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finite element for soil and rock analyses

PLAXIS SEMINAR‐ Taipei 2007 1

FINITE ELEMENT CODE FOR SOIL AND ROCK ANALYSES

Plaxis Vietnam 2008 1

f i n i t e e l e m e n t c o d e f o r s o i l a n d r o c k a n a l y s e s

C tit ti S il M d l St t l El t d Si l ti

PLAXIS SEMINARVietnam Seminar

2008

THE PLAXIS APPROACH

Constitutive Soil Models, Structural Elements and Simulation

V I S U A L I S E A N A L Y S E O P T I M I S E > T H E W A Y F O R W A R D

Dr WL CHEANG & Dr SW LEE

William W.L. CHEANG B.Eng (Hons), PG.Dip, M.Sc. Ph.D

Compiled by:

Regional Technical ManagerPlaxisAsia

Contributed

Erwin BEERNINKDennis WATERMAN

Erick SEPTANIKARonald BRINKGREVE

Siew Wei LEE

LAXIS PROFESSIONAL vers ion 8 .5 - PLAXFLOW vers ion 1 .5 - DYNAMICS module - 3-D FOUNDATION vers ion 2 .0 – 3-D TUNNEL vers ion 2 .0 – 3-D GEOTHERMIE vers ion 1 .



SOIL MODELS, STRUCTURAL ELEMENTS & BOUNDARY CONDITIONS IN PLAXIS INPUT

A. Constitutive Soil Models

1. Linear Elastic (LE)

2. Mohr‐Coulomb (MC)

3 Soft soil / creep model (SSM and SSCM)3. Soft soil / creep model (SSM and SSCM)

4. Hardening Soil model (HSM)

5. User‐defined Soil Models (USM)

B. Structural Elements

1. Geotextile element (membrane)

2. Beam (Plate) element

3. Node‐to‐node anchor3. Node to node anchor

4. Fixed end anchor

C. Recent Development with Embedded Inclusion



A SOIL MODELSA. SOIL MODELS



MOHR-COULOMB1. First order approximation of soil

2. Linear elastic perfect plasticity

3. Five material inputs

1. Friction angle (phi)g (p )

2. Cohesion (c’)

3. Dilatancy angle (psi)

4. Elastic modulus (E)

5. Poisson’s ration (nu)


SOFT SOIL & SOFT SOIL CREEP MODEL1. Inspiration from Cam‐Clay class of model

2. Primary compression for Normally consolidated soils

3. Stress dependent stiffness

4. Distinction between primary loading and unloading‐reloading4. Distinction between primary loading and unloading reloading

5. Memory of preconsolidation stress

6. Failure behaviour=Mohr Coulomb




SOFT SOIL & SOFT SOIL CREEP MODEL


HARDENING SOIL MODEL1. Second order approximation of soil

2. Advanced model for simulating soft and stiff soils (Shanz, 1998)

3. Input parameters




HARDENING SOIL MODEL


HS-SMALL1. Hardin‐Drnevich curve

2. Parameter: Go and γ0.7





B STRUCTURAL ELEMENTSB.STRUCTURAL ELEMENTS

SOIL MODELS, STRUCTURAL ELEMENTS

A. Structural Elements

1. Geotextile element (membrane)

2. Beam (Plate) element

3 Node to node anchor3. Node‐to‐node anchor

4. Fixed end anchor




STRUCTURAL ELEMENTS IN PLAXIS1. Plates and shells

2. Anchors

3. Geogrids (geotextiles)

4. Interfaces4. Interfaces

wall strip footing tunnel

13Plaxis Vietnam 2008

STRUCTURAL ELEMENTS IN PLAXIS

anchored wall cofferdamgeotextile wall

strut ground anchor




PLATES AND SHELLS

1. 3 or 5 noded line elements2. 3 degrees of freedom per node3. Elastic or elastoplastic behaviour4. To model walls, floors, tunnels


INPUT PARAMETERS FOR PLATES

1. Flexural rigidity (b=1 m)

2. Normal stiffness (b=1 m)12

3 bhEEI ⋅⋅=

( )

