vacuum preloading and vertical drains

11 Queau Lucile Artificial neural network development for stress analysis of steel catenary risers in touchdown zone

12 Sabetamal Hassan Finite element algorithms for dynamic analysis of saturated porous media

13 Siahaan Firman Micromechanically‐inspired stone column behaviour

14 Tor Lim Guan Sampling disturbance of an intermediate soil

15 Zhang Xue Particle finite element analysis of the granular column collapse

16 Zheng Jingbin Numerical modelling of spudcan and cone penetration in multi‐layer soils

Q & A session 2 (15 minutes)


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Keynotes


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8th TERZAGHI ORATION:

PROTECTING SOCIETY FROM LANDSLIDES – THE ROLE OF THE GEOTECHNICAL ENGINEER

Suzanne Lacasse1

1 Norwegian Goetechnical Institute (NGI) Email: [email protected]

ABSTRACT Protecting society from landslides and reducing exposure and risk to population and property are areas where the geotechnical profession can practice both the art and the science of engineering legated by Karl Terzaghi. The lecture presents the lessons learned from selected case studies of slope failures. Since factor of safety remains the main indicator in practice to ensure slope safety, the lecture also discuss the significance of factor of safety. Over the past decade, the geotechnical profession has moved in a direction of increased awareness of both its role and contribution to a safer society. The role of the geotechnical engineer is not only to act as solely a technologist providing judgment on factors of safety. The role has evolved to providing input in the evaluation of hazard, vulnerability and risk associated with landslides.


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2ND McCLELLAND LECTURE:

ANALYTICAL CONTRIBUTIONS TO OFFSHORE GEOTECHNICAL ENGINEERING

Mark Randolph1

1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT The theme of my recent McClelland Lecture is the contribution of analysis to offshore geotechnical engineering. Analysis should underpin design approaches that we use in day to day practice, gradually replacing purely empirical correlations as our understanding of the underlying processes improves. Simple analytical relationships educate us in respect of which are the controlling groups of parameters, and guide us in respect of studies based on numerical analysis or physical model tests. Results from more complex numerical analysis need to be interpreted and synthesised into design guidelines, capturing the key influences by means of appropriate dimensionless groups. The lecture will illustrate how this can be achieved for relatively complex problems in offshore geotechnical engineering. The application areas considered range from the axial response of piles, to seabed infrastructure associated with deep water applications, including shallow skirted foundations and pipelines. The emphasis throughout is on analytical solutions, including appropriately framed outcomes of numerical studies. Some of the material is retrospective, summarising key contributions in an effort to facilitate access, and thus help close the gap between theory and practice.


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Presentation Session 1:

Industry Impacts


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OFFSHORE SHALLOW FOUNDATION OPTIMISATION – INDUSTRY IMPACT

Susan Gourvenec

Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT This presentation will outline recent developments in offshore shallow foundation design methods and the integration of the new techniques in industry guidelines and practice.

Offshore shallow foundations are used for a variety of applications (Fig.1.), and present design challenges including multidirectional loading, often fully three-dimensional, and which may involve uplift. Offshore industry guidance recommends classical bearing capacity theory, based on idealized foundation and soil conditions with modification factors to account for more realistic boundary conditions. The method has been shown to predict conservative limit states for many conditions relevant to offshore shallow foundations.

Selected research projects focused on optimization of shallow foundation design for offshore applications will be presented (see reference list). The outcomes from this research will be shown, and the process of engagement with the committees developing international standards and industry stakeholders through JIPs will be discussed. In many situations, the new methods developed through research offer significant economic benefits, far beyond the cost of the foundation itself. For example, a need for a heavy lift vessel to install subsea foundations because they are too large for the pipe-laying vessel represents a significant additional project cost component.

Figure 1. Applications of offshore shallow foundations Figure 2. PLET foundation (Subsea7)

REFERENCES Chatterjee, S., Mana, D.K.S, Gourvenec, S. & Randolph, M.F. (2013 in press) Large deformation numerical

modeling of the short-term compression and uplift capacity of offshore shallow skirted foundations. ASCE J. Geotechnical and Geoenvironmental Engng.

Feng, X., Randolph, M. F., Gourvenec, S. & R. Wallerand (2013 aop) Design approach for rectangular mudmats under fully three dimensional loading, Géotechnique 10.1680/geot.13.P.051.

Gourvenec, S., Vulpe, C. & Murthy, T. (2013, accepted) A method for predicting the consolidated undrained capacity of shallow foundations on clay. Géotechnique.

Mana, D.K.S, Gourvenec, S. & Randolph, M.F. (2013) An experimental investigation of reverse end bearing of skirted foundations. Canadian Geotechnical Journal, 50(10): 1022-1033, 10.1139/cgj-2012-0428 *

Mana, D.K.S, Gourvenec, S. & Randolph, M.F. (2014, aop) Numerical modelling of seepage beneath skirted foundations subjected to vertical uplift. Computers and Geotechnics, 55: 150-157.

Mana, D.K.S, Gourvenec, S. & Randolph, M.F. (2013, aop) A novel method to mitigate the effect of gapping on the uplift capacity of skirted foundations – Gap Arrestors. Géotechnique 10.1680/geot.12.P.173 (aop)

Mana, D.K.S, Gourvenec, S. & Martin, C.M. (2013) Critical skirt spacing for shallow foundations under general loading. ASCE Journal of Geotechnical and Geoenvironmental Engineering. 139(9):1554-1566.


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PIPELINE STABILITY ON MOBILE SEABEDS

Scott Draper1, Liang Cheng2, Hongwei An2, David White3 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT The University of Western Australia now has three recirculating O-tube flumes which can be used to experimentally investigate seabed mobility and pipeline stability. In this talk will be briefly describe the award winning O-tubes together with a range of O-tube projects that are impacting subsea pipeline design offshore Australia. These projects range from the assessment of stability of existing pipelines, investigating design solutions for new pipelines and the development of a new industry design guideline. The last of these activities is being undertaken as part of the STABLEpipe Joint Industry Project, which includes operators Woodside Energy Ltd. and Chevron Australia, engineering consultants JP Kenny and The University of Western Australia (UWA). The O-tube facilities have been discussed recently by An et al. (2013) and Mohr et al. sub.

Figure 1. Pipeline on a mobile seabed.

REFERENCES An, H., Luo, C., Cheng, L., & White, D. (2013). 'A new facility for studying ocean-structure–seabed interactions:

The O-tube'. Coastal Engineering, 82, 88-101. Mohr, H., Draper, S. Cheng, L., White, D., An, H., & Zhang, Q. (2013). ‘The hydrodynamics of a recirculating (O-

tube) flume’, submitted to Continental Shelf Research.


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PERFORMANCE IMPROVEMENT OF RAIL TRACKS USING GEOSYNTHETIC AND RESILIENT INCLUSIONS: INDUSTRY

IMPACT

Sanjay Nimbalkar1 and Buddhima Indraratna2

1 Research Fellow, Centre for Geomechanics and Railway Engineering, University of Wollongong, Australia. Email: [email protected]

2 Professor of Civil Engineering and Research Director, Centre for Geomechanics and Railway Engineering, University of Wollongong, Australia. Email: [email protected]

ABSTRACT Rail tracks serve the principal mode of transportation for bulk freight and passengers. Among coastal areas in Australia, the high cost of track maintenance is the main issue due to poor drainage of soft coastal soils, ballast degradation, fouling (e.g. coal and subgrade soil), differential settlement of track, pumping of subgrade soils, and track misalignment due to excessive lateral movements. The prospect of high speed rail (HSR) to connect the Australia’s major east coast population centres is an exciting opportunity, however, more resilient tracks to support HSR are necessary to withstand the substantially increased vibration, and cyclic and impact forces. Installing geosynthetics and shock mats in the track substructure can led to significant attenuation of these forces and thereby mitigate ballast degradation (Selig and Waters 1994, Nimbalkar et al. 2012, Indraratna and Nimbalkar 2013). A full-scale field trial was conducted on instrumented track sections near Singleton, New South Wales to investigate the effects of these artificial inclusions on the performance of the track built on different types of subgrade soils (Indraratna et al. 2013). The performance of each section of track was monitored using sophisticated instruments (strain gauges, pressure cells, settlement pegs, transient displacement monitoring frame, fiber bragg grating sensors (Figure 1) and computer controlled data acquisition system). The finding suggested that geogrids can decrease vertical strains of the ballast layer and a few selected types of geogrids can be used more effectively with soft subgrade soils. The development of supplementary design tool aims to assist railroad industries in the design and maintenance addressing issues of ballast fouling, the role of geosynthetics and resilient inclusions. Such design tool based on engineering analyses is capable of replacing empirical approaches, creating a positive impact on industries.

Figure 1. Installation of fiber bragg grating sensors

REFERENCES Indraratna B and Nimbalkar S (2013) Stress-strain-degradation response of railway ballast stabilised with

geosynthetics, Journal of Geotechnical and Geoenvironmental Engineering 139(5): 684-700. Indraratna B, Nimbalkar S and Neville, T (2013) Performance assessment of reinforced ballasted rail track.

Proceedings of the Institution of Civil Engineers–Ground Improvement, (in press). Nimbalkar S, Indraratna B, Dash SK and Christie D (2012) Improved performance of railway ballast under impact

loads using shock mats. Journal of Geotechnical and Geoenvironmental Engineering 138(3): 281-294. Selig ET and Waters JM (1994) Track geotechnology and substructure management, Thomas Telford, London.


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SUCTION CAISSON INSTALLATION AND EXTRACTION IN ANGOLA CLAY

Christophe Gaudin, Conleth O’Loughlin, Mark Randolph, Susan Gourvenec, David

White, Muhammad Hossain1 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT Centrifuge tests have been carried out to investigate the behaviour and performance of suction caissons, anchored in natural clay from offshore Angola. The testing programme focused on investigating the influence of internal ring stiffeners, pullout rate and setup time on the uplift capacity. The initial testing programme includes six tests (three with stiffeners, three without) featuring self-weight and suction installation, two setup periods of one month and one year and extraction at model velocities of 0.001 mm/s and 0.4 mm/s. The interpretation of the centrifuge results, and comparison with a similar programme performed at IFSTTAR France on a slightly different model, provided insights notably into the importance of the internal stiffeners in the development of internal and external soil friction during penetration and extraction. It was demonstrated that soil back flow does not occur around the internal stiffeners leading to nil internal friction resistance and larger soil flow at the tip outside the caisson. The penetration resistance can be accurately predicted assuming a relatively low external friction factor and a bearing factor for the lowest stiffener increasing with depth to account for the increased extrusion of the internal plug. Due to large soil flow outside the caisson during penetration of caisson with stiffeners, set-up potentially results in a shear strength along the caisson skirt reaching values higher than the initial strength. In contrast, caisson without stiffeners exhibited limited set-up. Caissons extracted at 0.1 and 0.4 mm/s exhibited different pull-out resistance that is not related to drainage conditions, but to combined strain rate effects and consolidation occurring during slow pull-out as the maximum uplift capacity is mobilised.

Figure 1. Suction caisson model with and without stiffeners

Figure 2. Evolution of friction factors with time factor. Deq = 1.13 m, cv = 3 m2/yr

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.001 0.01 0.1 1 10 100

External and

internal frictio

n factors,

ex,

in(‐)

Time factor, Tv (‐)

Stif. Caisson ‐ Sealed Ext.Stif. Caisson ‐ Vented Extrac.Unstif. Caisson ‐ Sealed Ext.Unstif. Caisson ‐ Vented Extrac.External friction factorInternal friction factorInstallation friction factors

May be ignored due to excess pore pressures during installation. SeeSection 5.3.1


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Ballina Soft Soil Testing

Facility


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PROGRESS AT THE BALLINA FIELD TEST FACILITY

Richard Kelly1 1 Centre of Excellence for Geotechnical Science and Engineering. Email: [email protected]

ABSTRACT A National Soft Soil Field Testing Facility has been established near Ballina, NSW. The purpose of the facility is to perform validation testing for theoretical and laboratory scale developments. The facility became operational in January 2013. An initial site characterization was performed using MASW and ERI geophysical techniques along with in-situ tests performed by NewSyd, IGS and Douglas Partners. Piezocone tests with pore pressure dissipation and various cone capacities have been performed as well as cone penetrometer, seismic dilatometer, seismic cone, shear vane, a hydrostatic profile tool, T-Bar and piezoball penetrometer tests. The data is being used to assess strength, stiffness and permeability characteristics of the soil.

Figure 1. NewSyd performing in-situ testing An embankment has been constructed using conventional prefabricated vertical drains and a new Jute drain product from India. The purpose of the drains is to speed up the consolidation process. The purpose of using the Jute drain is to demonstrate whether it works and to compare costs with conventional drains. The embankment is 3m high by 95m long by 25m wide. It was constructed during July and August 2013 and as of 7 November 2013 had settled 700mm. The jute and conventional sections of the embankment have settled a similar amount.

Figure 2. Embankment at end of fill placement A large number of Shelby Tube and Piston samples were obtained during installation of the instruments. Testing will be carried out at UoN, UoW and UWA. The purposes of the tests are to


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characterize the geological history of the site, profiles of geotechnical parameters with depth and to obtain parameters for use in constitutive modeling of anisotropic, structured, viscous soft clays. Preliminary results show that the clays contain a vast array of organic compounds and that the soil and water chemistry has a measurable effect on the engineering parameters obtained from the tests.

Figure 3. Osterberg piston sampling Soil samples were taken at fixed radial distances from prefabricated vertical drains to assess smear effects. These samples are being tested and analysed at UoW. A second embankment will be constructed without the use of prefabricated drains in 2014 to act as a control for loading undisturbed soil and to compare rate of consolidation effects on the magnitude of settlement. A detailed site investigation and laboratory testing program will be developed. Consideration is being given to using geophysical tomography to develop 3D profiles of pore pressures and soil stiffness beneath the embankment periodically during consolidation. Self boring pressuremeter tests will be performed in collaboration with UWA early in 2014. Sherbrooke block sampling is being planned for June 2014 in conjunction with NGI. A linkage grant application has been submitted to construct a vacuum consolidation embankment. This work will be a collaboration between UoN, UoW, Coffey, Douglas Partners, the Indian Jute Board and Menard-Bachy. If successful, the work would proceed in the second half of 2014. Negotiations are being conducted with Ecoflex to perform field trials for environmental and temporary works performance on access tracks constructed from aggregate filled recycled truck tyres. Plans are in development to assess variability of soil parameters and to maximize the data obtained from geophysical, in-situ and laboratory tests using probabilistic methods as a collaboration between UoN and UWA.


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CONTRIBUTIONS OF UNIVERSITY OF WOLLONGONG FOR BALLINA FIELD TEST FACILITY

Buddhima Indraratna1, Cholachat Rujikiatkamjorn1, Richard Kelly2

1Centre for Geomechanics and Railway Engineering, University of Wollongong, Australia. (Email: [email protected])

2Coffey Geotechnics, Chatswood, NSW

ABSTRACT For the first time in Australia, a trial embankment with bio-degradable and synthetic drains was constructed at the Ballina Field Test Facility (BFTF), with the objective of evaluating the relative performance of the 2 drain systems. The following aspects will be discussed during the presentation.

1. Embankment Design for vacuum and non-vacuum embankments; 2. Analysis of Drain Installation Effects; 3. Deformation analysis and stability assessment to compare the performance of Jute Drains

and Conventional Drains using the following KPIs: a. Lateral displacement at embankment toe/max. settlement at the centre line, b. Sv/Sp concept for evaluating the effectiveness of Vacuum Preloading (Sv= settlement

at a given time due to vacuum pressure; Sp = Settlement due to surcharge alone for an equivalent total stress at the same time),

c. Beta-factor application to distinguish the performance between vacuum and non-vacuum embankments based on Port of Brisbane experience,

d. Drain Effectiveness: Strain based Consolidation vs PWP based Consolidation; e. Amount of swelling at toe f. Rate of PWP dissipation: soil-drain interface effects (drain unsaturation and clogging).

