lecture 4_helmut schweiger - 3d finite element analysis of deep foundations

1

PLAXIS Hong Kong Lectures / Hong Kong, 28 November 2011

Helmut F. SchweigerFranz TschuchniggComputational Geotechnics GroupInstitute for Soil Mechanics and Foundation EngineeringGraz University of Technology

3D FINITE ELEMENT ANALYSIS OF DEEP FOUNDATIONS - PRACTICAL APPLICATIONS

Finite Element Analysis of Deep Foundations

2PLAXIS Hong Kong Lectures / Hong Kong, 28 November 2011

THREE CASE HISTORIES

� DEEP FOUNDATION OF TWO ADJACENT HIGHRISE BUILDINGS (Vienna)

� DEEP FOUNDATION OF TOWER FOR SHOPPING MALL (Bucharest)

� JET GROUT FOUNDATION FOR RAILWAY STATION (Vienna)

OUTLINE OF PRESENTATIONDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

2



DEEP FOUNDATION OF TWO ADJACENT HIGHRISE BUILDINGS (Vienna)

� Project overview

� Soil conditions and its numerical modelling

� Finite element models

� Optimisation of the foundation concept

� Results

� Conclusion

DEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



Concept for foundation: deep foundation with diaphragm wall panels

~220m~168m

TWIN TOWERSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

3



TOWER ITOWER II

64.1 m

56.3 m58.9 m

24 m

TOWER I: 220 m high (around 60 stories) > constructed first TOWER II: 165 m highFoundation system: diaphragm wall panels

Undergroundcar parks

PROJECT OVERVIEW - TWIN TOWERSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



m

ref3ref

5050 p'cot'c'cot'cEE ��

�

��

��

�� Slab

Barrette length 30m

Constitutive model

Hardening Soil Model (HS):

m

ref3ref

urur p'cot'c'cot'cEE ��

�

��

��

��

m

ref1ref

oedoed pcotccotcEE ��

�

��

��

��

Hardening Soil Small Model (HSS):m

ref3ref

00 p'cot'c'cot'cGG ��

�

��

��

��

SOIL CONDITIONS AND ITS NUMERICAL MODELLING DEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

4



G0

�0.7

Parameter Gravel Sandy silt Fine sand E50

ref (kPa) 40 000 20 000 25 000 Eoed

ref (kPa) 40 000 20 000 25 000 Eur

ref (kPa) 120 000 50 000 62 500 pref (kPa) 100 100 100 m (-) 0.00 0.80 0.65 G0 (kPa) 150 000 62 500 78 125 �0.7 (-) 0.0001 0.0002 0.0002

Defintion of additional parameters for the HSS model:

G0: Correlation between small strain stiffness and stiffness at larger strains after Alpan (1970)

> Young's modulus interpreted as Eur

�0.7: Stiffness reduction curves after Vucetic and Dobrey (1991)

HS model

HSS model





5



> Both towers are modelled> Models consist of around 50 000 15-noded wedge elements

FINITE ELEMENT MODELS - OVERVIEWDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



Detailed model for Tower I

Barrettes with unit dimensions of 3.6 m x 0.6 m

Detailed model for Tower II

FINITE ELEMENT MODELS - DETAILED MODELSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

6



Detailed model Tower I:• Barrette length 25 m• uy,max of 80 mm• Interaction of towers • Differential settlements

> Optimisation of foundation system

Eccentric loads lead to large differential settlements

FINITE ELEMENT MODELS - FIRST RESULTS TOWER IIDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



Final design of deep foundation system

Arrangement of diaphragm wall panels for Tower I

Arrangement of diaphragm wall panels for Tower II

OPTIMISATION OF FOUNDATION CONCEPTDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

7



RESULTSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



• Barrette lengths between 20 m and 30 m • Maximum vertical displacements of both towers ~80 mm• Differential settlements reduced to acceptable value


8



Model details:

• No geometrical simplifications

• ~ 140 000 15-noded wedge elements

• Mesh refinement

• 300 diaphragm wall panels are modelled explicitly

Expectations:

• Validate first approach

• Detailed information about settlement trough between towers

_________________________________________________________Model Nr. El. Nr.Nodes Nr.El in 2D plane _________________________________________________________ 32 bit model 49 096 131 993 2 888 64 bit model 136 710 361 243 6 510 _________________________________________________________

RESULTS WITH 64 BIT CALCULATION KERNELDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



� 317 diaphragm wall panels


9



• Results are similar to first approach• Maximum vertical displacements ~80 mm• More accurate with respect to interaction and settlement trough between towers




