rcee research in civil and environmental engineering has been used from abaqus finite element...

RCEE

Research in Civil and Environmental Engineering

www.jrcee.com Research in Civil and Environmental Engineering 2013 1 (05) 287-299

REVIEWING PERFORMANCE OF PILED RAFT AND PILE GROUP

FOUNDATIONS UNDER THE EARTHQUAKE LOADS Ali Akbari a, Mohammad Nikookar b

*, Mahdi Feizbahr c

a Postgraduate of Geotechnical Engineering, Department of Civil Engineering, Amirkabir University of Technology, Tehran, Iran b Master of Science Student of Geotechnical Engineering, Faculty of Engineering University of Guilan, Rasht, Iran C School of Civil engineering, Engineering campus, University sains Malaysia,14300 Nibong tebal , Penang, MALAYSIA

Keywords A B S T R A C T

piled raft

pile group

load bearing proportion

earthquake load

superstructure response

The primary idea of using deep foundations was brought up, in order to control overall and differential settlement of foundation under heavy structural loads. However, gradually by increasing height of structures, using deep foundations were considered for controlling inversion and deal with shear loads resulting from the earthquake, more than was previously considered. But the important matter about deep foundations is to focus on shearing and bending forces in connection place of cap to piles, as well as their growth in depth, which it must be reviewed accurately for planning goals. There has been used from ABAQUS finite element software, to investigate interaction of soil, foundation, and structure. The studied soil is clay type with Mohr-coulomb behavioral model. There have been modeled structural sections with dual system performance in three storey type, including 10, 20, and 30 storey. The study aims to review transaction effect of soil, foundation, and structure for transferring shearing resulted from earthquake to superstructure, pile group loading share from horizontal loading and shearing and bending forces in piles.

1 INTRODUCTION

In the past, using reinforced mat foundation was the best option for dealing with heavy structural loads.

Gradually there was considered idea of using pile, in order to control and decrease settlement, because of

heavier structures and necessity to consider settlement in final designation and settlement limitation to the

allowable settlement. There was designed a traditional deep foundation with relative high safety factor for

* Corresponding author (E-mail: [email protected]).

ISSN: 2345 -3109

Ali Akbari et al - Research in Civil and Environmental Engineering 2013 1 (05) 287-299

288

the piles. In the past few decades, researchers concluded that it can be gained a more economical plan

than free pile group by taking into account the capacity of raft in the load and division safety factor

between raft and piles. Piled raft foundation is an economic and logical foundation in dealing with vertical

static loads because the load is transferred to the ground by both components of piles and raft; in addition,

it can be accessed to the same settlement by reducing number of piles in comparison with pile group

(Hemsly, 2000). With this idea, many attempts were performed to investigate piled raft behavior against

vertical static and dynamic loads. Behavior of piled raft foundation is relatively complicated during

earthquakes, because of dynamic interaction between raft, pile and soil. In areas with high activity of

seismicity such as Japan, the piles tolerate high load during earthquakes, especially when imposed

superstructure initial force will be high, which it is often the same. Stress is focused in head of pile, because

connection type between raft and piles is usually fixed. Dynamic behavior of piles depends on deformations

of foundation ground, in addition to the imposed inertia force from superstructure on the foundation. In

Huguken-Nambu earthquake, the piles were failed because of the ground deformations, even in absence of

superstructure (Horikoshi et al., 1996). Until 2003, there was ignored pile capacity in designing piles under

earthquake loads in Japan (Matsumoto et al., 2004).

Poulos and Davis (1980) offered a comprehensive relation for designing deep foundations under

static lateral load. In the past few decades, there were conducted significant researches to understand

behavior of deep foundations under dynamic loads. There were presented various methods to assess the

dynamic behavior of piles based on linear behavior of soil including subgrade reaction method (Tucker et

al., 1964), lumped mass idealization method (Prakash et al., 1973), Novak continuum approach (Novak,

1974). But results of dynamic and centrifuge laboratory experiments (Pak et al., 2003; Boominathan et al.,

2006; Puri et al., 1992; Anandarajah et al., 2001) show major difference between the observations and the

estimated parameters due to the resulted nonlinear behavior of soil and also due to separation between

pile shaft and soil under vibration. In recent year’s scholars such as Matsumoto et al., (2004), Nakai et al.,

(2004), Horikoshi et al., (2003) and Banerjee (2009) have studied performance of piled raft foundation by

using centrifuge laboratory tests and shaking table. Based on elasticity theory, Nakai et al., (2004) have

offered approximate analysis method for piled raft foundation under static lateral load, in order to

determine contribution of horizontal load in each component of raft and piles.