3. Element thickness bhEEA ⋅⋅=

EAEIhd 12==

h

b

h hb

b = 1 m in plane strainb = 1 meter in axisymmetry




PLATE WEIGHTS

1. Compensate for overlap:

2. For soil weight use:g

γunsat above phreatic level

γsat below phreatic level

realsoilconcrete dw ⋅−= )( γγ


PLATE WEIGHTS FOR TUNNELS

d real

1. Overlap is only for half the lining thickness

rinsideroutside

lining soil

r

( ) ( )1

( )outsideinside rrr += 21

( ) ( )realsoilrealconcrete ddw 21⋅−⋅= γγ




FIXED-END ANCHORS1. To model supports, anchors and struts

1. Elasto‐plastic spring element

2. One end fixed to point in the geometry,other end is fully fixed for displacement

3. Positioning at any angle

4. Pre‐stressing option


NODE-TO-NODE ANCHORS1. To model anchors, columns and rods

1. Elasto‐plastic spring element

2. Connects two geometry points in the geometry

i i3. Pre‐stressing option




ANCHOR MATERIAL PROPERTIES

Normal stiffness, EA (for one anchor) [kN]

Spacing, L (distance between anchors) [m]Spacing, Ls (distance between anchors) [m]

Maximum anchor force for compressionand tension, |Fmax,comp| and |Fmax,tens| [kN]


PRE-STRESSING OF ANCHORS

1. Defined in Staged construction phase

2. Both tension (grout anchor) or compression (strut) possible(g ) p ( ) p




GEOGRIDS

1. 3 or 5 noded line element2. Linear elastic behaviour3. No flexural rigidity (EI), only normal stiffness (EA)4. Only allows for tension, not for compression5. Soil/Geogrid interaction may be modelled using

interfaces


GROUND ANCHORS

1. Combination of node‐to‐node anchor and geogrid

2 N d d h h d (2. Node‐to‐node anchor represents anchor rod (no interaction with surrounding soil)

3. Geogrid represents grout body (full interaction with grid)

4. No interface around grout body; interface would l f l fcreate unrealistic failure surface




GROUND ANCHORS

axial forces in geotextile element

real distribution of axial forces in ground anchorInput geometry

Generated mesh

Axial forces in ground anchors


INTERFACES1. Soil‐structure interaction

1. Wall friction

2. Slip and gapping between soil and structure

2. Soil material properties p p

1. Taken from soil using reduction factor RinterCinter = Rinter * Csoiltan(φ)inter = Rinter * tan(φ)soil

2. Individual material set for interface




INTERFACESSuggestions for Rinter:

1. Interaction sand/steel = Rinter ≈ 0.6 – 0.7

2. Interaction clay/steel = Rinter ≈ 0.5

3 Interaction sand/concrete R 1 0 0 83. Interaction sand/concrete = Rinter ≈ 1.0 – 0.8

4. Interaction clay/concrete = Rinter ≈ 1.0 – 0.7

5. Interaction soil/geogrid = Rinter≈ 1.0(interface may not be required)

6. Interaction soil/geotextile = Rinter≈ 0.9 – 0.5 (foil, textile)


INTERFACES

1. Try to omit stress oscillations at corners of stiff structures

Inflexible corner points, causing bad

stress results

Flexible corner points with improved stress

results





C STRUCTURAL ELEMENTS (RECENT)Embedded Inclusions (Piles, Anchors and Soil Nails)

C.STRUCTURAL ELEMENTS (RECENT)

GENERAL DESCRIPTION OF EMBEDDED PILES

1. FE modeling

2. Current implementation

I3. Improvements

4. Ground anchor

5. Concluding remarks




FE MODELINGSCHEMATIZATION OF PILE-SOIL SYSTEM

Plateral

Paxial

Pile‐head

Pile behavior depends on:1 Soil type

QsQn

soil masses

Qs : mantle/skin shear forcesQn : mantle/skin normal forcesF : pile base shear forces