4. Model parameters and testing program required to capture the viscous behaviour of soft clay

(Visco-plasticity vs the Mandel-Cryer effect or time-lag caused by non-Darcian flow)


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CHARACTERIZATION OF SMEAR ZONE DUE TO INSTALLATION OF VERTICAL DRAINS

Cholachat Rujikiatkamjorn and Buddhima Indraratna

Centre for Geomechanics and Railway Engineering, Faculty of Engineering, University of Wollongong, ARC Centre of Excellence in Geotechnical Science and Engineering, NSW, Australia. [email protected]

ABSTRACT Much of Australian coastal areas contain soft soils and marine deposits. Stabilization of soft formation soils by applying a surcharge load alone is often time-consuming. The installation of prefabricated vertical drains (PVDs) can significantly decrease the preloading duration by reducing the drainage path length. In this study the characteristics and the extent of the smear zone due the vertical drain installation are investigated using a large-scale consolidometer and a novel mandrel-driving machine capable of working at installation rates in the range of usual practices. The permeability and compressibility of the soil collected at Ballina Field Test Facility after PVDs installation are investigated to determine the extent to which the soil surrounding the PVD had become disturbed. The effects of the vertical drain installation on soil disturbance are analyzed with a new elliptical cavity expansion theory and a finite element analysis. The finite element model has been applied to a case history from the Second Bangkok International Airport in Thailand and proves that the model can be applied to field conditions.


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NUMERICAL SIMULATION OF CPT CONE PENETRATION IN SAND

George Kouretzis1, Daichao Sheng1 and Dong Wang2

1ARC Centre of Excellence for Geotechnical Science and Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Australia. Email: [email protected]


ABSTRACT Several studies in the literature dwell into the finite element modeling of cone penetration in cohesionless soils. A common assumption among these studies is the simulation of the nonlinear soil response with simple constitutive models, such as the Drucker-Prager. This approach has some limitations, as such basic models cannot straightforwardly take into account the dependence of the sand shear strength and compressibility on its in situ density and mean stress level. A new perspective on the numerical simulation of cone penetration testing in sand is presented, based on an enhanced critical state model balancing between flexibility and computational robustness, which was implemented in ABAQUS/Explicit. Its main advantage, compared to similar studies employing simpler soil models, is that sand compressibility can be described with a single set of model parameters, irrespective of the stress level and the initial relative density. Calibration of the numerical methodology was based on back-analysis of published centrifuge experiments of cone penetration in medium and dense Fontainbleau sand. Accordingly, the methodology proved successful in predicting the development of cone resistance with penetration depth in similar centrifuge tests from independent laboratories. Additional results depicted in Figure 1 suggest that the methodology can provide the disturbance zone due to cone penetration, which will depend on the initial relative density of the sand. The practical outcome of this study, resulting from a series of "numerical CPT tests", is a discussion on the estimation of in situ soil parameters from the cone penetration resistance. Although the aim of this work is not to replace existing well-established empirical relations, it is shown that the numerical methodology is able to capture the effects of sand density on moving boundary problems.

Figure 1. Specific volume contours in the vicinity of the cone for (a) Dr=54%-initial specific volume v0=1.72 and

(b) Dr=89%-initial specific volume v0=1.59 (detail).

REFERENCES Kouretzis GP, Sheng D and Wang D (2014) Numerical simulation of cone penetration testing using a new critical

state constitutive model for sand. Computers and Geotechnics DOI: 10.1016/j.compgeo.2013.11.002


23


Multiphysics Modelling

and Moving Boundary

Problems


24

MODELLING CEMENTING MINEFILL

David Muir Wood1 and James Doherty2

1 Centre for Offshore Foundation Systems, The University of Western Australia and University of Dundee, United Kingdom. Email: [email protected]

2 Civil and Resource Engineering, The University of Western Australia Email: [email protected]

ABSTRACT Underground mining creates large voids known as stopes. To ensure regional stability, these voids are generally backfilled with a mixture of tailings and a binding agent such as cement. Water is added to the material to achieve a slurry like consistency to aid transport of the material to the stope. This material can be referred to as a cementing backfill. The proportion of cement used in the mix is usually relatively small: often around 3 to 10 percent by mass. However, because of the typical volumes involved, the actually quantity of cement used can be very large and optimising the cement content can have significant economic benefit. During filling, barricades are constructed to contain the backfill as it consolidates. The stresses generated on the barricades, and the subsequent deformation and stability of exposed faces, depend on volumetric changes and cement bonding occurring during this consolidation process. The cementing backfill is typical of many weakly cemented natural and man-made materials. The hydrating cement creates bonds between particles thus generating increased stiffness and strength. Deformation of the backfill tends to damage these bonds and eventually, given sufficient deformation, the backfill reverts to an uncemented granular material. It is appropriate to devise a constitutive model for the cementing material based on a successful model for uncemented soil. Cementation is conveniently described as an attraction which increases with time and decreases with damage according to evolution laws which, together with the relationships describing the uncemented soil, are inspired by appropriate experimental observation. Rates of loading and shearing influence the mechanical response: the strength of the cementation bonds is dependent on the stresses experienced at the time of hydration and curing. Natural weakly cemented materials have bond strengths which are controlled by the in-situ stresses which were present during their formation. There are two characteristics of the uncemented material that play a central role. Critical states, attained at very large shear strains, provide an asymptotic response and associated asymptotic stress state and fabric. As they are sheared, granular materialsare very aware of the density change required to bring them to such an asymptotic state. A mechanism for density change is thus also essential and is provided through a stress-dilatancy relationship or flow rule which itself is dependent on the extent to which hydration of the cement content has progressed with time.


25

APPLICATION OF MATERIAL POINT METHOD TO GEOTECHNICAL PROBLEMS

Wojciech T. Sołowski1

1 Priority Research Centre for Geotechnical and Materials Modelling, The University of Newcastle. Email: [email protected]

ABSTRACT The material point method is a relatively new numerical method for solving boundary value problems. It is particularly well suited for calculations where very large deformations and dynamics must be taken into account. In geomechanics it can be applied to simulate landslides, penetration problems and moving boundary cases. To validate the method a number of test computations were performed. Problems solved included the large penetration of a strip foundation in a purely cohesive soil (Sołowski & Sloan, in preparation), simulation of a dynamic soil exchange experiment (Sołowski et al. 2013, Figure 1) and simulations of collapsing piles of granular materials (Sołowski & Sloan 2013 a,b). Finding an accurate solution with the traditional finite element method would be very difficult for these problems. The results obtained with the material point method are quite satisfactory, and it is anticipated the procedure will develop into a valuable tool in geotechnics. The material point method is particularly well suited to cases involving very large deformations and/or dynamics.

Figure 1. Final stage of dynamic soil exchange simulation. Numerical results (black and white dots) are

overlapping experimental data (brown and grey material, respectively). See Sołowski et al. 2013 for details.

REFERENCES Sołowski WT, Sloan SW (2013a). Modelling of sand column collapse with Material Point Method. Proceedings of

the 3rd international symposium on computational geomechanics (ComGeo III), Krakow, Poland, 21-23 August, 2013: 698-705.

Sołowski WT, Sloan SW (2013b, accepted). Material point method modelling of granular flow in inclined channels. Applied Mechanics & Materials. Special volume containing proceedings of ACCM 2013 conference held in Sydney, Oct 3-4. (ISSN: 1660-9336).

Sołowski WT, Sloan SW, Kanty PT, Kwiecień S (2013). Numerical simulation of a small scale dynamic replacement stone column creation experiment. Proceedings of the Third International Conference on Particle-based Methods – Fundamentals and Applications PARTICLES 2013 M. Bischoff, E. Oñate, D.R.J. Owen, E. Ramm & P. Wriggers (Eds): 522-533.

Sołowski WT, Sloan SW (in preparation). Evaluation of material point method for use in geotechnics.


26

ASSESSMENT OF POST-COMPACTED SOIL USING A NON-DESTRUCTIVE APPRAOCH

Ana Heitor1, Buddhima Indraratna1,Cholachat Rujikiatkamjorn1

1 Centre for Geomechanics and Railway Engineering, University of Wollongong. Email: [email protected], [email protected] , [email protected]

ABSTRACT Conventional field compaction control methods are effective at the time of placement. However, their measurements are discrete and have a limited depth of investigation, which may not be suitable for post-construction compaction quality assessments of deeper fills or larger surface areas. In this situation, classical destructive geotechnical surveys (i.e. boreholes, cone penetration tests) are sought to evaluate the current fill conditions. Nevertheless, these methods often do not provide the level of required information because only certain locations are tested and high costs are usually incurred. The use of available non-destructive cost and time effective methodologies, such as shear wave velocity surveys (i.e. SASW, spectral analysis of surface waves or HVSR, horizontal-to-vertical spectral ratio), offers a valuable alternative to efficiently control compaction over large areas during post-construction stages, and locate areas within the existing earth structures where the soil was not sufficiently compacted. In fact, shear wave velocity (Vs) has been used in the past to evaluate the quality of compaction or the in-situ void ratio, but the effect of partial saturation has been consistently neglected. While this may not be a major concern for natural ground profiles where the ground water level (GWL) is close to the surface, for reclaimed fill areas it is a significant problem where the GWL is usually located deeper. This is particularly noteworthy, because high in situ Vs may not truly represent a higher degree of densification, as compacted soil is under unsaturated condition and suction plays an important role in controlling the soil stiffness.

This study showcases the effects of partial saturation in the implementation of a field methodology based on the propagation of shear wave velocity and suction for evaluating the compaction quality. It encompasses the use of the small strain range in relation to laboratory and field approaches to characterise the behaviour of materials under different compaction conditions, as well macrostructure characterization using X-ray CT-scan techniques. The small strain behaviour was characterized using Bender elements (Figure 1) for both as compacted and post-compaction conditions. Moreover, cycles of wetting and drying were also imposed to the compacted specimens in an effort to understand the effect of climatic variations on the measured Vs and its relative importance for the field site in Penrith.

A new empirical formulation for evaluating the current void ratio or degree of compaction based on shear wave propagation and suction or water content is proposed in this study. The performance of the methodology developed was first calibrated for site-specific silty sand soil in laboratory and then assessed for field site located in Penrith, in which the evaluation of the current compaction degree is of paramount importance for the future redevelopment of the site.

Figure 1. Illustration of a pair of Bender-extender elements mounted on a triaxial cell pedestal.


27

LARGE DEFORMATION ANALYSIS OF GEOMECHANICS PROBLEMS BY HIGH-ORDER ELEMENTS

Majidreza Nazem

ARC Centre of Excellence for Geotechnical Science and Engineering. The University of Newcastle, Australia Email: [email protected]

ABSTRACT This study addresses the application of high-order triangular elements for large deformation analysis of geotechnical problems using the Arbitrary Lagrangian-Eulerian method. Important aspects of the formulation, including the nodal recovery process and the remapping of state variables, are discussed. The efficiency of these elements is represented by analysing a few challenging problems in geomechanics, such as investigating the undrained response of a soil layer under a strip footing subjected to large deformations as well as dynamic penetration of an object into a soil layer, and comparing the analysis results with results obtained using quadratic triangular elements. The results indicate that high-order elements, such 15-node and 21-node triangular elements, not only decrease the computational time, but increase the accuracy of the numerical results. Preliminary studies indicate that high-order elements can be used in a wide range of geotechnical applications including analysis of consolidation of soils, problems involving soil-structure interaction, and soil dynamics problems.

REFERENCE Nazem M, Kardani M, Carter JP and Sloan SW (2013) On the application of high-order elements in large

deformation problems of geomechanics, Proceedings of the Third International Symposium on Computational Geomechanics, ComGeo III: 284-291


28

A NEW CRITICAL STATE MOHR-COULOMB SOIL MODEL FOR LARGE DEFORMATION ANAYLSIS OF SAND

Xu Li1, Yuxia Hu2 and David White3

1 School of Civil Engineering, Beijing Jiaotong University. Email: [email protected] 2 School of Civil, Environmental and Mining Engineering, The University of Western Australia. Email: [email protected]


ABSTRACT State transformation, leading to softening or hardening, is common in the shearing of sand. This has caused many difficulties in the description of soil behaviours in numerical simulations involving large deformation analysis, since the softening and hardening behaviours of soil can’t be ignored. To characterise the state transformation of sand, the current dilatancy angle and friction angle was linked with the soil state parameter, which is defined as the difference between the current void ratio and that at the critical state under the same stress status. This concept has been used to extend the classical Mohr-Coulomb constitutive model to incorporate state dependent dilatancy and friction of the soil. Then the Critical State Mohr-Coulomb (CSMC) model was implemented in a large deformation finite element (LDFE) framework (AFENA software). The newly developed CSMC model has been validated using soil element tests, such as biaxial and triaxial test. The strain-dependent hardening and softening features of the sand sample have been replicated. This CSMC model has also been used in the spudcan penetration into sand over clay soils. It is observed that the phase transformation of sand has significant influence on the penetration resistance of spudcan during large penetration. When a shear ban is formed in sand, its dilantancy angle reaches zero and the sand finds its critical state. The peak resistance of spudcan depends on the initial thickness of the sand layer and its initial stress state as well. A formula was proposed to predict the spudcan peak capacity for sand over clay soils. The CSMC model has shown a great potential to capture sand phase transformation during large deformation of sand in various soil-structure interaction problems.

(a) Dense sand over clay (b) Loose sand over clay

Figure 1. Spudcan penetration resistance in sand over clay soils

REFERENCES Carter, J.P. and Balaam, N.P. (1995). AFENA users manual: Geotechnical Research Center, University of

Sydney. Bolton, M.D. (1986). The strength and dilatancy of sands. Geotechnique, 36(1): 65-78.


29

CONSIDERATIONS ON THE DESIGN OF PLATE ANCHORS

Yinghui Tian, Christophe Gaudin, Mark Cassidy and Mark Randolph Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT With the depletion of shallow water oil and gas reservoirs, offshore engineering is moving into deep water, which demands effective and economic anchoring systems to moor floating facilities to the seabed. Plate anchors, used increasingly in recent years, have been proved to be a dependable deep water anchoring solution, particularly for mobile vessels. One popular installation method is to use suction caisson housing the plate anchors to penetrate to the designed depth. After the retrieval of the caisson, the anchor left in a vertical inclination and then rotated or “keyed” by tensioning the initial slack mooring chain attached to the padeye. The anchor travels upwards as well as rotates during the keying process. This upward movement, technically termed loss of embedment, tends to reduce the anchor bearing capacity as offshore sediments typically exhibit an increasing shear strength with depth. With the intention to reduce loss of embedment, a keying flap hinged to the anchor fluke is widely used in current industry practice, which was based on the assumption that the keying flap would rotate away from the shank and consequently has a larger resistance to prevent the anchor upward movement (Wilde et al. 2001; Brown et al. 2011). However, our research (Tian et al. 2013a) showed that the flap did not rotate until keying is completed. In contrast to the initial assumptions, the flap rotation is not governed by the shearing forces along the flap, but by the passive bearing pressure acting at the back of the flap. Consequently, the flap wants to rotate inward during the keying process, which is prevented by the strengthening ribs. The rotation of the flap outwards is only initiated once the keying is completed. Consequently, the flap, in its current design is detrimental on the bearing capacity of the anchor, due to the reduced projected anchor area and the change in failure mechanism resulting from the asymmetry geometry generated by the padeye offset. The authors proposed new anchor design schemes in an attempt to develop an optimal keying flap (Tian et al. 2013b). The existence of keying flap unintentionally leads to fortuitous a padeye offset, i.e. the padeye is below the anchor middle line with an offset distance. This offset has influence on the bearing capacity (detrimental) and loss of embedment during the keying process (beneficial). However, of more significant benefit is the potential of the anchor to dive, characterised by a re-embedment into deeper and stronger soil, increasing the anchor capacity. Our recent study (Tian et al. 2013c) has analytically examined the trajectory of a plate anchor under pull-out load from a combined loading yield surface assuming associated flow. The behaviour of plate anchors with different padeye offsets and under loads of varying inclinations can be predicted. It is demonstrated that there is an optimal padeye offset, that depends on the load inclination and that causes the anchor to increase its embedment, and hence its capacity, under pull-out.