Due to complexity of horizontal load sharing between raft and pile group in the piled raft foundation,

there is designed most piles for entire horizontal load which it very non-economic, because according to

Nakai et al., (2004) there can be decreased pile horizontal contribution 60 to 80 percent with regard to soil

type and recognizing foundation performance, which it requires more exact survey of horizontal load

mechanism in this type of foundation. Reaction of the two above introduced system will be certainly

different, under earthquake loads or any other dynamic load such as wind, with regard to their different

performance and interactions in dealing with vertical static loads. This study aims to describe some of these

differences. Figure 1 shows the apparent difference between piled raft and pile group foundation.


289

(a) (b)

Fig. 1 Schematic form of: (a) piles group, (b) piled raft

2 MODELING

Using and developing accurate numerical models for simulating any engineering structure is

inevitable because of high costs for constructing laboratory samples in a large scale. It can be studied role

of different parameters in different stages of loading by using a numerical model and laboratory samples

can be only used for modeling calibration. In this paper, it has been used from ABAQUS finite element

software to examine the interaction of soil – foundation – structure. The studied soil is clay type with Mohr-

coulomb behavioral model, which it have been used from un-drained shear resistance (SU), in order to

consider un-drained conditions under dynamic load. Physical and mechanical parameters of used materials

in this study are shown in Table 1.

Table 1 Mechanical and physical parameters of the used materials

Density:

γ, (kN/m3)

Young’s

modulus:

E, (MPa)

Poisson

ratio:

υ

friction

angle: ϕ,

(deg)

shear

strength: SU,

(kPa) Clay 19 40 0.45 0 60

Bedrock 20 500 0.3 45 0

Concrete 24 40,000 0.25 - -

Steel 78 210,000 0.2 - -

In dynamic studies, model boundaries should not be considered fixed, but should provide conditions

that part of energy from the vibrations is exited from the boundaries, which they are called transient

boundaries. There have been used infinite elements around the soil model, in order to consider transient

boundary conditions and semi-infinite mass. Infinite elements are defined as which connected nodes to

original model is the closed level and nodes of infinite side are open which it provides support fixed

conditions in far field. In this review, there have been used three types of foundations with arrangement

3×3, 4×4, and 5×5 with different length and diameter of piles, based on increasing number of storey,

according to specifications in Table 2. Structural part has been modeled with dual system function

(composition of brace and bending frame on earthquake direction) in three floor types of 10, 20 and 30 as

superstructure, by considering to importance of structural part in dynamical analysis, especially analysis

related to inertia. Specifications of the studied structure and foundation are shown in Table 3. Each

structure contains beam, column, brace and roof in each floor and there has been avoided from details

modeling such as how to connect, walls, base plates and etc, all connections have assumed fixed.


290

Table 2 Specifications of the studied structure and foundation

Storey number

Total weight (Ton)

Storey height

(m)

Number of

openings

Raft dimensions

(m2)

Raft thickness

(m)

Number of piles

Diameter of piles

(m)

Length of piles

(m)

Distance of piles

(m)

10 700 3 2 10×10 0.8 9 0.6 10 4

20 3,000 3 3 15×15 1.4 16 0.9 14 4

30 7,200 3 4 20×20 1.8 25 1.2 18 4

Thickness of soil layer has been selected 2 times of pile length and its width is 4 times of raft width. In

order to manner separate in the simulation of pile group and pail raft, cap has been located with the

distance of one-fifth of the raft thickness from the soil surface in the pile group to avoid direct contact with

the soil and just get involve to support horizontal and vertical load while in the pile raft, raft has been

defined in the contact interaction with the soil to consider the simultaneous behavior of raft and piles

under the load.

