gts seminar dubai 7dec2015

Int. Symposium on Recent Advances in High-Rise Buildings & Geotechnical Analysis 7-Dec-15, Knowledge City, Dubai

3D Numerical Analysis of Piled-raft FoundationsConcepts & Case studies

Ahmed Elkadi, PhD, M.ASCE

Contents

• Piled-raft systems

• Pile modeling & embedded piles

• Single pile calibration

• Parametric analysis

• Case studies

2

Piled Rafts (PR)

Advantages of a piled raft foundation

• Limitation of absolute and differential settlements

• Reduction in foundation tilting either due to load eccentricity or due to

irregularities in the subsoil

• Reduction in raft internal stresses

• Economical foundation option for

circumstances where the performance of

the raft alone does not satisfy the design

requirements.

• The addition of a limited number of piles

may improve the ultimate load capacity,

the settlement and differential settlement

performance, and the required thickness

of the raft. (H.G. Poulos, 2001)

Piled-rafts

3

www.geomarc.it

Piled Rafts

SSI of piled-rafts

Interaction influences:

• Pile-Soil interaction

• Pile-Pile interaction

• Raft-Soil interaction

• Pile-Raft interaction

Piled-raft system

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Piled Rafts

• Ensure that the foundation does not undergo excessive displacementsServiceability limit state (SLS) => rmax < r allowable & qmax < q allowable

• Overall stability should be insuredUltimate limit state (ULS) => Rg* ≥ S* (geotechnical)

Design issues

Key questions:• The relative proportion of load carried by the raft and piles

• Which pile configuration reduces total and differential settlements

3D Nonlinear FE Analysis

5

Piled Rafts

Desirable characteristics for the analysis of piled rafts

• Pile groups subjected to vertical load and

moments in both horizontal directions

• Realistic (nonlinear) soil behavior

• Non-linear soil-pile interface behavior

• Different pile types within group

• Raft/cap stiffness incorporated

• Structure stiffness incorporated

Analysis characteristics

TC18 report, 2001

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Model Simulation

Geotechnical reportSoil profiles

Lab (e.g. Triaxial tests)

Field (e.g. CPT, SPT,..)

Calibration

Foundation Design

• Serviceability Limit State

• Ultimate Limit State

Determine the required structural parameters

Structural Design

Pile TestsSLT, DLT, RLT, PDA

Numerical Model

• Parametric Study

• Load Combinations

7

Three pile modeling approaches are available:

• Solid Element Model

• Beam-Solid Connectivity Model

• Line-to-Solid Interface Model (embedded pile)

Pile modeling in DIANAPile modeling concepts

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Model = Soil (solid) + Pile (solid) + Interface (surface)

Nodal connectivity is required on pile outer surface

Solid element modelSolid element model

9

Points of attention for solid element models:

• Model definition and mesh-generation could be elaborative for large number of piles

• Many elements in model large computation times

• Pile forces and moments are not directly available in post-processing

• Interface behavior: elastic, nonlinear elastic, coulomb friction, and user-supplied material

Surface interface elements for solid-to-solid connection:

Solid element modelSolid element model

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Model = Soil (solid) + Pile (beam) + Interface (line)

Nodal connectivity is required along pile length

Beam solid connectivity modelBeam-solid model

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Points of attention for beam-solid element models:

• The nodal compatibility requirement makes geometry modeling and

meshing of the soil elaborative.

• For piled rafts with large number of piles, this technique produces

large models large computation times

Line interface elements for beam-to-solid connection:

Beam solid connectivity modelBeam-solid model

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Model = Soil (solid) + Pile (beam) + Interface (line-to-solid)

No nodal connectivity required => well-suited for PRs

Line-to-soild interface model

Sadek & Shahrour (2004):

A three dimensional embedded beam element for reinforced geomaterials

Shear interaction between beam element and surrounding soil.

Line-to-solid (Embedded pile) model

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Elementary Coordinate

y

z

x

Line-to-solid interface elements for beam-to-solid connection:

Characteristics of line-to-solid interface modeling in DIANA:

• Pile and soil geometries and meshes can be specified independently

• Intersections of line and solid elements are calculated automatically

• Nonlinear friction-slip properties for line-solid interface elements

• Mesh refinement requirements for the soil are minimum

reduced computation time

Line-to-soild interface modelLine-to-solid (Embedded pile) model

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Model = Soil (solid) + Pile (beam) + Interface (point-to-solid)

Axis of pile tip bearing

y

z

x

Pile tip sping

Point-to-soild interface model

Characteristics of point-to-solid interface modeling in DIANA:

• The pile tip can be arbitrarily placed in the solid element

• Nonlinear properties for point-solid interface elements

Point-to-solid (Embedded pile) model

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Relative Slipdisplacement

Shaft friction forceper unit length of pile

1

Ks

Ultimate shear force, qu

Input parameters:

• Ultimate shear force, qu [kN/m]

per unit length of the pile, at reference depth

• Shear Stiffness Modulus, Ks [kPa]

Linear elastic penalty stiffness of the interface

in the longitudinal direction of the pile.