Figure 1. Illustration of plate anchor installation (courtesy of Intermoor)


30

REFERENCES Wilde, B., Treu, H. and Fulton, T. (2001). "Field Testing of Suction Embedded Plate Anchors". Pro. 11th Int.

Offshore and Polar Engr. Conf. Brown, R. P., Wong, P. C. and Audibert, J. M. (2011). "Sepla Keying Prediction Method Based on Full-Scale

Offshore Tests". Proc. of the 2nd Int. Symp. on the Frontiers in Offshore Geotechnics: ISFOG2 2010, Perth, Australia, Taylor and Francis Group.

Tian, Y., Gaudin, C., Cassidy, M. J. and Randolph, M. F. (2013a). "Considerations on the Design of Keying Flap of Plate Anchors." Journal of Geotechnical and Geoenvironmental Engineering ASCE, 139(7):1156-1164

Tian, Y., Gaudin, C., Cassidy, M. J. (2013b). "Improving Plate Anchor Design with a Keying Flap." Journal of Geotechnical and Geoenvironmental Engineering ASCE, under review.

Tian, Y., Cassidy, M. J. and Gaudin, C. (2013c) "The Influence of Padeye Offset on Plate Anchor Re-embedding Behaviour." Geotechnique Letters


31


Georisk


32

RECENT ADVANCES IN CENTRIFUGE MODELLING OF OFFSHORE FOUNDATIONS

Mark Cassidy1, Yinghui Tian, Conleth O’Loughlin, Christophe Gaudin, Britta Bienen


ABSTRACT An abbreviated version of our 2013 Zheng Guo-Xi Lecture at Zhejiang University, China, will be presented. This will highlight recent technical advances in our centrifuge facilities. The aim is to stimulate cross-node interest in developing new apparatus and physical testing programs to underpin the CGSE research agenda. The original abstract to the Zheng Guo-Xi Lecture is provided below, with the written version available as Cassidy et al. (2013). This paper presents some recent advances in apparatus and methods for geotechnical centrifuge testing of offshore foundations at the University of Western Australia. We operate a 3.6 m diameter fixed-beam machine capable of accelerating modelling experiments of 200 kg up to 200 times Earth’s gravity and a 1.2 m drum centrifuge that can operate at up to 400 gravities. This paper describes innovative actuators developed to measure combined vertical, horizontal and moment loads in our beam and drum centrifuges. These allow independent movement in three degrees of freedom. A description is also provided of the application of sophisticated load control algorithms that can follow complex and irregular cyclic loadings from offshore storms on geotechnical infrastructure, and test results for an unburied pipeline are presented. Recent visualisation and buckling experiments on offshore pipelines are discussed. The last centrifuge advancement discussed is the use of piezoelectric and MEMS accelerometers to measure the rapid penetration of deep water dynamically installed anchors. As we look to the future we are commissioning a new 10 m diameter beam centrifuge rated at 240 g-tonnes. This paper provides details of this new centrifuge, which will be ready for use in 2015, and describes the benefits to offshore foundation modelling that this larger centrifuge will provide.

REFERENCES Cassidy, M.J., Tian, Y., O’Loughlin, C., Gaudin, C., Bienen, B. (2013). Recent advances in centrifuge modelling

of offshore foundations. Proc. of 7th National Chinese Symposium on Physical Modelling in Geotechnics. Hangzhou, PR China. (Invited Keynote).


33

PROBABILISTIC ANALYSIS OF DRY SOIL MIX COLUMNS

Jinsong Huang1, Richard Kelly2, Scott W. Sloan1 1 The Australian Research Council Centre of Excellence for Geotechnical Science and Engineering,The University of Newcastle

2 Coffey Geotechnics

ABSTRACT The mechanical properties of dry soil mix columns can be highly variable but conventional design methods are deterministic in nature. Variability is often implicitly controlled by acceptance criteria in the construction specification, but it is often unclear what the acceptance criteria should be. This paper shows that both simple and advanced probabilistic methods can be readily used by practicing engineers to assess the performance/failure of dry soil mix columns. Reliability-based design methods and examples are given for the design of column strength as well as by adjusting the column spacing to achieve a target probability of unacceptable performance or failure. Further analyses of capital expenditure and maintenance cost are provided for guidance in optimising designs. Lastly, a simple design chart is developed to provide guidance with respect to acceptance criteria required to achieve desired performance.

Figure 1. Design chart for adoption of mean and variance to achieve design probabilities of failure.


34

FAILURE MECHANISM AND BEARING CAPACITY OF FOOTINGS BURIED AT VARIOUS DEPTHS IN SPATIALLY

RANDOM SOILS

Lisa Jinhui Li1 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT Many civil engineering problems are solved using buried structures, such as spudcan and anchors. Routine analysis on these structures is generally limited to uniform soils or soils with strength increasing linearly with depth. In reality the soil properties vary from place to place due to a combination of geologic, environmental and physical-chemical process. The bearing capacity of a footing can be greatly overestimated without accounting for the inherent random heterogeneity of soils. This presentation is to show how the spatial variability of the random soils affects the failure mechanism and the ultimate bearing capacity of the foundations buried at various depths. A non-linear finite element analysis combined with random field theory is employed to explore the foundations in random soils. Different possibilities of shear failures resulting from different spatial patterns of soils are demonstrated, which explained the significant discrepancy between the random soils and the uniform soils. Results show that the inherent spatial variability of soil shear strength can drastically modify the basic form of the failure mechanism. The effect of the spatial pattern of the soils on the development of shear planes was also investigated, with the coefficients of variation for the bearing capacity demonstrated to be closely related to the shear plane length. The statistical bearing capacity results are then used to estimate the failure probability of the foundations. Finally the factors of safety are proposed for practical engineering design according to different levels of failure probability. This study provides a thorough understanding on the failure mechanisms of the footings in random soils, especially where structures can penetrate deeply into soils in offshore engineering.

Figure 1. Strain for a surface footing in random soils and in uniform soils. The blue regions indicate weaker soil and the red regions indicate stronger soil. The grey regions indicate strain development.

REFERENCES Cassidy, M. J., Uzielli, M., and Tian, Y. H. (2013). “Probabilistic combined loading failure envelopes of a strip

footing on spatially variable soil.” Comput. Geotech., 49, 191-205.

In random soils In uniform soils


35

DETERMINATION OF MATERIAL PROPERTIES BY INVERSE ANALYSIS

Jim Hambleton1

1 Priority Research Centre for Geotechnical and Materials Modelling, The University of Newcastle, Australia. Email: [email protected]

ABSTRACT Estimating material properties such as shear modulus, undrained shear strength, softening/hardening modulus, and strain-rate parameters from in situ test data involves two key steps. First, a forward model that possibly relates a measured quantity to a controlled quantity (e.g., force versus displacement) is proposed. Second, the material properties in the forward model are regarded as unknowns, and these are estimated by fitting the forward model to the test data, typically using least-squares regression. Values of the material properties providing the least error between the forward model and data are deemed the best estimates of the true material properties. While this general procedure is well known, basic questions arise as to how accurately the material properties can be determined and whether multiple sets of material properties may lead to the same response (non-uniqueness). A general framework for addressing these questions is hampered by the problem-specific nature of inverse analysis, although some aspects of the theory have been developed (e.g., Frank 2007). This talk discusses techniques for assessing the viability of inverse analysis from direct examination of the forward model. A specific example involving estimation of apparent cohesion and friction angle based on indentation of a cylinder (Hambleton and Drescher 2012) is considered.

REFERENCES Frank PM (1978) Introduction to System Sensitivity Theory. Academic Press, New York. Hambleton JP and Drescher A (2012) Approximate model for blunt objects indenting cohesive-frictional materials.

International Journal for Numerical and Analytical Methods in Geomechanics 36(3): 249-271.


36

PREDICTING THE SETTLEMENT OF SHALLOW FOUNDATIONS ON SAND

James P. Doherty1 and David Muir Wood2

1 School of Civil and Resource Engineering, The University of Western Australia. Email: [email protected]

2Centre for Offshore Foundation Systems, The University of Western Australia and University of Dundee, United Kingdom. Email: [email protected]

ABSTRACT

Predicting the settlement of shallow foundations on sand should be a routine task for the geotechnical engineering profession. However, a number of footing settlement prediction competitions have demonstrated that there is presently no well-accepted or reliable solution for this very common problem. This is attributed to the continued pursuit of solutions that rely on some tenuous link between penetration test data and ground stiffness and then use this ground stiffness model to estimate the footing settlement via some elastic solution which incorporates (often only very approximately) the actual boundary conditions of the foundation. It also appears that the wide availability of sophisticated soil models in commercial finite element software has done little (if anything) to improve the situation. It is suggested that this is due either to a lack of confidence or to an absence of guidance in the application of such advanced soil models.

We present here a soil model that is simple enough to be applied in routine engineering practice, but sophisticated enough to capture the important mechanical responses of sand relevant to the footing settlement problem. The description of the elements of the model indicates its hierarchical origins as an extended Mohr-Coulomb model, with the intention that it should lie very close to if not actually within the engineer’s comfort zone. In order to emphasise the applicability of the model a detailed description of the process adopted for deriving model parameters is described. The model is used to describe the material at the UWA test site which has been characterised using laboratory and in-situ test data which were made available for a recent competition for the prediction of settlement of footings. The success of the model in describing the load-settlement response is demonstrated.


37

3 Minute Postgraduate

Student Thesis

Presentations: Day 1


38

QUANTITATIVE RISK ASSESSMENT OF RAINFALL-INDUCED LANDSLIDES

Abid Ali1*, Jinsong Huang1, Andrei V. Lyamin1, Scott W. Sloan1, Mark J. Cassidy2

1 Centre for Geotechnical Science and Engineering, The University of Newcastle, Australia. 2 Centre for Geotechnical Science and Engineering, The University of Western Australia.

*Email: [email protected]

ABSTRACT Rainfall-induced landslides are a serious geohazard in many parts of the world causing widespread casualties and damage to properties. These landslides are common in regions where soil is initially unsaturated as matric suction contributes to soil shear strength and thus slope stability (Fredlund et al., 2007). As rainwater infiltrates, matric suction reduces and soil unit weight increases and consequently the slope becomes less stable (White et al., 2012). To accurately predict landslide risk it is imperative to have a sound understanding of the factors responsible for triggering landslides. Amongst these factors, saturated hydraulic conductivity has been identified as a very important parameter for seepage and stability problems in unsaturated soils (Tsaparas et al., 2002, Rahardjo et al., 2007, Rahimi et al., 2010). Since soil is a spatially variable geomaterial, the saturated hydraulic conductivity will also vary spatially. Moreover, the nature of collapse mechanisms have been found to affect the failure depths with failures due to development of positive pore water pressure being deeper than those due to loss of matric suction for the same rainfall intensity. How the spatial variability of hydraulic conductivity will affect the landslide risk when collapse mechanisms are different is the aim of the present research work. By modelling the saturated hydraulic conductivity as a random field coupled with the newly developed framework of quantitative risk assessment of landslides ((Huang et al., 2013) where consequence is assessed individually for each failure), the landslide risk of two infinite slopes having different collapse mechanism (as illustrated in Figure 1 - left) is investigated. Both slopes have the same initial pore water pressure distribution and differ only in the value of their soil cohesion (which results in different collapse mechanisms) with the other soil properties being exactly the same. The pore water pressure due to rainfall infiltration is obtained by performing seepage analysis in HYDRUS 1D ((Simunek et al., 2005) which solves the modified form of Richard’s equation for 1D flow). The resulting pore water pressure distribution is then substituted in analytical expression to estimate the factor of safety and to find the first passing time of failure and first passing depth of failure (which is the failure consequence). Several thousand simulations are performed in a Monte-Carlo framework to assess the risk for different degrees of spatial variability for the two slopes. The study shows that if soils are highly variable with depth, there would be more shallow failures and consequently the risk would be less. Furthermore, the risk associated with failures due to development of positive pore water pressure is greater than for failures due to loss of matric suction for the same degree of spatial variability and rainfall intensity. Lastly, it is also observed that the risk is maximal for a certain degree of spatial variability. Though the infinite slope model has its own limitations, the results provide important insights into landslide risk assessment. Future studies are aimed to understand the effect of rainfall pattern on rainfall-induced landslide risk as well as investigating the rainfall-induced landslide risk for 2D slopes by considering limit analysis techniques.


39

Figure 1. Left - Pore water pressure profile, initially and at failure for the two cases. Histogram of failure depths for same spatial correlation lengths for Case 1 (middle) and Case 2 (right).

REFERENCES Fredlund, D. G. & Rahardjo, H. 2007. Soil mechanics for unsaturated soils, John Wiley & Sons, Inc. Huang, J., Lyamin, A. V., Griffiths, D. V., Krabbenhoft, K. & Sloan, S. W. 2013. Quantitative risk assessment of

landslide by limit analysis and random fields. Computers and Geotechnics, 53, 60-67. Rahardjo, H., Ong, T., Rezaur, R. & Leong, E. 2007. Factors controlling instability of homogeneous soil slopes

under rainfall. Journal of Geotechnical and Geoenvironmental Engineering, 133, 1532-1543. Rahimi, A., Rahardjo, H. & Leong, E.-C. 2010. Effect of hydraulic properties of soil on rainfall-induced slope

failure. Engineering Geology, 114, 135-143. Simunek, J., Van Genuchten, M. T. & Sejna, M. 2005. The HYDRUS-1D software package for simulating the

one-dimensional movement of water, heat, and multiple solutes in variably-saturated media. HYDRUS Software Ser, 1, 240.

Tsaparas, I., Rahardjo, H., Toll, D. G. & Leong, E. C. 2002. Controlling parameters for rainfall-induced landslides.

Computers and Geotechnics, 29, 1-27. White, J. A. & Singham, D. I. 2012. Slope stability assessment using stochastic rainfall simulation. Procedia

Computer Science, 9, 699-706.

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40

COMBINED VERTICAL, HORIZONTAL AND MOMENT CAPACITY OF AN HYBRID FOUDNATION SYSTEM

Steven Cheng

Centre for Offshore Foundation Systems, The University of Western Australia, Email: [email protected]

ABSTRACT This report describes a programme of centrifuge tests conducted at the Centre for Offshore Foundations Systems (COFS) within the University of Western Australia in November 2012. This experimental series is part of the ARC Linkage Project between the project partner COFS and Keppel in order to investigate the feasibility and performance of a hybrid foundation system, composed of a skirted mat and a suction caisson. The Stage 1 modelling programme aimed to investigate the feasibility of suction-induced preloading through a series of beam centrifuge test on a hybrid foundation, as well as the performance of preloaded foundations through a series of drum centrifuge tests on a skirted mat foundation. Stage 2 was performed in order to extend the investigation to i) the influence of the caisson skirt length on the plug behaviour, ii) the influence of the mat/caisson area ratio on the performance of the suction-induced preloading, iii) the performance of the suction-induced preloading in comparison to a direct preloading and iv) the feasibility of reverse pumping extraction of the foundation unit. Stage 3 (this report) contained centrifuge experiments which investigated the bearing capacity of the hybrid foundation under combined vertical (V), horizontal (H) and rotational (M) loads. Different hybrid foundation designs were used to investigate the caisson influence in terms of varying diameters and skirt lengths. As a result, combined VHM yield envelopes was generated. It’s found that increases in vertical capacity are more influenced by an increase in the diameter than an increase in skirt length. In contrast, larger capacity increases in the horizontal and rotational directions can be gained by extension than by enlarging. Finally, comparison was drawn to other published date. The experimental results of the skirted mat were compared to other published data of skirted mats whose results were obtained from either numerical or experimental investigations. In summary, the influence of caisson parameters on the combined load capacity of a hybrid foundation was successfully investigated which will provide a good base for further complementary investigations.