There has been considered mechanical contact between soil surface and foundation components

including raft bottom, piles shaft and end by introduction a surface for transferring shear tension between

two contact surfaces, by using penalty frictional formulation and normal behavior for superficial

introduction for transferring normal tensions. Due to symmetry in geometry and loading, there have been

modeled all models symmetry for time reduction, and displacement constraints have been used in

symmetry surface (displacement and rotation have been closed in symmetry and other both directions

respectively). Figure 2 represents modeling and meshing a foundation 3×3 piles array in the ABAQUS

software.

(a)

(b)

(c)

(d)

Fig. 2 Foundation model with 3×3 piles array in the ABAQUS software:

(a) Foundation meshing and the field soil, (b) array type of piles

(c) Model configuration, (d) three-dimensional model


291

There have been applied loading in three steps contain geostatic step for considering in situ soil

tensions, static for gravity loading resulting superstructure weight, and dynamic time history loading as

horizontal acceleration in bed rock 2 meter thick to the model. There has been used from explicit approach

for analysis in the above three steps. There has been used from time history acceleration earthquake in

Kobe with PGA= 0.371 g and effective duration of 16 seconds (time 2 to 18 seconds) recorded in the Nishi-

Akashi station for dynamic loading. Time history of this acceleration is shown in Figure 3. This record has

been selected because of suitability for start effective time (analysis time reduction), relatively slow end,

and average PGA.

Fig. 3 recorded time history acceleration of Kobe earthquake in Nishi-Akashi Station

3 VALIDATION

There has been used from results of experimental study of Matsumoto et al., (2004) for validation of

modeling in ABAQUS software. In this study, some shaking table experiments on piled raft foundation with

three different type of superstructure has been done, which there has been used from superstructure with

a maximum height in validation of finite element model. Used soil in the study is dry sand soil with Young’s

modulus 70 MPa and internal friction angle 45 degrees. Dynamic loading has been applied as harmonic

acceleration with frequency 5Hz and PGA=0.1 g in model bed. According to Table 3, cited in original article,

finite element model has been modeled by using laboratory conversion coefficient to prototype model.

Figure 4 represents modeling of laboratory sample in ABAQUS software.

Table 3 coefficients of conversion laboratory model to real model (Prototype model)

length gravity acceleration tension force

λ 1 1 λ λ 3

-0.4

-0.2

0

0.2

0.4

0 2 4 6 8 10 12 14 16

Accele

ration

(g

)

Time (S)

Kobe Earthquake PGA=0.371 g


292

(a)

(c) (b)

Fig. 4 (a) meshing foundation plan, (b) meshing cross-section foundation, (c) the dimensions and configuration

of finite element model

Figure 5(a) shows time history of total shear force in finite element model. In this figure, numeric

value of shear force has become to laboratory model by using force coefficients in Table 3. In laboratory

model, maximum shear force at foot structure has been reported almost equal to 30N and it is 28.5N in

finite element model. Figure 5(b) shows time history of shear force at piles head for finite element model.

Maximum shear force of piles is almost to 12N and it is 12.5N in finite element model. Results of modeling

finite element and laboratory test do not show significance difference.

(b) (a)

Fig. 5 Time history for: (a) total shear force, (b) piles head shear force in finite element model

-30

-20

-10

0

10

20

30

0 1 2 3

Ho

rizo

nta

l L

oad

(N

)

Time (S) -30

-20

-10

0

10

20

30

0 1 2 3

Ho

rizo

nta

l L

oad

(N

)

Time (S)


293

4 ANALYSIS RESULTS

Figure 6 shows time history of shear force at the base of structure for 10 storey structure. As figure

shows, Recorded maximum shear force in pile group is greater than compound raft. This reduction in

response of piled raft foundation system is due to raft surface to surface contact with soil which this

contact is more maintained and foundation sliding is less, so superstructure based on piled raft foundation

receives less response from earthquake. In pile group, shear force is transferred to soil by only piles with

less contact surface in relation with the piled raft, while in the piled raft; part of this force is transferred to

soil by raft and another part by piles. This performance difference leads to different response to these two

types foundation under earthquake loads. Maximum acceleration and shear force values for two types of

foundations and three types of structures are shown in Table 4. It can see that by increasing number of

storey, response of superstructure based on pile group to earthquake is more than the piled raft.