• Normal Stiffness Modulus, Kn,Kt [kPa]

Linear elastic penalty stiffness of the

interface in the transversal direction.

These input parameters are best extracted from

SLT results after separating shaft friction and

base bearing behavior from the total response

Pile element parameters

Pile bearing capacity is input in pile elements and not a result of the calculation!

=> Deformation behavior

Line-to-solid interface model

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Pile element parametersPile element parameters

Skin tractions

ts = qs/length = ks * (Du) ≤ qult

tn = qn/length = kn * (Du)

tt = qt/length = kt * (Du)

Displacement

Specified bearing capacity

Force

Relative

displacement

Tau

Base (tip) bearing capacity

qb = kb * (Du) ≤ qbult

Relative

displacement

Tip

capacity

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VerificationSingle pile analysis of the Alzey Bridge pile loading test

The pile load test was conducted by Sommer & Hammabach in 1974 to optimize the foundation

design of Alzey Highway Bridge in Germany (El-Mossallamy 1999)

Calibration analysis

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VerificationSingle pile analysis of the Alzey Bridge pile loading test

Calibration analysis

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O. Elkadi (2011):

M.Sc. Thesis “Performance of Piled Raft Systems”

Parametric study

D = 1.3m

S/D = 2, 3 & 4

L/D = 5 & 20

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Parametric study

29MM 15MM

D = 1.3m S/D = 2 L/D = 5

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30MM

D = 1.3m

S/D = 2, 3 & 4

Pile Group L/D=20 Pile Group L/D=5

Parametric study

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Piled-raft L/D=20 Piled-raft L/D=5

D = 1.3m

S/D = 2, 3 & 4

Parametric study

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Parametric study

% load carried by raft for different pile layouts in piled-raft

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Single pile vs pile group vs piled raft

Group behavior and pile-raft interaction reduce on the one hand the stiffness

of the piles and increase on the other hand their bearing capacity

Pile behavior

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Single pile vs pile group vs piled raftPile behavior

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Piled raft foundation

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Case study: Pentominium Tower

Hyder Consulting, 2008

Worlds Tallest residential building

• > 100 stories tall (>500m)

• Preliminary design 233 piles

• 1.5m Diameter

• 46-51m long


28


Kamiran et. al., Proceedings ICE, Civil Engineering, 162, Nov. 2009

Site Investigation

Field

• 8 boreholes 80-125 deep boreholes

• standard penetration testing

• packer permeability testing

• pressuremeter testing at 3 m intervals in

three of the boreholes

• geophysics (cross-hole, cross-hole

tomography and down-hole testing)

Lab

• cyclic undrained triaxial

• cyclic simple shear

• stress path triaxial testing

• resonant column

• constant normal stiffness testing


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Model statistics

• 102355 nodes

• 6260 beam elements

• 22200 plate elements

• 3520 interface elements

• 6250 pile interface elements

• 162184 solid wedge elements

• 10 Load cases

MIDAS GTS software


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User defined Nonlinear elastic &

Nonlinear Elasto-plastic soil models

MIDAS GTS software

Kamiran et. al., Proceedings ICE, Civil Engineering, 162, Nov. 2009


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Final design from 36MN => 32MN

Pile length of 42m

Hyder: “complex geotechnical finite element analysis has been carried

out, which has been validated using standard geotechnical calculation

techniques. The application of such testing and analysis approach has

resulted in a cost-effective and optimised foundation design solution.”

Piled raft foundation>250 Piles of 1.5m diameter

Varying length: 35m edge, 65m center

Marine sediments underlayed by Sandstone

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Case study: Kingdom Tower

Langan International

https://commons.wikimedia.org/wiki/File:Jeddah_tower.jpg#/media/File:Jeddah_tower.jpg


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Deformed shape of vertical displacement

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DIANA SOFTWARE


Axial forces in the piles

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Introduction to Pile Analysis

Q & AShams Tower, 2006

Palazzo Versace & D1 Tower, 2007

FAD Towers, 2007

gts seminar dubai 7dec2015

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