41

PIEZOBALL TESTING AT THE BALLINA SOFT SOIL RESEARCH SITE

Cathal Colreavy1*; Conleth O’Loughlin1, Mark Randolph1

1 Centre for Offshore Foundation Systems, The University of Western Australia. * Email: [email protected]

ABSTRACT Full flow penetrometers have been increasingly used during site investigations, particularly in soft offshore soils. The T-bar was originally developed as a site investigation tool in the centrifuge (Stewart & Randolph, 1991) and later scaled up for use in the field, both onshore and offshore (Randolph et al., 1998). The ball penetrometer was later proposed to reduce the chance of the load cell being subjected to bending. The main advantage of full flow penetrometers over the traditional cone are:

improved accuracy in soft soils due to the larger projected area; minimal corrections required for overburden and pore pressure; theoretical solutions exist for deducing the shear strength from the penetration resistance; the ability to measure the remoulded penetration resistance directly by cycling the device over

a small depth range. More recently, pore pressure elements have been added to these full flow devices, in particular the ball. Measurement of pore pressure during piezoball (a ball with pore pressure measurement) tests gives information on the degree of drainage during penetration. In addition, measurement of pore pressure during dissipation tests may be used to estimate the consolidation properties of the soil. Many different sensor positions have been experimented with in previous studies. Most have focused on the equator position, however recent centrifuge tests suggest that the mid-face is the most appropriate filter location during piezoball tests, particularly during dissipations. A recent centrifuge study at UWA (Mahmoodzadeh & Randolph, 2013) proposed measuring pore pressure at two locations on the piezoball as a means of assessing the consolidation properties.

For this research, a piezoball penetrometer was built in-house at UWA (Figure 1). The ball, which has a diameter of 60 mm and a shaft diameter of 20 mm (As/Ap = 0.11) measures pore pressure at three different locations; the tip, mid-face and equator. In total there are nine pore pressure sensors, four around the equator circumference, four around the mid-face and one at the tip. The sensors are located as close as possible to the surface to minimize saturation problems.

Figure 1. UWA triple element piezoball

To test this new penetrometer, field trials were carried out at the Ballina soft soil testing facility. Testing included penetration, dissipation and cyclic remoulding tests along with comparative piezocone tests. Initial analysis of results has shown that the piezoball performed well. One particularly promising finding is that the piezoball reached 50 % dissipation faster than the piezocone during dissipation tests.


42

Further tests are planned for the site next year. These tests will include variable rate tests to assess the degree of drainage on the piezoball pore pressure generation and subsequent dissipation.

REFERENCES Mahmoodzadeh, H and Randolph, M.F. (2013) Penetrometer testing – effect of partial consolidation and

subsequent dissipation response. Submitted. Randolph, M.F., Hefer, P.A., Geise, J.M. & Watson, P.G. (1998) Improved seabed strength profiling using the T-

bar penetrometer. Offshore Site Investigation and Foundation Behaviour ’98, Science and Underwater Technology, London, 221 – 236.

Stewart, D.P. and Randolph, M.F. (1991) A new site investigation tool for the centrifuge. Proc. Int. Conf. on Centrifuge Modelling – Centrifuge 91, Boulder, Colorado, 531 – 538.


43

DYNAMIC SIMULATION OF SUBMARINE LANDSLIDES AND ITS CONSEQUENCES ON PIPELINES WITH

MATERIAL POINT METHOD

Youkou Dong1, Dong Wang, Mark Randolph 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT Submarine landslides are common on continental slopes, especially where with weak layers. In recent decades, many cases of submarine landslides were reported (Weaver et al. 2000), among which the largest ones spanned several thousand km2 (Hampton et al. 1996). Although some national and international projects (Masson et al. 2006) were directly related to them, unfortunately, submarine landslides remain to be an open issue. The probability of occurrence of submarine landslides along the routes raises the running risk of pipeline systems in their life span. Once submarine landslides occur, the moving masses in the areas exert extremely significant impact forces on the downstream pipelines, arousing severe damage or even failure of the pipelines (Locat 2001; Locat and Lee 2002). Therefore, estimation and control of impact forces from submarine landslides are necessary for pipeline design and routing. It is generally accepted that the processes of submarine landslides involve three stages: slope failure of intact soil, debris flow of remoulded soil and turbidity current of heavy fluid (Boukpeti et al. 2012). My thesis will focus on the second stadge, in which strain-softening and rate-dependency effect should be considered. MPM is developed by Sulsky et al. (1995) from particle-in-cell (PIC) method (Harlow 1963), which is extensively used in fluid dynamics. When compared with conventional mesh-dependent numerical methods, MPM is better to capture extremely-large deformation in Offshore Engineering. Similar to arbitrary Lagrangian Eulerian (ALE) method, MPM takes advantage of both Lagrangian and Eulerian meshes, which makes MPM has an excellent balance between accuracy and efficiency.Some verification work of MPM in Offshore engineering is done, such as strip and circular shallow footing case and T-bar and Ball penetration case. And some research in free falling penetrometer is on-going to explore the behavior of MPM in high speed penetration problems, in which strain-softening and rate-dependency effect are considered. The used MPM software, named as Uintah, developed by Utah University, is a 3D code. To reduce the required computational resources of plane strain problems, the 2D framework was developed, which was verified and 40% - 50% of computational time was saved by 2D framework from 3D. GPU parallelisation was also tried. The CUDA, a GPU parallelisation platform released by NVIDIA Corporation, was incorporated into my MPM program recently coded on Windows platform. However, it is found that GPU paralleling did not show as much advantage as expected. The reason is the overall cost induced by frequent and heavy saving and reading of GPU and CPU memories.

REFERENCES Boukpeti N, White DJ, Randolph MF, and Low HE (2012) The strength of fine-grained soils at the solid-fluid

transition. Geotechnique 62(3): 213-226. Hampton MA, Lee HJ, and Locat J (1996) Submarine landslides. Rev. Geophys. 34: 33 - 59. Harlow FH (1963) The particle-in-cell method for numerical solution of problems in fluid dynamics. Proceedings of

Symposia in Applied Mathematics 15: 269 - 288. Locat J (2001) Instabilities along ocean margins: a geomorphological and geotechnical perspective. Mar. Pet.

Geol. 18(4): 503 - 512. Locat J, and Lee HJ (2002) Submarine landslides: advances and challenges. Can. Geotech. J. 39: 193 - 212. Masson DG, Harbitz CB, Wynn RB, Pedersen G, and Lovholt F (2006) Submarine landslides: processes, triggers

and hazard prediction. Phil. Trans. R. Soc. A 364: 2009 - 2039. Sulsky D, Zhou SJ, and Schreyer HL (1995) Application of a Particle-in-Cell Method to Solid Mechanics.

Computer Physics Communications 87: 236-252. Weaver PPE, Wynn RB, Kenyon NH, and Evans J (2000) Continental margin sedimentation with special

reference to the Northeast Atlantic margin. Sedimentology 47: 239-256.


44

DESIGN OF WORKING PLATFORMS FOR TRACKED PLANT

Seyed Nima Salimi Eshkevari1

1 Ph.D Student, The University of Newcastle Australia. Email: [email protected]

ABSTRACT Drilling rigs and other tracked plant such as crawler cranes are commonly used on construction sites with their size and capacity continuing to grow with advancements in technology. Safe operation of such plant requires reliable ground support and temporary working platforms are often utilised to provide a safe working environment on sites with weak subgrades. The guideline BR-470 published by (Building Research Establishment 2004) is currently used for the design of working platforms for tracked plant. Although the guideline has been found to be successful in achieving its primary goal i.e. provision of safe working platforms to support tracked plant, Some users have reported unnecessarily (uneconomically) large platform thicknesses following the presented approach in the guideline (Corke and Gannon 2010).The guideline contains a very simplified semi-empirical design approach that places some restrictions that limit its application. A more rigorous geotechnical design approach can help to improve the economy of designs, as well as facilitating to account for the influence of quality assurance on design outcomes. The existing research aims to apply the concepts of reliability analysis to develop a design method that accounts for the quality of geotechnical data obtained from different test methods. The load and resistance factors will be determined so that consistent level of reliability is obtained for all design conditions. This requires us to evaluate and apply the uncertainties of all variables including soil parameters obtained from specific tests, to reliability analysis of the problem. The uncertainties of geotechnical parameters estimated by certain in-situ test methods are to be determined for the purpose of this research. The in-situ test methods considered in this study are Cone Penetration Test (CPT), Plate Load Test (PLT) and Field Vane Shear Test (FVT) for subgrade materials and in-situ dry density and PLT for working platform materials.

In the proposed Reliability Based Design approach, the ultimate bearing capacity of working platforms is considered as the limit state to be prevented by a satisfactory margin of safety. Punching shear failure has been selected as the most critical failure mode for working platforms under tracked plant loading. Rigorous punching shear coefficients using Finite Element limit Analysis program are obtained to improve prediction of ultimate limit state loadings. The uncertainties of punching shear coefficients obtained using this numerical analysis technique, will be evaluated and applied into reliability analysis of the problem.

The outcomes of this research will provide practitioners an improved design method that does not suffer from limitations of the existing design method. It also provides resistance factors based on the test method used to estimate geotechnical properties of materials. Using this method, the designers may compare the costs of additional site investigations with the savings from less conservatively designed platforms, and decide the most economical site investigation plan for each site.

REFERENCES Corke, D. and J. Gannon (2010). "Economic design of working platforms for tracked plant." GROUND

ENGINEERING 43(2): 29-31.


45

EFFECTS OF INSTALLATION ON THE CAPACITY OF HELICAL ANCHORS IN CLAY

C. Todeshkejoei

ARC Centre of Excellence for Geotechnical Science and Engineering, The University of Newcastle, Australia. Email: [email protected]

ABSTRACT Helical anchors (Fig. 1) are deep foundations that can be used in either tension or compression. The use of helical anchors has widely expanded both offshore and onshore during recent years due to their advantages over traditional pile, which mainly derive from its method of installation. These advantages include cost effectiveness, capacity to resist loads immediately after installation, little or no vibration during installation, smaller crews and equipment, low weight of construction equipment, and ease of installation in all-weather conditions. Despite recent development in using helical anchors, engineers and designers still lack confidence, as existing design methods are highly questionable and inadequate. Moreover, current design methods have been found to be excessively under- or over-conservative, which has in turn limited the use of helix anchors for civil engineering infrastructure projects. Almost none of the current methods and studies for assessing the uplift capacity of helical anchors account for disturbance of the soil due to installation, which may explain the main deviations between measured and predicted capacities. In other words, existing methods basically consider the uplift capacity of pre-embedded helical anchors. This study utilizes analytical, numerical, and physical models to understand the three key phases during installation and pullout in fully saturated clay: (1) the installation process itself, in which the helical anchor is twisted into the ground, (2) recovery of soil strength over time due to reconsolidation, and (3) undrained uplift capacity of helical anchors based on the heterogeneous strength profile due to soil disturbance. Installation and uplift tests will be conducted using the beam centrifuge located at The University of Western Australia. As the interaction between a helical anchor and soil during installation is incredibly complex, and modelling the process using conventional numerical techniques (e.g., finite element method) is significantly challenging, this study explores the viability of approximate analytical techniques and models. The installation model provides information on the extent of the disturbed zone and magnitude of shearing experienced by the soil. In particular, the model offers a framework for assessing the ratio of rotation to vertical displacement during installation, with each scenario inducing different degrees of soil disturbance. This study will improve the understanding of the impact of the installation process on the ultimate capacity of helical anchors in fully saturated clay, and it will lead to safer and more economical designs, as well as possible technological breakthroughs. The main outcome of this study will be an easy-to-use yet reliable framework from which design engineers can estimate the capacity of helical anchors with regard to the time-dependent effects of installation.

Figure 1: (a) schematic of a dual-helix anchor and (b) installation of helical anchor (image courtesy of Viking Helical Anchors).

(a) (b)


46

NUMERICAL ANALYSIS OF DYNAMIC COMPACTION OF SOILS

Javad Ghorbani1, Majidreza Nazem2

and John P. Carter3 1 Centre for Geotechnical and Materials Modelling, School of Engineering, The University of Newcastle. Email:

[email protected] 2 Centre for Geotechnical and Materials Modelling, School of Engineering, The University of Newcastle. 3 Centre for Geotechnical and Materials Modelling, School of Engineering, The University of Newcastle.

ABSTRACT In unsaturated soils media, which include solid particles, water, and air, the water content has a significant impact on their dynamic response, and particularly their capability for dynamic compaction. The ability of soil in absorbing the applied energy strongly depends on its degree of saturation. This research mainly focuses on the finite element simulation of partially saturated soils behavior under dynamic loading to investigate the effect of water content on the densification properties of a soil mass. To achieve this goal, a finite element routine within the framework of the generalised Biot’s theory is developed and implemented into an in-house program with capillary pressure, displacement and pore water pressure as separate degrees of freedom. The fundamental equations are derived based on the mass conservation law and linear momentum balance of all three phases in an isothermal environment. In addition, a few experimental equations are employed to demonstrate hydraulic conductivity and drainage characteristic of the soil, such as suction-saturation relationship and permeability dependency on suction. The time integration of global equations is performed by application of Generalised-alpha method. The accuracy of finite element code will be verified by comparing numerical results with analytical solutions and experimental data. Finally, the effect of large deformations and boundary condition changes will be considered in the simulation of dynamic compaction problem, and the effect of water content as well as the applied energy on soil compaction will be studied in detail.


47

LONG-TERM PERFORMANCE OF SUCTION EMBEDDED PLATE ANCHORS IN PERMANENT MOORING SYSTEMS

Chao Han1 Dong Wang2 Christophe Gaudin3 Mark Cassidy4

1 PhD candidate, Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

2 Assistant Professor, Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]"

3 Research Associate Professor, Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

4 Winthrop Professor, Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT Suction Embedded Plate Anchors (SEPLAs) are one of recently developed taut-leg anchoring systems, which borrow the concept of suction caissons for precise positioning and that of plate anchors for cost efficiency. The plate anchor is normally rectangular in shape, and is initially slotted vertically inside a suction caisson. The suction caisson is penetrated to the target depth and then retrieved, with the SEPLA being left in the soil. The mooring chain attached to the padeye is tensioned and the anchor is led to rotate until an inclination approximately perpendicular to the chain direction. The installation process is termed keying, which induces a loss of embedment depth referencing to the original position. It has been extensively studied by a number of experimental and numerical analyses.

In the early applications prior to 2004, SEPLAs were designed to moor MODUs (mobile drilling units) for short-term purpose. Their monotonic capacity resisting extreme weather conditions has been investigated by preceding researchers, finding a bearing factor in the range of 10 – 14 for deeply embedded anchors, dependent on the shape and ambient soil properties. Thereafter with the deployment of the first permanent SEPLA in 2006 for a floating production unit in the Gulf of Mexico, research attention has been drawn to its long-term behaviour, i.e. its performance against sustained and cyclic loadings generated by tides, waves, seasonal currents and storms.