Layout 1:� 75 Piles � Diameter 1.5m� Spacing 6*D� Pile lengths are based on executed

deep foundation with panels (20 - 30m)� Piles are modelled as continuum (VP)

with interfaces:

� Also modelled with Embedded Piles (EP)

Alternative foundation concept: Piled raft foundation

Soilinter cRci SoilSoilinter � tantanRtan i

Soilinter � �� ii otherwise,0.1Rfor0

COMPARISON OF DIFFERENT FOUNDATION SYSTEMSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

10



An embedded pile is a pile which consists of:

� Beam elements� Properties: E, �, d� d determines the elastic zone in the soil around the beam

� Special interface elements� Properties: skin resistance, base resistance

ts Shear stress in axial direction

tn /tt Normal stress in n/t – direction

Ks Elastic shear stiffness

Kn/Kt Elastic normal stiffness in n/t - direction

up Displacements of the pile

us Displacements of the soil





Layout 2:� Piles modelled with Embedded

Piles concept (EP) � Diameter 1.5 m� Pile spacing is reduced in high

loaded regions� Pile lengths are based on

executed deep foundation with panels (20 - 30 m)


11




Layout 2:� Piles modelled with Embedded

Piles concept (EP) � Diameter 1.5 m� Pile spacing is reduced in high

loaded regions� Pile lengths are based on

executed deep foundation with panels (20 - 30 m)

Maximum vertical displacements:� Tower I: ~ 90 mm� Tower II: ~ 90 mm




Maximum settlements:• Barrettes: 7.6cm• Layout 1 (VP): 12.0 cm• Layout 1 (EP): 12.0 cm• Layout 2 (EP): 8.7 cm

• Shallow foundation: 18.7cm

Barrettes

Shallow foundation


12



Comparison of different foundation systems

� Maximum settlements

� �KPP:

Layout 1: 0.42Layout 2: 0.80

tot

PileKpp R

R �

~ 50 %




Settlements / differential settlements

> Road - up to 40 mm tolerable

> Highway - Point A 14 mm

Inclination 1/900

> Train - Point B 18 mm

Most critical area

Inclination 1/600

> Measurements during construction are necessary

A

B


13



� Same length of deep foundation elements > high differential settlements.

� Final foundation concept with lengths between 20 - 30 m improves the settlement behaviour of the towers.

� In most calculations the geometry is simplified in that way that only one tower is modelled in detail and the foundation of the second tower is modelled with a homogenized block.

� The computed maximum vertical displacements are about 80 mm. The settlements of the highway and the railway lines are in the order of 15-20 mm.

� With a 64 bit calculation kernel both towers are modelled in detail. The settlements are similar to the simplified models but the settlement trough between the towers is more accurate.

CONCLUSION FOR THIS PROJECTDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



DEEP FOUNDATION OF TOWER FOR SHOPPING MALL (Bucharest)

� Project overview

� Soil conditions


� Optimisation of the foundation concept

� Results

� Validation of numerical model

� Conclusion


14



Sky Tower

SHOPPING MALL WITH SKY TOWER DEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



SHOPPING MALL WITH SKY TOWER DEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

15



Results of interest:

1. Maximum differential settlements (of 2.6 m thick slab)

2. Arrangement of diaphragm wall elements

• Dimensions 93.4 x 61.7 m

• Height more than 130 m

• Excavation depth 20.4 m

• Slab thickness 2.6 / 1.5 m

• Point loads up to 14 900 kN

PROJECT OVERVIEW - SKY TOWER DEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



Silty Clay

Constitutive models:

• Diaphragm wall panels > Mohr-Coulomb

• Floors > Linear elastic

• Soil layers > Hardening Soil Model

Sand

~20.5 m

~25.0 m

60.0 m

Soil profile:

• Core drillings down to -60 m

• Altering layers of sands and silty clays

• Drained conditions are assumed

Assumption: Alternate layers continue

SOIL CONDITIONS DEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

16



All models consist of about 52 000 15-noded wedge elements

Core walls Floors (Top-down method)

FINITE ELEMENT MODELSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



~ 94 m

~ 62 m

Cross section

2D "Plane strain" model

Layout 1 Top view Deep foundation elements in 3D

FE model


17



uy,max~ 95 mm

uy,diff.max ~ 50 mm

uy,max~ 165 mm

uy,diff.max ~ 100 mm

• 2D (plane strain) model overestimates differential settlements by approx. 100%

• 2D representation of geometry / load situations not possible

• 2D representation of deep foundation elements not possible

> For optimisation of foundation concept full 3D modelling is required




• Maximum settlements between 85 - 100 mm

• Differential settlements between 42 - 60 mm

• Full connection of barrettes difficult to achieve in practice

Circle of panels beneath concentrated loads

Discontinuous barrettes Different lengths of barrettes


18



Final foundation concept

Top view

25 m

15 m

FE discretization of panels




• Maximum vertical displacements of about 105 mm

• Maximum differential settlements of about 65 mm

• Between point A and C (2.6 m thick slab) differential settlements of about 47 mm