Fig. 6 Time history of shear force at the base of structure in 10 storey structure

Table 4 Comparison of maximum acceleration and shear force in types of foundation and structure

Foundation

type Pile group Piled raft

Number of

storey

max

acceleration m/s2

Max shear

force

kN

max

acceleration

m/s2

max

shear force

kN

10 5.62 1,170 5.38 1,028 20 3.94 3,990 3.53 3,810

30 3.32 6,840 2.84 5,970

Figure 7 represents the comparison of settlement in raft center during an earthquake for two types of

foundations. According to the figure, settlement in piled raft is less than pile group, because of compound

performance of raft and piles. Settlement difference for 10 storey superstructures is not considerable,

during and after earthquake, while this difference will be significantly increased by increasing number of

-1200

-800

-400

0

400

800

1200

0 2 4 6 8 10 12 14 16

Sh

ear

Fo

rce

at

Str

uct

ure

Bas

e (k

N)

Time (S)

Case PR SFmax = 1028 kN

-1200

-800

-400

0

400

800

1200

0 2 4 6 8 10 12 14 16

Shea

r F

orc

e at

Str

uct

ure

Bas

e (k

N)

Time (S)

Case PG SFmax = 1170 kN


294

storey, and piled raft shows suitable performance for settlement, especially when number of storey is

increased.

Fig. 7 Comparison of settlement in raft center during earthquake: (a) 10 storey structure, (b) 20 storey structure,

(c) 30 storey structure

Table 5 shows horizontal load bearing proportion of piles in pile group and piled raft. In pile group,

there is transferred horizontal loading to soil only through piles, because of lacking cap connection to soil

(default propose in pile group analysis), so 100% of horizontal load is transferred to soil by piles; while in

piled raft, part of this force is transferred to soil by raft and another part by piles. It is observed that

regardless of horizontal load bearing proportion of raft or cap in pile group foundation made increasing in

piles vertical and lateral (shear) designing forces. This Represents use of piled raft bearing concept is

economic method in vertical and horizontal loading.

-3

-2

-1

0

0 2 4 6 8 10 12 14 16

Set

tlem

ent

(mm

)

Time (S)

(a)

Case PR

Case PG

-25

-20

-15

-10

-5

0

0 2 4 6 8 10 12 14 16

Set

tlem

ent

(mm

)

Time (S) (b)

Case PR

Case PG

-150

-120

-90

-60

-30

0

0 2 4 6 8 10 12 14 16

Set

tlem

ent

(mm

)

Time (S)

(c)

Case PR

Case PG


295

Table 5 Percent of piles horizontal load in pile group system and piled raft

Foundation type

Number of storey pile group piled raft

10 100 45.6

20 100 66

30 100 69.3

Figure 8 shows horizontal load bearing proportion of P1 and P2 piles in two types of foundations and

three types of structures. As it can be seen piles have less horizontal load bearing proportion because of

anticipating raft in horizontal loading. Also it can be seen that horizontal load bearing proportion of P1 pile,

which it is located the most distance from foundation center, is more than P2 central pile, which this

distance is increased by increasing number of storey. In the figure, decreasing horizontal load bearing

proportion of each pile is resulted by increasing number of storey, because as it is seen in table 5,

increasing number of piles and piles diameter will increase group role in horizontal loading.

Fig. 8 Horizontal load bearing proportion of P1 and P2 piles: (a) 10 storey structure, (b) 20 storey structure, (c) 30

storey structure

Figure 9 shows changes of vertical loading for P1 and P2 piles in a 10 storey structure during

earthquake. As it can be seen, vertical loading oscillations are very low for both two foundations types, in

central pile (P2), while this oscillation is very high in corner pile (P1). Despite large vertical load oscillation

of pile P1 in the group pile, because of high initial value of vertical load on P1, this pile will not under

tension during earthquake. Therefore piles in piled raft system are more critical than piles in pile groups in

terms of the tension. These results are also similar for increasing number of storey.