This project is to investigate SEPLA behaviour for permanent mooring applications, and in particular to study the foundation response to various levels of sustained and cyclic loadings. In terms of sustained loading, numerical LDFE analyses incorporating commercial FE software ABAQUS into ‘remeshing and interpolation technique with small strain’ strategy, have been conducted, to capture the consolidation effect of soil within the duration of load. The constitutive relationship of soil is defined by Modified Cam-Clay model to represent normally consolidated kaolin clay. Numerical results indicate that the SEPLA foundation can hold a sustained load approximately up to 80% of its monotonic capacity, without excessive anchor movement. Also in this loading range, the undrained strength of soil is increased within the duration of sustained load, due to the dissipation of excess pore pressures. Subsequently, experimental tests were carried out in state-of-the-art centrifuge facilities. With the aid of Particle Image Velocimetry technique and load measurement, visualisation of soil flow mechanism around anchor and determination of capacity variation were both enabled. Nevertheless, future work that entails validating numerical methods in accordance to test results, and broadening available experimental database is still necessary.

Further research effort needs to cover the aspect of cyclic loading as well. It includes physically modelling the real-life loading conditions, and developing a reliable numerical approach accounting for cyclic effect in clay foundation, for instance strength degradation and pore pressure accumulation. At last, a refined SEPLA design scheme would be established at their mutual support.


48

ESTIMATING THE PUNCH-THROUGH POTENTIAL FOR SPUDCAN PENETRATING SAND OVERLYING CLAY

Pan Hu, Dong Wang, Sam Stanier and Mark Cassidy Centre for Offshore Foundation Systems, The University of Western Australia

ABSTRACT In the jack-up industry the punch-through failures continue to be a major problem during installation of jack-up platforms, when unexpected sudden and rapid penetration of spudcan foundations occurs. The hazardous sand overlying clay soils have been encountered in many worldwide region of active jack-up operation. Prediction of the full penetration-resistance profile is the prerequisite in assessing the risk of punch-through failure of spudcan foundations on sand overlying clay. Simple methods are needed to predict the full penetration resistance profile for a spudcan penetrating through sand overlying clay that can be used routinely in site-specific assessment of jack-up safety (Osborne et al. 2006). A large deformation finite element analysis method, the Coupled Eulerian-Lagrangian approach is used to model the complete penetration resistance profile of a spudcan on sand overlying clay. The punch-through behaviour observed in the experiments is replicated and the penetration resistance profiles from the numerical analyses are generally a good match to the experimental measurements. The penetration resistance in the underlying clay layer is well predicted using a simple linear expression for the bearing capacity factor for the spudcan and underlying sand plug. This expression is combined with an existing modified failure stress dependent model for predicting peak resistance to form a simplified method for prediction of the full penetration resistance profile. This new method provides estimates of the vertical penetration that the spudcan will run during the punch-through event. It is validated against both loose and dense sand centrifuge tests.

REFERENCES Osborne, J., Pelley, D., Nelson, C. and Hunt, R. (2006). “Unpredicted jack-up foundation performance.”

Proceedings of the 1st Jack-up Asia Conference. Singapore.


49

NUMERICAL AND EXPERIMENTAL STUDY OF PARTICLE MIGRATION IN SOILS

Pooya Karambakhsh1, Daichao Sheng2, Kristian Krabenhoft2, Klaus Thoeni2,

Richard Merifield2

1 ARC Centre for Geotechnical Science and Engineering, The University of Newcastle. Email: [email protected]

2 ARC Centre for Geotechnical Science and Engineering, The University of Newcastle

ABSTRACT The flow of water in soils may result in migration of smaller particles into coarser materials leading to stability or performance problems such as piping and internal erosion in embankment dams (Richards and Reddy, 2007) and levees (El Shamy and Aydin, 2008) and mud pumping in ballasted railway tracks (Haque et al., 2007). In this study the problem of particle migration in soils is to be investigated using Discrete/Distinct Element Method (DEM) coupled with a numerical model of fluid based on Navier-Stokes equation (Batchelor, 2000). This method will provide the means to demonstrate soil behavior in particle-scale and the interaction between the fluid and the soil particles through buoyancy and drag forces (Figure 1). Moreover, due to the limited experimental data on some infiltration problems such as mud-pumping in which the pore water pressure change under cyclic loading leads to the travel of fine particles to the coarser layers (Alobaidi and Hoare, 1996), laboratory tests will be carried out on the samples obtained from specific sites with the history of the occurrence of the problem. In both numerical and physical modeling, involving parameters such as representative particle-size ratio and initial void ratio (relative density) of soil layers as well as initial and change in pore pressure and characteristics of applied loads will be investigated. Finally, the gathered database of numerical and experimental results will be employed to propose a constitutive model capable of representing the observed behavior.

Figure 1. Mud pumping under cyclic load

REFERENCES ALOBAIDI, I. & HOARE, D. J. 1996. The development of pore water pressure at the subgrade-subbase interface

of a highway pavement and its effect on pumping of fines. Geotextiles and Geomembranes, 14, 111-135.

BATCHELOR, G. K. 2000. An Introduction to Fluid Dynamics, Cambridge University Press. EL SHAMY, U. & AYDIN, F. 2008. Multiscale Modeling of Flood-Induced Piping in River Levees. Journal of

Geotechnical and Geoenvironmental Engineering, 134, 1385-1398. HAQUE, A., KABIR, E. & BOUAZZA, A. 2007. Cyclic filtration apparatus for testing subballast under rail track.

Journal of Geotechnical and Geoenvironmental Engineering, 133, 338-341. RICHARDS, K. & REDDY, K. 2007. Critical appraisal of piping phenomena in earth dams. Bulletin of Engineering

Geology and the Environment, 66, 381-402.


50

EFFICIENT SEQUENTIAL AND PARALLEL ITERATIVE SOLVERS FOR LARGE-SCALE GEOTECHANICAL

PROBLEMS

Omid Kardani1, Andrei V. Lyamin2, Kristian Krabbenhøft3

1 Centre for Geotechnical and Material Modelling, The University of Newcastle. Email: [email protected] 1 Centre for Geotechnical and Material Modelling, The University of Newcastle. Email: [email protected]

1 Centre for Geotechnical and Material Modelling, The University of Newcastle. Email: [email protected]

ABSTRACT The aim of this project is to development iterative solution schemes to tackle to common problems for large scale geotechnical stability problems; that is impractical virtual memory requirements and prohibitive computation times. With problem the size of problem increasing beyond few millions of variables the use of direct solvers becomes more and more prohibitive (both in storage and computational times) even on modern computers. This motivates the study on possible employment of iterative methods to solve systems of equations resulting from Interior Point Method (IPM)-based formulations of the original geotechnical problem. However, iterative solvers are not efficient without preconditioning techniques for difficult (highly ill-conditioned) problems, such as those which occur at last iterations of IPM methods. This motivates using appropriate preconditioners to enhance the efficiency of the iterative solution schemes. While the sequential implementation of the developed iterative solution scheme offers a robust solver in this application and can effectively resolve the problem of virtual memory requirements [15], the computation time is still a drawback for this approach. Even the finest implementation of such iterative scheme still requires too many iterations and the only remedy seems to lay in a parallel implementation of the preconditioned solver and use of Graphic Processing Units (GPU) [16]. Therefore, the implementation is extended to parallel computational environment using a GPU. This extension brings about significant acceleration of the computations (up to 50 times). In fact, the results from parallel computations suggest that by exploiting an optimized approach for implementation on GPU using CUDA, achieving a state-of-the-art solution algorithm able to efficiently solve large scale geotechnical problems in acceptable computation times, hence fulfillment of the second aim of the project.

REFERENCES O. Kardani, A. V. Lyamin, K. Krabbenhøft,” A Comparative Study of Preconditioning Techniques for Large Sparse

Systems Arising in Finite Elements Limit Analysis”, Accepted for publication, International Journal of Applied Mathematics, 2013.

O. Kardani, A. V. Lyamin, K. Krabbenhøft, ” On Implementation of Robust Iterative Solution Schemes for Large Scale Problems in Computational Geomechanics”, Under revision.

O. Kardani, A. V. Lyamin, K. Krabbenhøft, “Implementation of Parallel Preconditioned Conjugate Gradient on GPU for Large Sparse and Highly Ill-conditioned Systems Arising in Computational Geomechanics”, Submitted.


51

INVESTIGATION OF PLOUGHING PROCESS IN SAND WITH APPLICATION TO OFFSHORE GEOTECHNICS

Elaheh Kashizadeh1,*, James Hambleton1, Samuel Stanier2, David White2

1 Priority Research Centre for Geotechnical and Materials Modelling, The University of Newcastle, Australia 2 Centre for Offshore Foundation Systems, The University of Western Australia, Australia

* Email: [email protected]

ABSTRACT Numerous applications in geomechanics involve large displacement of soil by lateral movement of an object across a soil surface, i.e., ploughing. In applications such as earthmoving and trenching, the ploughing phenomenon is plainly visible, although the phenomenon can also play an indirect role. For example, significant volumes of soil can be displaced when pipelines buckle laterally, and tractive devices on off-road equipment often rely on grousers or cleats that plough the soil as a result of slippage. While some level of understanding of ploughing in clay can be derived from the literature on machining of metals, studies concerned with ploughing in sands are limited. Also, there is a basic lack of understanding of the unsteady regime in which the object first begins to slide across the surface with prescribed force or penetration. In this transient regime, the shape and location of material boundaries, as well as the stress and strain fields, evolve in time. As a consequence, basic quantities of interest such as horizontal (drag) force and penetration also change in time. Generally the aim of this research is to build a basic understanding of ploughing processes in dry sand beginning with fundamental aspects of the process for simple object geometries and progressing the applications. A rigorous, effective numerical approach for modeling the ploughing process has been developed which draws on concepts from the kinematic method of limit analysis to obtain a realistic pattern of deformation within each increment of plough displacement. This method will be extended to account for softening/hardening and dilatation/compaction, depending on the initial state of the sand, and the results will be validated by experimental tests (Fig.1).

One application of particular interest in this thesis is the influence of ploughing on the lateral capacity of shallow offshore foundation systems, especially so-called grillage foundations consisting of a series of grilles that are pushed through the soil when the horizontal load becomes sufficiently large. In this case, ploughing is beneficial insofar as the accumulation of displaced material ahead of the grilles tends to increase the lateral capacity of the foundation. However, the questions arise as to how much the accumulated material contributes to the capacity, how much lateral displacement occurs prior to achieving the desired resistance and whether there exists an optimal configuration for generating the maximum horizontal force. This research will theoretically and experimentally examine the effects of blade thickness and blade spacing which they are the critical factors to design the grillage. For a thick blade the effect of interface friction at the base of the blade must be characterized. Unlike the failure mechanism occurring with a thin blade, which might be represented very well by a single, straight failure surface, a major task for the thick blade is to identify appropriate failure mechanisms, which may resemble those observed in bearing capacity problems for downward blade trajectories.

Figure 1. Typical PIV test setup for a ploughing test


52

THE NUMERICAL SIMULATION OF CONTACT PROBLEMS USING A THIRD MEDIUM

M Khishvand1, M Nazem1

1 ARC Centre of Excellence for Geotechnical Science and Engineering, The University of Newcastle. Email: [email protected]

ABSTRACT The numerical simulation of contact problems is nowadays a standard procedure in many engineering applications. Several approaches have been developed over the last four decades to formulate contact constraints for numerical simulation methods. The contact constraints are usually formulated using either the Lagrange Multiplier method and the Penalty approach, or their variants of both methodologies. Recently, a new contact scheme, based on a space filling mesh in which the contacting bodies can move and interact (Figure 1), has been developed in solid mechanics (Wriggers et al 2013). The contact constraints are satisfied by changing the property of the medium that imbeds the bodies coming into contact, according to the movements of the bodies. Within this approach the medium will be formulated as a material with changing characteristics. In this thesis, a new finite element formulation that is based on this innovative idea will be developed for tackling the soil-structure problems. The bodies coming into contact are assumed to undergo finite deformations. This new approach is followed that provides an alternative method to treat contact problems and avoids the complexity of the classical schemes related to the exact formulation and enforcement of the contact constraints. The main goal of this research is to investigate the potential merits of third medium contact concept for finite element analysis of geotechnical problems in which soil interacts with an object (or structure) and is subjected to finite strains.

Figure 1. Setup (left) and discretization (right) of the contact problem (Wriggers et al 2013)

REFERENCES Wriggers P, Schroder J, Schwarz A (2013) A finite element method for contact using a third medium.

Computation Mechanics, Published online 30 March 2013


53

PIPELINE ON-BOTTOM STABILITY AND THERMAL EXPANSION MANAGEMENT: THE INTERACTION OF

SEDIMENT TRANSPORT, GEOTECHNICS AND STRUCTURAL BEHAVIOUR

Simon H. F. Leckie1, Liang Cheng2, Scott Draper3, David J. White4

1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected] 2 School of Civil & Resource Engineering, The University of Western Australia, Perth, Australia. 3 Centre for Offshore Foundation Systems, The University of Western Australia, Perth, Australia. 4 Shell EMI Chair of Offshore Engineering, The University of Western Australia, Perth, Australia.

ABSTRACT Scour induced embedment and spanning of an offshore pipeline can influence it’s on-bottom stability, thermal expansion management, span integrity and flow assurance. While extensive physical and numerical modelling has been carried out on scour and sediment transport around pipelines, accounting for these effects in design has been made difficult by a lack of published data on observations from the field.

In this work, historical pipeline monitoring data is used to track the evolution of scour induced embedment and spanning for several pipelines on Australia’s North West Shelf. Up to 7 years of pipeline integrity monitoring data for each pipeline are being processed and the resulting embedments and pipeline spanning summarised. While the work is ongoing, early results provide useful insights into both the rate at which scour and sediment transport occur and the possible presence of a long term pseudo-steady state where a balance of spanning and embedment is reached. New insights into the mechanisms by which scour initiates and self-burial occurs are also emerging.

Sediment transport around pipelines typically results in an undulating profile of embedment and spanning. Analytical techniques are being used to determine the stability (and more generally the resistance to lateral loading) of pipelines, taking into account pipeline rigidity, loading scales and seabed profile variation.

Figure 1. Typical mobile seabed profile around a pipeline


54

RATE EFFECTS ON THE UPLIFT CAPACITY OF SKIRTED FOUNDATIONS ON CLAY

Xiaojun Li1*, Christophe Gaudin1, Yinghui Tian1, Mark J. Cassidy1

1 Centre for Offshore Foundation Systems, The University of Western Australia. * Email: [email protected]

ABSTRACT A series of centrifuge tests have been carried out to investigate the uplift resistance of skirted foundations under increasing displacement rates in lightly over-consolidated kaolin clay. The uplift rate investigated covered drainage conditions ranging from fully drained to fully undrained conditions. Typically circular and square shaped skirted foundations have been fabricated to make a comparison. Uplift load, displacements and pore pressures at the foundation invert were monitored during testing. The results provided insights into the generation of suction at the foundation inverts and its contribution to the uplift resistance of foundations and their associated failure mechanisms in clayed soil. Backbone curves for both the circular and square foundations were established which enable to predict the level of suction generation and uplift resistance of foundations under drained, partially drained and undrained soil conditions (Fig.1). It was observed that fully undrained conditions were achieved at uplift rate approximately one order of magnitude higher than rates associated with undrained conditions for foundation compression. At this higher velocity (compared to foundation in compression), the soil strength is enhanced by viscous effect. Alternatively, a complementary numerical model is currently in developing to simulate suction generation underneath the foundations and its impact on uplift resistance.