Final foundation concept

Contour lines of settlements Vertical displacements of slab


19



• FE analysis predicts slightly higher settlements > conservative estimate of parameters• Effect of small strain stiffness is not taken into account

FE model Measurements vs calculation

VALIDATION OF NUMERICAL MODEL (IN SITU LOAD TEST)DEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



SHOPPING MALL WITH SKY TOWER Step 3:Excavation to -15.00 mConstruction of slab at -5.0 m


20



Step 4:Construction of base slab




� Due to the geometrical layout 2D analysis proved to be too conservative.

� A number of 3D calculations have been performed investigating different arrangements of diaphragm wall panels.

� The final concept of the deep foundation consists of two discontinuous circlesbeneath the highly loaded areas.

� The computed maximum settlements are 105 mm. The expected maximum differential displacements are about 65 mm.

� The numerical back analysis of an in situ load test shows reasonable agreement � The soil parameters have been estimated prior the test and no adjustments have

been made.

CONCLUSION FOR THIS PROJECTDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

21



JET GROUTING FOUNDATION FOR RAILWAY STATION (Vienna)

� Project layout

� Soil profile


� Validation of numerical model

� Results

CONTENTDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



~ 500m

~ 400m

PLAN VIEW

PROJECT OVERVIEW - PLAN VIEWDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

22



175m

100m

PROJECT OVERVIEW - ARRANGEMENT OF JET GROUT COLUMNSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



8 m - gravel3 m - sandy silt

14 m - sand

11 m - sandy silt

2 m - sand

10 m - stiff base layer

TYPICAL CROSS SECTION - SOIL PROFILEDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

23



� Three different models for the sensitive zones

Zone A: => 130 embedded piles => 51376 elements

Zone B: => 615 embedded piles => 47464 elements

Zone C: => 114 embedded piles => 47022 elements

FINITE ELEMENT MODELDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



Zone B: => 615 embedded piles => 47464 elements

Model without soil - top view

Model without soil - bottom view

EMBEDDED PILE MODEL FOR ZONE BDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

24



> 615 Piles

� Different pile lengths

� Different pile inclinations

> Rest is modelled as blocks

EMBEDDED PILE MODEL FOR ZONE BDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



inclined embedded piles

vertical displacements - uy,max = 63 mm

EMBEDDED PILE MODEL FOR ZONE B - RESULTSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

25



� Global settlement behaviour of construction is obtained because complete structure is modelled including areas with significant different load intensities.

� Detailed results in the zone where the embedded piles are modelled.

� Benefit of zone B (embedded piles): spacing, pile length and pile diameter can be modified with reasonable effort.

EMBEDDED PILE MODEL FOR ZONE B - RESULTSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)



MODEL - 64 bit calculation kernel

General results:• Global settlement behaviour of construction is obtained because complete structure is modelled including

areas with significant different load intensities. Maximum vertical displacements of ~65 mm.• Detailed results in the zones where the embedded piles are modelled. • "Embedded pile zones": spacing, pile length and pile diameter can be modified with reasonable effort.

64 bit calculation kernel:• Validation of first approach. • More than 1200 jet grout columns are modelled - more accurate results with respect to interaction of

sensitive zones.


26



� In many cases the geometry can be simplified in that way that only sensitive parts are modelled in detail > yields global settlement behaviour.

� With a 64 bit calculation kernel less simplifications are necessary. The settlements are similar to the simplified models.

� When creating embedded piles no additional geometry points are created � embedded piles do not influence the finite element discretisation.

� With embedded piles it is possible to model a high number of deep foundation elements with different spacings, inclinations and lengths.

� When using the HSS model the influence of the model boundary condition is reduced and a more realistic settlement behaviour can be obtained.

CONCLUSIONSDEEP FOUNDATION (Vienna) DEEP FOUNDATION (Bucharest) JET GROUT FOUNDATION (Vienna)

PLAXIS Hong Kong Lectures / Hong Kong, 28 November 2011

Helmut F. SchweigerFranz TschuchniggComputational Geotechnics GroupInstitute for Soil Mechanics and Foundation EngineeringGraz University of Technology

3D FINITE ELEMENT ANALYSIS OF DEEP FOUNDATIONS - PRACTICAL APPLICATIONS

lecture 4_helmut schweiger - 3d finite element analysis of deep foundations

Documents