3.6

5.4

1.3 2

0

2

4

6

8

10

12

Case PR Case PG

Pro

po

rtio

n o

f la

tera

l lo

ad

car

ried

by

eac

h p

ile

(%)

(c)

P1 P2

5.1

9.4

3

6.4

0

2

4

6

8

10

12

Case PR Case PG

Pro

po

rtio

n o

f la

tera

l lo

ad

car

ried

by

eac

h p

ile

(%)

(b)

P1 P2

5.8

11

4

9.1

0

2

4

6

8

10

12

Case PR Case PG

Pro

port

ion o

f la

tera

l lo

ad

car

ried

by

eac

h p

ile

(%)

(a)

P1 P2


296

Fig. 9 Changes of vertical loading for P1 and P2 piles in 10 storey structure during earthquake

Figure 10 shows maximum bending moment and shear force diagram of P1 pile in maximum shearing

and bending moment. Time of maximum shear force and maximum bending moment is different, because

effect of initial forces due to superstructure weight in static step. As it can be seen, amount of bending

moment and shear force will be decreased significantly, by increasing depth in both two foundation types

and three structures types, and the force is transferred to soil through the first several meters, which it

shows domination of inertia-based transaction on kinematic transaction. Maximum bending moment and

shear force in piles head that connected to pile group system are bigger than piled raft, because of the cap

not participating in vertical and horizontal loading. The general forms of the bending moment and shear

force variations along the pile in both types of foundation are roughly similar. Variations and suddenly

distance not observed.

-500

0

500

1000

1500

0 2 4 6 8 10 12 14 16

Ver

tica

l L

oad

on P

ile

hea

d (

kN

)

Time (S)

Case PR P2 P1

0

500

1000

1500

0 2 4 6 8 10 12 14 16

Ver

tica

l L

oad

on P

ile

(kN

)

Time (S)

Case PG P2 P1


297

(a)

(b)

(c)

Fig. 10 maximum bending moment and shear force diagram of P1 pile: (a) 10 storey structure, (b) 20 storey structure, (c) 30 storey structure

-10

-8

-6

-4

-2

0

-200 0 200 400

Dep

th (

m)

Max Shear Force (kN)

Case PR

Case PG

-10

-8

-6

-4

-2

0

-200 0 200 400

Depth

(m

)

Max Bending Moment (kN.m)

Case PR

Case PG

-14

-12

-10

-8

-6

-4

-2

0

-200 0 200 400 600

Dep

th (

m)


Case PR

Case PG

-14

-12

-10

-8

-6

-4

-2

0

-400 0 400 800 1200

Depth

(m

)


Case PR

Case PG

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

-200 0 200 400 600 800

Dep

th (

m)


Case PR

Case PG

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

-400 0 400 800 1200 1600

Dep

th (

m)


Case PR

Case PG


298

5 CONCLUSION

Compound raft foundation is the most complete of foundation system which it provides an economic

plan to deal with structural heavy loads and lateral loads such as earthquake, by considering capacity of raft

bearing and pile group components. In recent decades, this type of foundation has been increasingly used

for transferring heavy structural loads to the soil in different types, including connected and disconnected

pile, compound pile raft, and floating piled raft. In this paper, performance of piled raft and pile group

under earthquake loading have been studied by ABAQUS finite element software, and its results are:

The imposed shear force on foundation by superstructure under earthquake loading in is less than pile

group. In other words, made structure on piled raft foundation receives less response from earthquake.

After earthquake, foundation settlement in pile raft foundation is less than the pile group, and

settlement difference in two systems will be significantly increased by increasing number of storey.

In pile group, entire shear force is transferred to the soil by piles, while in the piled raft foundation; part

of this force is transferred to soil by raft and another part by piles.