0.1 1 10 100 1000 100000

2

4

6

8

10

12

14

Without viscous effect

Nor

mal

ised

upl

ift re

sist

ance

, qm/q

ref

Normalised veloctiy, V = vD/cv

Circular Square

With viscous effect

Figure 1. Backbone curves for foundation uplift

REFERENCES Li X, Gaudin C, Tian Y, Cassidy MJ (2014) Rate effects on the uplift capacity of skirted foundations on clay. 8th

International Conference on Physical Modeling in Geotechnics (ICPMG 2014), Perth, Australia (accepted).


55

3 Minute Postgraduate

Student Thesis

Presentations: Day 2


56

SIMPLIFIED RISER-SOIL INTERACTION MODEL FOR FATIGUE DESIGN OF STEEL CATENARY RISERS

Jiayue Liu1, Mehrdad Kimiaei1, Mark Randolph1,


ABSTRACT Steel catenary risers (SCRs) provide a technically feasible and commercially efficient solution for the offshore field developments in deep waters. Due to the nonlinear nature of the riser-soil interaction, dynamic response and fatigue design of SCRs in the touchdown zone (TDZ) are among the most complicated engineering challenges. The main aim of this project is to establish a simplified framework for structural analysis of SCRs through in-depth study of the nonlinear riser-soil interaction stiffness. The main methodology is to utilize numerical simulations. Using general finite element software such as Abaqus, this project is to develop a numerical tool for dynamic analysis of SCRs using nonlinear riser-soil interaction models. User defined elements will be developed to capture complex hysteretic nonlinear riser-soil interaction in the TDZ using existing nonlinear soil models (Randolph and Quiggin (2009), and Aubeny and Biscontin (2009)) as shown in Fig. 1. Through delicate analyses and simplification of the nonlinear soil model, spatially varying and single-value equivalent linear soil stiffness models will be proposed to replace the nonlinear one, thereby establishing a robust and more practical model for fatigue design of SCR in the TDZ. Fatigue life of different SCR systems using nonlinear and equivalent linear soil models under different input key parameters will be calculated and compared. Trench formation and its impact on the fatigue performance and global response of SCRs will be investigated.

(a) (b)

Figure 1. Soil model characteristics (a: Randolph and Quiggin 2009; b: Aubeny and Biscontin 2009)

REFERENCES Aubeny, C., and Biscontin, G. (2009). Seafloor-riser interaction model. International Journal of Geomechanics,

9(3), 133-141. Randolph, M., and Quiggin, P. (2009). Non-linear hysteretic seabed model for catenary pipeline contact.

Proceedings of the 28th International Conference on Ocean, Offshore and Arctic Engineering, ASME, Honolulu, USA, Paper OMAE2009-79259.


57

INTERPRETATION OF CONE PENETRATION DATA IN LAYERED CLAYS

Hongliang Ma1


Supervisors: Yuxia Hu and Shazzad Hossain

ABSTRACT The cone and ball penetrometers are site investigation tools for soil characterisations. When multilayer soil is encountered, the interpretation of the site investigation data obtained from the penetrometers becomes challenging. This is because there are no existing design guidelines for the accurate extraction of soils parameters from the site data. The successful completion of this project will provide such guidelines to the industry to assist foundation design. My PhD thesis is aiming to provide such guidelines, through LDFE analyses following remeshing and interpretation technique with small strain model (RITSS). A number of different combinations of soil profiles will be considered and analysed, including soft-stiff-soft, stiff – soft - stiff and multi-layered deposits. The strong soil layer can be either stiff clay or sand and the soft layer is soft clay. In the presentation, I will mainly talk about the difficulties and challenges that maybe encountered of this project.

Figure 1. In situ testing equipments

REFERENCES Hu, Y. & Randolph, M. F. (1998a). A practical numerical approach for large deformation problem in soil. Int. J. Numer Anal. Methods Geomech. 22, 5, 327–350. Zhou, M., H. Liu, et al. (2013). "Behaviour of ball penetrometer in uniform single- and double-layer clays."

Géotechnique 63(8): 682-694.


58

NUMERICAL MODELLING OF SUBMARINE LANDSLIDE AND ITS IMPACT TO OFFSHORE INFRASTRUCTURE

USING THE MATERIAL POINT METHOD (MPM)

Jiajie Ma1, Dong Wang1, Mark Randolph1 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT Oil and gas developments in deeper waters pose increasing challenges for offshore geotechnical engineers. One of the major concerns is potential geohazards for subsea infrastructures such as pipelines. Submarine landslides could impact nearby pipelines and lead to severe damage or even failure. Geohazards assessment for new offshore developments therefore tends to be dominated by establishing the potential for submarine landslides, the probability of impact, and assessing the potential consequences. To the designer’s perspective, the quantitative characterisation of velocity, run-out distance of the slide and the impact force between slide and pipeline is of great importance as these information facilitates the design process that could avoid disastrous event. Numerical modelling of submarine landslide is challenging. As the sliding material is remoulded and entrained by water, it transforms into a debris flow and eventually a turbidity current, impact pipelines in its path. This poses significant problems for numerical simulation as the sliding material undergoes extreme deformation and the constitutive properties are history dependent. The Material Point Method (MPM), a particle based mesh-free method (Sulsky et al. 1995), is advantageous over traditional Arbitrary Lagrangian Eulerian (ALE) methods in both accuracy and efficiency when applied to extreme deformation analysis. The open source Uintah MPM software is used as the starting point for code development. A new penalty contact algorithm termed Geo-contact is developed to facilitate large deformation analysis with smooth, partially or fully rough geotechnical contact conditions (Ma et al. 2013). The transition of sliding material, from soft clay to fluid like debris flow, is approximated using elastic-plastic constitutive law with Von-Mises yielding criterion, considering rate dependency and strain softening. The MPM software is verified in three quasi-static plain-strain simulations and the results agree well to limit analysis of plastic theory and Large Deformation Finite Element (LDFE) simulation results, demonstrating its accuracy and validity. The MPM is then applied to landslide simulation. An initial verification of modelling a block sliding on rigid surface shows that in simulations dominated by tangential behaviour, the contact interface and resistance acting on it should be clearly defined. The Geo-contact algorithm is modified to facilitate sliding simulation and applied to total stress landslides simulation with intact soil strength on slopes created by rotation of gravity. The results agree well to LDFE in terms of velocity, run-out distance and slide shape. The same simulations are performed with a slope discretised by material points, predicting very close results except a slight discrepancy due to additional resistance induced by the stepping contact interface. The Geo-contact algorithm is then applied to model centrifuge landslide experiment (UWA-SM3 analysis) with rate dependent and softening soil. The advantage of MPM is further demonstrated via quantitative evaluation of impact resistance in flow-pipe interaction simulation (Figure 1). The slide enters the model with an initial velocity and height, runs along a rigid slope, and impact an embedded pipeline. The impact resistance comes from both soil deformation and drag effect of the flow. It is found that different initial flow height and velocity leads to different failure mechanism and resistance on the pipeline, offering an insight for assessing the impact consequence.


59

Figure 1. Flow-pipe impact model

REFERENCES Ma J, Wang D, Randolph MF (2013). A New Contact Algorithm in the Material Point Method for Geotechnical

Simulations, International Journal for Numerical and Analytical Methods in Geomechanics International, under review

Sulsky D, Zhou S, Schreyer HL. (1995) Application of a particle-in-cell method to solid mechanics. Computer Physics Communications 87: 235-252.


60

SLIDING BEHAVIOUR OF A MOBILE FOUNDATION

Michael Cocjin1, Susan Gourvenec2, David White2, Mark Randolph2 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

2 Centre for Offshore Foundation Systems, The University of Western Australia

ABSTRACT Mudmats are a type of shallow foundation employed to support offshore pipelines. This research project investigates a type of mudmat designed to skid on the seabed to accommodate pipeline movements. Subsea pipelines experience fluctuations in temperature during periodic start up and shut down operations, causing the pipe to expand and contract over the lifetime of the facility. Typically a pipeline is constrained at each end by a mudmat, which is designed to remain stationary and resist pipe loads with the thermal expansion of the pipe being instead accommodated by controlled pipeline buckles at some points along its length. Constraints on foundation size and increasingly demanding environmental and operational conditions make a foundation that is designed to slide more attractive. A key design feature of a mobile foundation is stability under applied vertical loads and overturning moments, i.e. minimal settlement, pitching or rolling, such that horizontal translation is the only motion. With pipe expansion and contraction, the supporting foundations slide back and forth across the same patch of seabed and the soil in the vicinity of the foundation experiences repeated shearing. Under undrained shearing, normally and lightly overconsolidated clays, which are typical to deepwater seabed soils, generate positive excess pore water pressure. This reduces the effective stress and therefore reduces the strength of the soil. When the pipeline is stationary (either during shut down or in operation), which can be for periods of a few weeks to several months, the excess pore water pressures dissipate leading to consolidation and an increase in the undrained shear strength. A series of centrifuge tests were carried out on a model mudmat foundation (Figure 1) sliding on the surface of soft, normally consolidated clay. Operational loading cycles involving repeated sliding (remoulding), and intervening periods of operational shutdown (reconsolidation) were replicated in the tests. Increases in horizontal resistance with sliding episodes were quantified indicating the potential long-term benefits of soil consolidation on sliding capacity, as portrayed in Figure 2. Associated foundation rotations during the sliding and shutdown episodes were also investigated.

Figure 1. Model foundation in centrifuge test set-up Figure 2. Effect of re-consolidation

REFERENCES Cocjin M., Gourvenec S., White D., and Randolph M.F. (In preparation). Observations of a sliding foundation.


61

MESH OPTIMISATION METHODS FOR SOLVING LARGE DEFORMATION GEOTECHNICAL PROBLEMS

M.H. Moavenian1*

Principal supervisor: M. Nazem1

Co-supervisors: J.P. Carter1 and A.J. Abbo1 1 ARC Centre of Excellence for Geotechnical Science and Engineering, the University of Newcastle, Newcastle, NSW,

Australia. *Email: [email protected]

ABSTRACT Despite recent advances in the finite element method, mesh distortion due to large deformations may still occur in some problems such as penetration of objects into soil, numerical simulation of spudcans installation, and footings subjected to large deformations. In order to overcome this drawback, robust remeshing techniques are required. This research is mainly related to studying current mesh optimisation techniques and investigating their performance in geomechanics problems.

In the first stage of this research a few robust remeshing techniques will be implemented within a finite element framework, including the Reference Jacobian Method, Spring Analogy Method, and methods based on Radial Basis functions. Although these methods are widely used to tackle mesh distortion in large deformation problems of fluid mechanics, their application in geomechanics problems has not been addressed in the literature. Then, the capability of these remeshing methods in eliminating mesh distortion in large deformation problems of geotechnical engineering as well as their advantages and disadvantages will be comprehensively studied. This project also aims to deliver a robust mesh refinement strategy tailored to targeted problems.


62

THE HYDRODYNAMICS OF A RECIRCULATING (O-TUBE) FLUME

Henning Mohr1, Scott Draper1, David White1


ABSTRACT An O-tube flume (described recently by An et al. (2013)) is a horizontal closed-circuit flume that can be driven by an inline impeller-type pump to produce steady and/or regular and irregular oscillatory flow over a mobile seabed. In this study, we investigate the hydrodynamics of an O-tube. First, we derive a dynamic equation to explain the coupling between pressure and flow rate within the tube. This result allows for better interpretation of experiments focused on physical phenomenon such as seabed liquefaction. It also allows for improved control of the flow within the O-tube since it can be used to predict the non-linear interaction between unsteady flow (i.e. waves) and steady flow (i.e. currents). Example measurements of flow rate taken from an O-tube are given for comparison with the dynamic equation. Secondly, we present velocity measurements to give a detailed description of the flow field within the tube, including turbulence, secondary flows and flow asymmetry. Finally, we conclude the paper by providing two example applications of the facility to study sediment transport and scour, and we discuss possible future applications of an O-tube flume.

Figure 1. Photograph of the Mini O-tube (MOT) at the University of Western Australia

REFERENCES An H, Luo C, Cheng L and White D (2013) A new facility for studying ocean-structure–seabed interactions: The

O-tube. Coastal Engineering 82: 88-101.


63

THE VARIATION OF NC FOR A SPHERE PENETRATING SOFT SOIL

John Morton 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT Geotechnical acquisition of the undrained shear strength, su of surficial seabed deposits is typically expensive and requires a submersible penetration rig. Therefore, various free-fall penetrometers have been proposed as a cheaper alternative to traditional underwater penetration tests. An instrumented spherical penetrometer is one such option that has been investigated in this research.

(Morton et al. 2013) (in press) details over 99 offshore field trials with a 0.25m diameter predominately steel sphere. Inside the sphere, an inertial measurement unit is used to record forces acting on the sphere during freefall in water and dynamic embedment of the soil.

During penetration the soil flow mechanism changes from surface heave to deep flow-round. This transition is associated with a variation of the normalized bearing capacity factor, Nc. This has been shown for full flow penetration devices where the transition depth depends on the dimensionless strength ratio, ′ , where γ' is the effective unit weight of the soil and D is the diameter of the penetrometer. Higher strength ratios are associated with an open cavity behind the sphere and a delay in the transition to a steady Nc (White et al. 2008, Zhou et al. 2013). In order for the sphere to accurately measure the su in this shallow zone, knowledge of the variation of the soil flow mechanism from a shallow to a deep mechanism is required.

This research addresses this problem through centrifuge tests, carried out at 100g using the beam centrifuge at UWA. The test database comprises of tests on a freefalling sphere and also on a ball penetrometer penetrating at a constant rate. In order to investigate a range of strength ratios, the ball penetrometer was varied in diameter from 11.3 - 40mm and the su was varied by using a normally-consolidated and an over-consolidated kaolin sample. The tests aimed to both measure the variation of Nc and also observe the approximate transition point where the failure mechanism changes from a shallow to a deep failure mechanism, using an underwater camera focusing on the point of impact with the soil. For the freefall tests, a high frame rate camera was required. Figure 1 shows a series of images captured at 2500 frames per second of a sphere freefalling in water and impacting the soil at an impact velocity of 18m/s. The images of the sphere/ soil interaction permits the degree of hole closure to be determined and allows for an assessment of the static and dynamic transition from surface heave to a deep flow-round failure mechanism.


64

Figure 1. Sphere impacting kaolin

REFERENCES White DJ, Gaudin C, Boylan N & Zhou H (2010) Interpretation of T-bar penetrometer tests at shallow embedment

and in very soft soil. Canadian Geotechnical Journal 47( 2): 218-229. Zhou M, Hossain MS, Hu Y and Liu H (2013) Behaviour of ball penetrometer in uniform single- and double-layer

clays. Géotechnique 63(8): 682 –694.


65

IMPROVEMENT OF SOIL STABILITY ALONG RAIL CORRIDORS THROUGH NATIVE VEGETATION – BIO

ENGINEERING

Muditha Pallewattha Centre for Geomechanics and Railway Engineering, Faculty of Engineering, University of Wollongong, ARC Centre of

Excellence in Geotechnical Science and Engineering, NSW, Australia.

(Supervisors: Prof Buddhima Indraratna, Dr Ana Heitor)

ABSTRACT Current demand of the infrastructure facilities along metropolitan areas has been lead to construct earth structures, major highways and railways on soft soils. In relation to above facts civil engineering is in a more challenging situation to discover more cost effective, reliable and sustainable methods for ground improvement. In that case green corridor concept or ground improvement using native vegetation can be considered as a more effective method. Even though this is considered as a new idea, the use of vegetation in hill slopes to prevent erosion and to give some stability had been started in centuries back without proper engineering quantification or design.