1. Soil type

2. Stress state

3. Pile geometry

4. Pile type (Steel, concrete, timber…etc.)

5. Installation

31

Fn

FsPile‐base

Fs : pile‐base shear forcesFn : pile‐base normal forces

Plaxis Vietnam 2008

FE MODELINGVOLUME PILE APPROACH

τtτs

σr

32

R dα

Plaxis Vietnam 2008



FE MODELINGEMBEDDED PILE APPROACH

pile

soil

pile

tskin

Ffoot


Skin & Base Interaction

Skin interaction model Base/Foot interaction model

kt

s

t

Skin stiffness:ks : axial stiffnesskn&kt : lateral stiffness

kskn

ks

kt

kn

ktkb

Base stiffness:kb : base/foot stiffness

34

≤≤nSkin tractions:ts = qs/length = ks (us

pile‐ussoil) tmax

tn = qn/length = kn (unpile‐un

soil)tt = qt/length = kt (ut

pile‐utsoil)

≤

kskn Base/Foot force:

0 Fb = kb (ubpile ‐ ub

soil) Fmax

Plaxis Vietnam 2008



Skin & Base InteractionLinear skin traction model & Maximum base/foot force

Bearing Capacity:

½ (Ttop+Tbot)*Lpile + Fmax

Ttop

Lpile

35

Fmax

Tbot

Plaxis Vietnam 2008

Mesh‐dependent behavior

P

Bearing capacityfine meshcoarse mesh

Due to soil failure inside “pile region”

36

y

Plaxis Vietnam 2008



ImprovementsI. Objective behavior (less mesh‐dependent )

“Pile zone” is defined based on the volume of pile (=πR2*L)Any small (soil) element that falls inside pile zone will be forced to remain elastic

2 R

37

Embedded Pile Element

Plaxis Vietnam 2008

ImprovementsII. Layer‐dependent maximum (allowable) skin traction model

Maximum skin traction as function of depth will be generated automatically for different layers according to

t { avg t φ }* Rts { σhavg tan φi + ci }*2πR

with:φi interface friction angle (Rp* φsoil)ci interface cohesion (Rp* csoil)σh

avg average lateral compression at the pile(based on soil stresses around the pile)

≤

38

extra control paramaters:tsmax user‐defined maximum valueΔσh user‐defined pre‐stress to include local confinement

Plaxis Vietnam 2008



ImprovementsIII. Multi‐linear curve of maximum (allowable) skin traction

Ttop

Ty1

ytop

y1

Ty3

Ty2

Ty4

y2

y3

y4

39

Ty5y5

Plaxis Vietnam 2008

Improvements IV. Installation Effects

Pile centre

Pile zone

Disturbed/influenced soil region

void ratio eσ may decrease/increase




Ground anchors

A Ground anchor model consists of* anchor* grout body

B

wall

grout body

anchor

FE modeling:Anchor 2‐noded springGrout body embedded pile

(allowing sliding)

41

C

Plaxis Vietnam 2008

CONCLUDING REMARKS ON EMBEDDED ELEMENTS

1. Embedded pile has been implemented in 3DF

* Bearing capacity : maximum skin traction & base/foot resistance

1 Mesh effect is handled by including elastic region inside “pile zone”1. Mesh effect is handled by including elastic region inside pile zone

2. Current version appears capable of predicting “objective” failure behavior

* layer‐dependent (maximum) skin tractions will be included

1. Future version

* multi‐linear curve of maximum skin tractions (available in 3DF V2)

* installation effects (case studies & field experience)

G d h d l b d E b dd d il h

42

1. Ground anchor model based on Embedded pile approach

Plaxis Vietnam 2008



PILED RAFT FOUNDATION FOR A STORAGE TANK

CHARACTERISTICS

1.No. of Piles 122

2.Pile Diameter 400mm

3.Pile Lengths 5m and 8m (staggered)

4.Raft 1000mm THK

5.Raft Size

6 Tank Diameter 10m6.Tank Diameter 10m

7.TankThickness 1000mm

8.Soil Layered



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