The horizontal load bearing proportion of pile is increased by distance pile from foundation center.

Oscillation of vertical loading in central piles is very low and in corner piles is very high, which it is less

in the used piles in pile group, in comparison with piled raft.

With increasing depth of pile, bending moment and shear force are significantly reduced and force

transmission from pile to soil is done in a few meters of pile.

References

Anandarajah, D., Zhang, J., Gnanaranjan, G. & Ealy, C. (2001). Back-calculation of Winkler foundation parameters for

dynamic analysis of piles from field-test data. Proc. NSF International Workshop on Earthquake Simulation in

Geotechnical Engineering , 1–10.

Banerjee, S. (2009). Centrifuge and Numirical Modeling of Soft Clay-Pile-Raft Foundations Subjected to Seismic

Shaking, PhD Thesis, National University of Singapore.

Boominathan, A. & Ayothiraman, R. (2006). Dynamic response of laterally loaded piles in clay. Proc., Inst Civil Eng-

Geotechnical engineering 2, 199–202.

Hemsly, J.A. (2000). Design application of raft foundation, piled raft foundation projects in germany, Tomas Telford

Publishers, 323-410.

Horikoshi K., Ohtsu H. & Kimura M. (1996). Investigation of piles damages by the 1995 Hyogoken-Nambu Earthquake.

Tsuchi-to-Kiso, 44, 27-29 (in Japanese).

Horikoshi, K., Matsumoto, T., Hashizume, Y. & Watanabe, T. (2003). Performance of Piled Raft Foundation Subjected

to Dynamic Loading. IJPMG-International Journal of Physical Modeling in Geotechnics. 51-62.

Lanzo, G. & Vucetic, M. (1999). Effect of Soil Plasticity on Damping Ratio at Small Cyclic Strains, Soils and Foundations,

39, 131-141.

Matsumoto, T., Fukumura, K., Horikoshi, K. & Oki, A. (2004). Shaking Table Tests on Model Piled Rafts in Sand

Considering Influence of Super structures. IJPMG-International Journal of Physical Modeling in Geotechnics 3, 21-

38.


299

Matsumoto, T., Fukumura, K., Pastsakorn, K., Horikoshi, K. & Oki, A. (2004). Experimental and Analytical Study on

Behaviour of Model Piled Rafts in Sand Subjected to Horizontal and Moment Moading. IJPMG-International

Journal of Physical Modeling in Geotechnics 3. 1-19.

Nakai, Sh., Kato, H., Ishida, R., Mano, H. & Nagata, M. (2004). Load Bearing Mechanism of Piled Raft Foundation

During Earthquake. Proc. third UJNR Workshop on Soil-Structure Interaction, march 29-30, menlo Park, california,

USA.

Nakai, Sh., Mano, H. & Matsuda, T. (2004). An Analysis for Stress Distribution of Piled Raft Foundations under Seismic

Loading. Department of Urban Environment Systems, Chiba University. Meeting on Piled rafts.

Novak, M. (1974), Dynamic stiffness and damping of piles. Can Geotech J 11. 574–598.

Pak, R.Y.S., Ashlock, J.C., Abedzadeh, F. & Turner, N. (2003). Comparison of continuum theories with measurements

for piles under dynamic loads. Proc. Sixteenth ASCE Engineering Mechanics Conference. University of Washington,

Paper No: 156.

Poulos, H.G., Davis, E.H. (1980). Pile Foundation Analysis Design, John Wiley & Sons Inc., New York.

Prakash, S. & Chandrasekaran, V. (1973). Pile foundations under dynamic lateral loads. Proc. Eighth International

Conference on Soil Mechanics and Foundation Engineering, Vol. 2, 3/31. Moscow. 199–202.

Puri, K.V., Prakash, S. (1992). Observed and predicted response of piles under dynamic loads. Geotechnical Special

Publication 34, ASCE, 153–169.

Tucker, R.L. (1964). Lateral analysis of piles with dynamic behavior. Proc. Conference on Deep Foundations, 1. Mexico.

157–171.

rcee research in civil and environmental engineering has been used from abaqus finite element...

Documents