To improve soil stiffness, to stabilize slopes and to control erosion, bio engineering aspects of native vegetation in relation with geotechnical engineering is being tried to use for some extent during past few decades. At the same time several researches had been conducted to quantify the effect of native vegetation on shear strength of soil, but most of the engineering project are still reluctant to use the green corridor concept due to lack of proper knowledge regarding actual quantification and design methodologies.

When the ground improvement using native vegetation is considered, tree roots provide mechanical strengthening to the soil by anchoring effect of the main roots and by the improvement of cohesion due to hair roots. At the same time tree roots improve the matric suction of soil by means of root water uptake in conjunction with the transpiration of the tree.

Most of the previous researches which have been carried out to quantify the mechanical strength applied by tree roots on soil are mainly based on the empirical equations. In many cases equations are developed for one tree species or one soil condition and due to the experimental interpretation of the equations it is difficult to modify the results to suit for another condition and it cause the limited usability of those. At the same time the effect of transpiration of tree on soil hasn’t been considered properly. To accommodate above hindrance UOW model has been developed to quantify the suction effect of tree on soil due to transpiration.

Root based suction of a tree affects the shear strength and pore water pressure dissipation of soil, apart from that it may alter the failure criteria of the root system from saturated stage to unsaturated stage. Therefore the root based suction and the mechanical behaviour of the root system has to be investigated together.

Main objective of the study is to develop a reasonable prediction model for shear strength improvement of soil by native vegetation; consists of coupling effect of the integrated mechanical strengthening and increment of suction induced by tree roots. Main components of the study are mathematical interpretations with some of the experimental coefficients of the conceptual model and performing numerical analysis, determining experimental coefficients which are related to mathematical model by experiments and validation of model by field investigations.


66

CREEP MODEL CAPTURING VACUUM PRELOADING

Pankaj Baral Centre for Geomechanics and Railway Engineering, Faculty of Engineering, University of Wollongong, ARC Centre of

Excellence in Geotechnical Science and Engineering, NSW, Australia.

(Supervisors: A/Prof Cholachat Rujikiatkamjorn, Prof Buddhima Indraratna)

ABSTRACT Due to low bearing capacity and high degree of compressibility of soft soils, they often cause excessive and differential settlement with time. These soft soils are widespread in the coastal region of eastern Australia, where the population is increasing fast and many infrastructures have to be constructed. In order to accelerate the consolidation process especially on these soft soils, application of an effective ground improvement technique is essential. A system of Prefabricated Vertical Drains (PVDs) combined with vacuum and surcharge preloading is considered to be cost effective and environmentally friendly ground improvement technique, in which a vacuum pump is used to generate suction. A vacuum nearly equal to 100 kPa can generate preloading pressure equivalent to 3 -4 m height of surcharge. The main purpose of vertical drain used in this system is to speed up soil consolidation process by shortening the drainage path. If large creep settlement is likely to be occurred after the construction, it may cause infrastructure, to collapse and a concern may arise regarding stability and serviceability of the infrastructure. As soft soil settlement could not be fully described by current soil consolidation theory, there are a lot of hypotheses which defines creep settlement of the soft soil. Some of the hypothesis suggests that creep occurs only after the end of primary consolidation (EOP) whereas, some suggests that creep occurs during whole consolidation process. However, the magnitude of creep settlement is dependent on stress history and stress level in the ground as well as the soil type and its anisotropy. An exact definition and expression for the creep settlement in soft soil consolidation is still lacking and needs to be investigated.

This research formulates an expression for the creep settlement for anisotropic radial consolidation of soft soil facilitated with PVDs considering the effect of vacuum and surcharge preloading. In addition, based on extensive laboratory tests, field studies, and numerical analysis, an appropriate creep model, revised guidelines and design charts facilitating increased construction rates and reduced maintenance costs will be developed, which will be beneficial for the researchers working on soft ground improvement.


67

SOFT SOIL IMPROVEMENT USING VACUUM PRELOADING AND VERTICAL DRAINS

Darshana Perera

Centre for Geomechanics and Railway Engineering, Faculty of Engineering, University of Wollongong, ARC Centre of Excellence in Geotechnical Science and Engineering, NSW, Australia.

(Supervisors: Prof Buddhima Indraratna, A/Prof Cholachat Rujikiatkamjorn, Dr Richard Kelly)

ABSTRACT Presence of soft clay poses huge challenge for infrastructure construction due to its inherent qualities such as high compressibility and low shear strength. Unfortunately most of the Eastern coast of Australia is covered with very soft soils. Therefore applying sound soil improvement methods is imperative in order to safeguard of the structure. Vacuum preloading with prefabricated vertical drains (PVD) is one of the popular method to improve the soft soils. Use of PVD will reduce the drainage path of consolidation and use of vacuum pressure will further accelerate the consolidation process. Vacuum pressure will apply a negative pore water pressure so it will increase the effective stress immediately. Moreover, due to the inward movement of soil resulted by vacuum pressure lateral displacement of soil will be reduced. This will improve the stability of the embankments and it can be built in lesser time. The slender PVD is driven in to the ground using a steel mandrel. This will create a disturbed zone around the drain. Lateral permeability of the disturbed zone (Smear Zone) is lower compare to the surrounding undisturbed region. This will retard the consolidation and the accurate determination of the extent and characteristics of this smear zone is extremely important in a soil improvement project. Even after the excess pore water pressure is dissipated the secondary consolidation will continue and soil will settle under constant effective stress. Knowledge of this time dependent long term consolidation is very limited when vacuum pressure with PVD is been used to improve the soil. In my PhD I am investigating about smear Zone and its characteristics when vacuum pressure is applied and the effects of it to the long term consolidation of the clay. A modified Rowe cell is designed and built in UOW to conduct small scale testing of Ballina clay samples. This test focuses on how the clay responds to the vacuum pressure with PVDs. A sintered Bronze drain is used to simulate the band drain and vacuum pressure will be applied to the air tight consolidation chamber. Undisturbed large scale sample testing is planned to use for the investigation of more realistic smear zone characteristics. In addition, samples collected from the Ballina site around the installed drain will be used to establish the disturbed zone. Another thing which draws the attraction is the rate effects of the Ballina clay. Constant rate of strain test is currently conducted to examine how the consolidation curve would change in different rates of strain. Aim of this work is come up with an analytical solution which can predict the long term behaviour more accurately when vacuum pressure with vertical drains is use to improve the soft clay. Then this method can be incorporated to computer software to obtain a user friendly design procedure and design charts.


68

ARTIFICIAL NEURAL NETWORK DEVELOPMENT FOR STRESS ANALYSIS OF STEEL CATENARY RISERS IN

TOUCHDOWN ZONE

Lucile M. Quéau1, Mehrdad Kimiaei, Mark F. Randolph 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT The recent deep-water developments are pushing riser technology to the limit. Risers are pipes that conduct hydrocarbons from subsea to sea surface and the most common approach is to use steel catenary risers (SCRs) due to their conceptual simplicity. A schematic representation of an SCR is shown in Figure 1. SCR fatigue design is challenging, in the touchdown zone (TDZ) in particular due to the contact between riser and seabed. A limited understanding of the influence of some parameters on SCR response, pertaining to the SCR geometry and structural properties, environmental loading and the seabed characteristics, contributes to a high level of incertitude in their design. The functional relationships between the input parameters and fatigue damage should be evaluated by performing sensitivity studies to increase the confidence in the use of SCRs. So far, only a few sensitivity studies have been published with limited applications, considering small ranges and/or investigating only a selection of input parameters. These studies also failed to examine the potential interactions between these parameters, potentially leading to misinterpretation.

This research aims to provide quantitative guidance on how each parameter affects the fatigue damage in the TDZ by means of numerical simulations.

At first, the dimensional analysis technique is applied as it is a pre-requisite to conduct pertinent sensitivity studies. It consists in establishing the complete list of independent design parameters impacting SCR fatigue damage and grouping them, based on their units, into dimensionless groups. Suitable dimensionless groups are proposed and validated through a series of numerical tests, comparing the response of similar SCR systems defined by appropriate scaling of parameters.

Once validated, the dimensionless groups are used to test the robustness of published results and extend the ranges of the input parameters varied in the sensitivity studies. A large database of over 50 000 cases is created to examine a wide range of SCR applications. Calculations are automated by developing an interface linking a marine analysis software and an optimisation software through various programming languages. The results of these simulations are used to investigate the relative effects of the input dimensionless groups and their interactions on the maximum stress range in the TDZ, which controls the fatigue damage. An approximation of the maximum stress range is defined using the response surface method with artificial neural network and successfully approximates over 99 % of the cases of the database with an accuracy of 5 %. This approximation benefits SCR fatigue design by providing a simple tool that can be used to plot design charts, which indicate the effect of the variation of the input parameters on the fatigue damage. For instance, the design chart presented in Figure 2 shows the effect of the dimensionless amplitude of the loading on the fatigue damage for various values of the normalised water depth.


69

Figure 1. Side view of an example steel catenary riser configuration.

Figure 2. Example of design charts.


70

FINITE ELEMENT ALGORITHMS FOR DYNAMIC ANALYSIS OF SATURATED POROUS MEDIA

H. Sabetamal1*, M.Nazem1, S.W.Sloan1 and J.P.Carter1

1 ARC Center of Excellence for Geotechnical Science and Engineering, *Email: [email protected]

ABSTRACT Numerical analysis of offshore geotechnical problems has been one of the utmost active research fields during recent years. The significance feature of Offshore Geomechanics lies on the existence of hydrodynamic/cyclic loading, large deformations, extreme soil-structure interactions and significant changes of conditions of seabed soil due to installation of offshore structures. This study presents the development as well as implementation of a computational scheme in the framework of the finite element method aiming to analyse a few important offshore applications. The numerical scheme accounts for large deformation dynamic behaviour of saturated soils as well as frictional interface of two phase saturated porous media, using higher order contact elements. It also utilises appropriate absorbing boundary conditions and advanced adaptive time-stepping schemes for coupled consolidation analyses. The computational framework is particularly employed to simulate the installation, consolidation and pull out capacity of dynamically penetrating anchors (DPAs) as well as problems of pipeline-seabed interaction. Due to low permeability of soft seabed soils, the installation process of DPAs and laying process of a pipe is usually considered to be undrained, and consequently, excess pore pressures are generated. The time scale of consolidation is important for estimating the pipeline-soil response as well as the holding capacity of DPAs under different events. The analysis of aforementioned offshore applications comprises dynamic loading, installation effects, consolidation, and frictional soil-structure interaction. In particular, the geotechnical analysis of pipeline should comprise some assessment of the drainage conditions around the pipe as it moves, which can considerably influence the forces at the interface between soil and pipe. However, the mechanisms that control consolidation and drainage are not usually incorporated since they complicate numerical treatments of contact constraints. The developed computational framework is able to provide reliable solutions for these problems while addressing realistic but complicating features.

(a) (b)

kPa

Figure 1. Numerical modeling of a DPA installation into MCC soil: a-Deformed mesh; b- Excess pore pressure contour map

REFERENCES Sabetamal H, NAZEM M, Carter JP and Slown SW (2014) Large deformation dynamic analysis of saturated

porous media with applications to penetration problems. Computers and Geotechnics, 54: 117-131 Sabetamal H, NAZEM M and Carter JP (2013) Numerical Analysis of Torpedo Anchors.

Proceedings of the Third International Symposium on Computational Geomechanics: ComGeo III, Krakow, Poland.

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71

MICROMECHANICALLY-INSPIRED STONE COLUMN BEHAVIOUR

Firman Siahaan

Centre for Geomechanics and Railway Engineering, Faculty of Engineering, University of Wollongong, ARC Centre of Excellence in Geotechnical Science and Engineering, NSW, Australia.

(Supervisors: Prof Buddhima Indraratna, A/Prof Cholachat Rujikiatkamjorn)

ABSTRACT The rapid demands for roads and railways in Australia warrant the need to build those infrastructures over problematic soft clay. Stone column is one of primary methods for the improvement of soft clay over which many infrastructures are built. To date, stone column designs have been based on analytical and numerical methods in which micromechanical aspects of column particulates have been largely ignored and its load transfer often oversimplified. This research is aimed to review and possibly revise the design method through understanding of micromechanical behaviour of stone column and its load transfer with surrounding soft clay under embankment loading. Stresses and contact distributions within column during its critical short-term loading are studied. Effects of particle morphology on stress-strain behaviour of column are investigated. These outcomes will then be easily extended to the behaviour of stone column during subsequent long term loading which primarily involves consolidation of surrounding clay. The research works include laboratory experiments, numerical modelling and possible limited field testing. By understanding stress distribution and load transfer, analytical model inspired by arching theory will also be developed. As part of laboratory experiments at University of Wollongong, fully-instrumented model stone columns are being tested using large-scale triaxial apparatus. Some main instruments include miniature soil pressure gauges and pore-pressure transducers. Fibre-Bragg Grating (FBG) optical sensors will be innovatively used to obtain accurate measurements of lateral column deformation throughout the testing. Earlier tests have indicated marked effects of particle size distribution of column on the overall behaviour of a single unit cell comprising granular column and soft clay. Testing using large-scale Ballina clay retrieved in field with minimal disturbance will also be conducted. Additionally, a full CT scans on selected specimens are proposed to be carried out to study the clogging of stone column due to its radial interaction with soft clay. Behaviour of Constant Normal Stiffness (CNS) interface will be simulated using large-scale CNS Direct Shear apparatus. Stone column is numerically-simulated using three-dimensional Discrete Element Method (3D DEM) in which the vertical column boundary is modeled as stress-controlled particles. These boundary particles were tracked and their forces constantly updated in the simulation to take into account the CNS behaviour in the column-clay interface prior to the yielding of stone column or clay. Particle angularities are fully represented in the simulation using clumps. To efficiently model surrounding soft clay including its interaction with column, two-dimensional Finite Element analysis will be used in conjunction with 3D DEM. To calibrate and validate numerical models, data from laboratory experiments and field performance such as field plate load testing are used. This novel research is expected to pioneer the understanding of micromechanical aspects of stone column and its load transfer mechanism. This will provide much-needed theoretical basis to improve current design method and be well-aligned with the key objective of ARC Centre of Excellence in Geotechnical Science and Engineering.


72

SAMPLING DISTURBANCE OF AN INTERMEDIATE SOIL

Guan Tor Lim1 1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT Substantial work and research have been done in the past on the assessment of sampling disturbance mainly on tube penetration disturbance during sampling process and stress relief disturbance when a soil sample is removed from the ground or tube. However, most of the general studies (e.g., Baligh et al., 1987 and Clayton et al., 1998) have focused on clay behaviour with an implicit assumption of undrained condition. Soil is assumed to behave as incompressible fluid in their model that caused soil to be considered as being saturated, homogenous and isotropic with no shear strength properties.

Intermediate soils have engineering properties between sand and clay and the behaviour of these soils is still not well understood. This research will consider the effects of sampling disturbance on intermediate soil. Analytical and numerical modelling will be carried out based on the partially drained condition, accounting for soil’s volume change. In addition, other assumptions such as methods used to drive the sampler, friction between the sampler and the soil and the geometry of the samplers will be taken into consideration to study the soil. Sample disturbances from the processes of drilling, storing, time of storage, trimming process and specimen set up will be included in the entire process of disturbance simulation. This simulation is vital to assess typical procedure of laboratory testing that produces disturbance on the sample indirectly. The changes in soil fabric due to sampling disturbance will be quantified using some available techniques from previous research on soil fabric of clay and sand.

A preliminary study on carbonate silty sand from the North-West shelf of Australia subjected to sampling disturbance (tube penetration and stress relief) in undrained condition has been done. The results have shown that the value of residual shear strength measured after sampling disturbance without reconsolidation is not significantly affected by the sampling disturbance, regardless of the level of disturbance. Behaviour of intermediate soil when subjected to sampling disturbance in partially drained condition is still unknown. It is predicted that the soil will have a higher strength and stiffness due to reduction in void ratio compared with its in-situ properties. Throughout the process of disturbance simulation, the changes in soil fabric during sampling disturbance will be studied and understood as well.

This project aims at making some valid recommendation on adequate sampling methods and creating practical procedure to obtain reliable soil properties or parameters for laboratory testing. Besides, this project aims to better understand the real behaviour of intermediate soil including fabric changes possibly induced by sampling disturbance under partially drained condition.

REFERENCES Baligh, MM, Azzouz AS and Chin, CT 1987, ‘Disturbances due to “ideal” tube sampling’, Journal of Geotechnical

Engineering 113(7): 739-757. Clayton, CRI, Siddique, A and Hopper, RJ 1998, ‘Effects of sampler design on tube sampling disturbance –

numerical and analytical investigations’, Geotechnique 48(6): 847–867.


73

PARTICLE FINITE ELEMENT ANALYSIS OF THE GRANULAR COLUMN COLLAPSE

Xue Zhang1, Kristian Krabbenhoft 2, Daichao Sheng 3

1 Centre for Geotechnical Science and Engineering, The University of Newcastle. Email: [email protected] 2 Centre for Geotechnical Science and Engineering, The University of Newcastle. Email:

[email protected] 3 Centre for Geotechnical Science and Engineering, The University of Newcastle. Email: [email protected]

ABSTRACT The collapse of granular columns is reproduced by an axisymmetric version of the particle finite element method (PFEM). The granular medium is represented by a simple rate-independent plasticity model and the frictional contact between the granular flow and its rigid basal surface is accounted for. In the developed version of the PFEM, the resulted governing equations of the boundary value problem are cast in terms of an optimization problem and solved using mathematical programming tools. The agreement between the model and the experiment is generally satisfactory although the friction angle of the granular material, as well as the exact interface conditions between the base and granular material, are shown to have a relatively significant influence on the results.

Figure 1. Velocity distribution of granular columns with initial aspect ratio at time ̅. (a)a 0.5, ̅ 1.0 (b)a1.0, ̅ 1.0 (c)a 5.0, ̅ 0.8 (d)a 0.5, ̅ 1.2. The grey line corresponds to the initial configuration while the

black line is the final deposit. Colors are proportional to the module of velocity (cm/s).


74

NUMERICAL MODELLING OF SPUDCAN AND CONE PENETRATION IN MULTI-LAYER SOILS

Jingbin Zheng1

1 PhD student, Centre for Offshore Foundation Systems, The University of Western Australia Email: [email protected]

Supervisors: Shazzad Hossain, Dong Wang, Mark Randolph, Mark Cassidy

ABSTRACT Penetration resistance of spudcan foundations is commonly assessed in the framework of bearing capacity formulations (SNAME 2008; ISO 2011). However, the difficulty in obtaining high-quality soil samples from deeper water sites with complex soil environment and the absence of predictive approach for calculating the complete penetration profile in multi-layer soils have impaired the applicability of conventional bearing capacity models plugging in shear strength parameters gleaned mostly from laboratory test data. Instead, the idea of deriving the spudcan penetration resistance directly from the results from in situ penetrometer testing, for example, the cone penetration test (CPT), has been initiated. This presentation describes the PhD project which focuses on the development of a new design procedure for assessing spudcan penetration resistance profile in multi-layer soils directly from the CPT test profile. The methodology is based on the Large Deformation Finite Element (LDFE) approaches, by which numerical analyses are performed to simulate spudcan and cone penetration in multi-layer soils. The difficulties and problems encountered in the study will be presented for discussion.

REFERENCES ISO (2011) Petroleum and Natural Gas Industries – Site Specific Assessment of Mobile Offshore Units – Part 1:

Jack-Ups, International Organization for Standardization, ISO 19905-1. SNAME (2008). Recommended Practice for Site Specific Assessment of Mobile Jack-Up Units, T and R Bulletin

5-5A, 1st Edition – Rev. 3, Society of Naval Architects and Marine Engineers, New Jersey.


75

Abstracts Not Presented


76

CONSOLIDATION OF SOFT SOIL STABILIZED BY VERTICAL DRAINS UNDER CYCLIC LOADINGS

Buddhima Indraratna1, Xueyu Geng1 and John Carter2

1 Centre for Geomechanics and Railway Engineering, Faculty of Engineering, University of Wollongong, ARC Centre of Excellence in Geotechnical Science and Engineering, NSW, Australia. [email protected]

2 Faculty of Engineering and Built Environment, ARC Centre of Excellence for Geotechnical Science and Engineering, University of Newcastle, Newcastle, NSW 2308, Australia

ABSTRACT Railways have technical and economic advantages over other forms of transport. Therefore, railway traffic has gained a specific and irreplaceable position in recent years. In order to meet demands of rapid urbanization along coastal areas, railways will inevitably be constructed on soft soil subgrade. It is imperative to understand the behaviour of soft clay subgrade subjected to cyclic loads when a new rail track is designed or an existing one is under maintenance. When the soft clay subgrade is subjected to the cyclic loading, excess pore pressures and axial strains accumulates, resulting in a decreased bearing capacity of the subgrade. Moreover, in comparison with sands, clays usually exhibit creep deformations in the form of prolonged settlements of buildings and geotechnical structures, retarded dissipation of pore water pressure in the subsoil, and slow slippage of natural slopes and embankments. The low bearing capacity and high compressibility of these soils affect the long term stability of buildings, roads, rail tracks, and other forms of major infrastructure. Therefore, soil stabilization is required to prevent unacceptable differential settlement and damage to infrastructure. To improve the subgrade, prefabricated vertical band drains (PVDs) is used increasingly among a variety of ground improvement techniques. The installation of PVDs introduces a short radial drainage path to dissipate the excess pore pressure so that the soft clay subgrade becomes more stable when subjected to heavy train loads. This study presents a model for partially drained radial consolidation under cyclic loading. This is achieved through the application of radial consolidation theory and the internal generation of excess pore pressure predicted by an undrained cyclic model. This study demonstrates theoretically the ability of vertical drains to prevent excess pore pressure accumulating to a critical value under cyclic loading.

Figure 1. (a) Large-scale triaxial rig; (b) soil specimen

(a) (b)


77

THE ROLE OF COMPRESSION BEHAVIOUR IN CONSTITUTIVE MODELLING OF SOFT SOILS

Chao Yang1, Daichao Sheng and John P Carter

1 Centre of Excellence for Geotechnical Science and Engineering, The University of Newcastle. Email: [email protected]

ABSTRACT Critical State Soil Mechanics, including the Modified Cam-Clay (MCC) constitutive model have laid the foundations of modern soil mechanics. Recent developments on the constitutive modellling of soft soils have greatly extended the MCC model and covered the initial overconsolidation state by using different scaling schemes within yield surface, incorporated soil structure by introducing an enlarged yield surface, accounted for anisotropic fabric by rotating the yield surface, and even considered time-dependency by defining stress states outside the yield surface. However, a more complete and more accurate description of the compression behaviour in the compression plane has been generally neglected, which leads to inconsistent model predictions even with these more complex formulations. This study aims to emphasize the role of a proper description of the compression behaviour in accurately predicting the shear strength of soft soils. Two important aspects of soft soils, the effects of soil structure and of anisotropic fabric, will be investigated by reconsidering the compression behaviour of soft soils. Plastic anisotropy of soft soils can be described by an inclined yield surface and an associated rotational hardening law. To establish a rational rotational hardening law is crucial for natural soils. Constant stress ratio consolidation tests illustrate parallel normal compression lines for each different value of the earth pressure coefficient (K). The relative positions of those parallel lines actually define the relative inclinations of the yield surfaces at different values of K, provided that the shape of the yield surface has been determined. Meanwhile, the values of plastic dilatancy obtained from the consolidation tests can be used to describe the plastic potential surface at various stress conditions. The rotational hardening laws for both the yield and plastic potential are naturally achieved, and constitutive models are thus formulated for anisotropically deposited soft soils. A thorough validation with experimental data lends substantial support to the adopted approach. Natural soft soils behave differently from laboratory reconstituted soils. One possible reason can be attributed to the difference in soil structure, a combination of fabric and inter-aggregate bonds. Natural soils generally possess highly non-linear normal compression lines, thus exhibiting an increased preconsolidation stress, higher shear strength and a substantial increase in stiffness after yielding. A direct and simple description of the compression behaviour of natural soils is proposed by properly defining the change of compressibility with large deformation. Satisfactory predictions are obtained when only this improved description of compression behaviour is included in constitutive models. This approach provides great flexibility for simulating different types of soils, such as soft clays, residual soils, weak rocks, and artificially cemented soils. In some cases the behaviour ofreconstituted samples cannot be regarded as the ‘instrisic’ state, but the proposed approach is able to deal with such situations. The combination of the above approaches to plastic anisotropy and soil structure is able to provide more accurate predictions of the mechanical response of soft soils in engineering practice. Further details can be found in Yang et al. (2013-a, b, and c).

REFERENCES Yang C, Carter JP and Sheng DC (2013) Description of compression behaviour of structured soils and its

application. Canadian Geotechnical Journal. Under review. Yang C, Sheng DC, Carter JP, Pineda J and Kelly R (2013) From compression behaviour to plastic anisotropy of

reconstituted soft soils. GeoShanghai 2014 International Conference on Geotechnical Engineering, 2014, Shanghai, China. Under review.


78

Yang C, Sheng DC Carter JP and Kelly R (2013) An anisotropic elastoplastic constitutive model for structured soils. 8th European Conference on Numerical Methods in Geotechnical Engineering, 2014, Delft, The Netherlands. To be submitted.


79

ESTIMATION OF SPUDCAN PENETRATION RESISTANCE IN LAYERED SOILS FROM FIELD PENETROMETER DATA

AND QUANTIFICATION ONF PUNCH-THROUGH RISK Stefanus Safinus

Centre for Offshore Foundation Systems, University of Western Australia

Supervisors: Assoc/Prof M. Shazzad Hossain, W/Prof Mark Randolph and W/Prof Mark Cassidy

ABSTRACT

Punch-through incidents of offshore mobile drilling rig, often occur during jacking, mostly attributable to inadequate site investigation or geotechnical assessment. Process for interpretation of soil data and assessment the spudcan performance usually prolong the response time to a potential punch-through event. Plus, subjectivity often plays its role in determining the resistance mobilisation mechanism, causing variation in the penetration profile prediction. Answering this problem, a systematic assessment method for soil parameters and spudcan penetration in highly-layered soils is developed, utilising field piezocone data as input. The approach is developed based on the mechanism observed in centrifuge simulation. Discrete analysis is chosen to handle the irregular / highly layered strength profile. The alteration of strength profile due to the penetration process is considered to capture the actual soil condition. Spudcan and piezocone penetrations in one- to four-layer soils, consisting of siliceous/calcareous sand and clay/silt, were simulated in centrifuge to study the penetration responses in different soil type and strength profile. Validation of the design approach has been performed against numerous field (InSafe JIP database) and centrifuge data. Yet refinement for accuracy will be continually done, through back-calculation of more penetration data with different soil and strength variation. Automated in a computer program and feed by real time penetrometer data from the integrated jack-up installation system, this prediction tool can provide critical information for the jack-up operator in assessing punch-through potential and early planning of mitigation strategy. This project is undertaken with the industry partner Keppel Offshore and Marine, Singapore under the ARC Linkage Project LP110100174.

Figure 2 Integrated jack-up installation system


80

A DETAIL STUDY OF FLUID-SOIL-RISER INTERACTION AT TOUCH DOWN ZONE

Ehssan Zargar1, Mehrdad Kimiaei2, Mark Randolph3, Scott Draper4

1 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected] 2 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

3 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected] 4 Centre for Offshore Foundation Systems, The University of Western Australia. Email: [email protected]

ABSTRACT A detail investigation of Fluid-Soil-Riser interaction at Touch Down Zone (TDZ) will be done in this research. An SCR (Steel Catenary Riser) laid on the sea floor and connected to a floating vessel at the sea surface is subjected to cyclic loads due to motions of the floating vessel and the hydrodynamic loads acting directly on the riser. These cyclic forces lead to an oscillatory motion in the riser. In the TDZ this can result in a complex interaction between three different domains existing there: the riser, the seawater around the riser, and the seabed soil underneath the riser. An oscillating riser section in the TDZ will penetrate into the seabed sequentially, which can lead to plastic deformation of the soil. At the same time, the riser excitations displace fluid and induce vortices in the seawater which will impose extra shear stress on the sea floor. This can lead to soil erosion and sediment transportation in the TDZ. Ultimately plastic deformation and soil erosion will alter the seabed profile in the TDZ and, in turn, the new seabed profile will affect the fluid flow around the pipe and formation of vortices in the seawater as well as the riser motion and its interaction with the soil. This complex trilateral interaction between the riser, fluid and the soil repeats during every cycle of the riser motion. The principal motivation of this study is to obtain a more accurate estimation of fatigue life of risers in the touch down zone, which is highly sensitive to nonlinear response of the riser in the touch down zone. The aim of this project is to get a better understanding of the complex interaction between three different domains in the TDZ: riser, fluid and soil.

Figure 1. A schematic view of the study


81

Group Workshop

Sessions


82

Session 1: Industry Impacts (Leaders: Marc Senders & Christophe Gaudin)

Research Questions Examples

SI Tools Pipeline Design FE Development

Transfer: How are research

outcomes from the CGSE are transferred to and applied by industry?

Relevance: What and who

determine research topics?

Does the CGSE include in its research priorities topics relevant to industry?

How do we balance short term/long term projects and industry needs/funding?

Impact: How do we measure

and report impacts of the CGSE research in industry practice?


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Session 2: Field Testing – Methods & Interpretation (Leaders: Richard Kelly & Buddhima Indraratna)

Research Questions:

1. What are the primary tell-tale indicators or factors to be determined through field measurements to establish the onset of soft soil embankment instability and how can they be quantified?

2. What form of constitutive model should we adopt (or not) and what suite of laboratory tests do we need to develop the model?


84

Session 2: Field Testing – Methods & Interpretation (Cont.)

3. How to establish strategic direction/integration/collaboration with other research areas such as Georisk and Moving Boundaries?


85

Session 3: Soft Soil Constitutive Models & Laboratory Testing

(Leaders: David Muir Wood & Antonio Carraro) Research Questions:

1. What information do we presently have to characterise the Ballina soft soil and how does it compare with other soils worldwide?

2. What laboratory tests will provide most information relevant to analysis of the embankment response? How much overlap of tests should there be at the three universities? What starting state and history should be used? Should centrifuge tests be included?


86

Session 3: Soft Soil Constitutive Models & Laboratory Testing (Cont.)

3. Numerical modelling needs to be enlightened by a constitutive model. Can we identify

aspects of behaviour to model before we start laboratory testing?


87

Session 4: Georisk (Leaders: Mark Cassidy & Jinsong Huang)

Research Questions:

1. Which method is more important for promoting reliability based design in geotechnical engineering, simple probabilistic methods such as FORM or more complicated methods such as Random Finite Element Method. Which should we be perusing in CGSE?

2. What are the advantages of the random limit analysis software, compared to RFEM, and what other applications can we apply either too?


88

Session 4: Georisk (Cont.)

3. What should we be adopting from structural probability analysis for application in geomechanics?

4. What probabilistic assessment do we wish to characterise the Ballina site (to advise on in-situ test layout and physical testing design)?

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