Computer modeling of micropile systems with ZSoil Computer Modeling of Micropile... · Computer...
Transcript of Computer modeling of micropile systems with ZSoil Computer Modeling of Micropile... · Computer...
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Computer modeling of micropile systems with ZSoil
Andrzej TrutyAleksander Urbanski
Politechnika Krakowska
Kraków, 2014
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What is ZSoil ?
FEM software for solving 2D/3D static/dynamicsoil-structure interaction problems
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Main ZSoil capabilitiesStatics (short/long term) and transient dynamics forsingle and two-phase (partially saturated)media+structures
Stage construction and excavation analysis isallowed in the real time scale (including consolidationand/or creep effects)
Strong deformation discontinuities between thestructure-subsoil or structure-structure can beintroduced via Coulomb type interfaces
Small strain stiffness of soils can be represented by acomplex but easily calibrated nonlinear constitutivemodels (Hardening Soil-small (HSs) model forinstance)
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Why do we need FEM modeling of micropilesystems?
FEM models alow to analyze coupledmicropile-foundation-subsoil systems(rehabilitation of foundation of an existing building)
Serviceability and ultimate limit states can beanalyzed
FEM modeling helps to understand all interactionsbetween the micropile-foundation-subsoilcomponents
All kind of nonlinearities can be included (in micropileitself, subsoil, interfaces)
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Sources of nonlinearities inmicropile-subsoil-structure system
concrete
Steel elementsubsoil
micropile
Subsoil behaves in a nonlinear mannerInterface micropile-subsoil is probably the source ofstrongest nonlinearityIn some cases reinforcement-concrete interface canbe activatedConcrete can crack (if bending is activated)Other ?
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Sources of uncertainties in FEM models ofmicropile-subsoil-structure system
Subsoil: stress history (overconsolidation), initial porepressures, stiffness
1 Geostatic conditions (Ko in situ)2 Level of saturation3 Dilatancy (usually ψ =
16÷ 1
4φ′ in triaxial tests)
Micropile-subsoil interface: effect of micropileinstallation and dilatancy
1 During installation radial stresses increase locallynear the micropile (we add an axisymmetric stressfield into the general 3D state)→ K effect
2 Friction angle in the interface depends strongly on thetechnology
3 Strains are large (but only locally)
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Effective stress analysis in ZSoil (static case)Overall equilibrium: σtot
ij,j + fi = 0Effective stress principle σtot
ij = σ’ij + S p δij
Fluid flow continuity: S ·εkk + vF
k ,k − c·p = 0
Darcy velocity vFi = −kij kr (S)
(− pγF + z
), j
kr (S) function kr =(S − Sr )3
(1− Sr )3
S(p) (van Genuchten )
S(p) = Sr +1− Sr1 +
(α
pγF
)21/2
c(p) storage function c = c(p) = n(
SKF
+dSdp
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Effective stress analysis in ZSoil: possibledrivers
Quasi-undrained analysis→ short loading time, lowpermeability (in statics)
Steady state drained analysis→ long loading time
Transient case→ tracing pore pressure disipation inreal time
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Consequences of effective stress analysis
Parameters for soil constitutive model must beeffective→ c′, φ′
Undrained (su) or transient values of strengthparameters c, φ are naturally embeded in the theoryonce the consolidation driver is used and properelasto-plastic model is used
Cohesion results from suction pressure or effect ofcementation
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Soil constitutive models: M-C vs HSs
Elasto-plastic M-C model(frequently used in practice)
Ultimate limit states: YES
Serviceability limit states: NO (most often)
Elasto-plastic model HSs(since last few years quite often used in practise)
Ultimate limit states: YES
Serviceability limit states: YESTechnical report: R. Obrzud, A. Truty. THE HARDENING SOIL MODEL - APRACTICAL GUIDEBOOK Z Soil.PC 100701 report
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HSs model: 2 plastic mechanisms
f f
Double hardening model
Cap surfaces and failure cone (M-C) in principal stress space
p’
Isotropic hardening mechanism:cap yield surfaces p ydescribed with van Ekelen’s formula
Mohr-Coulombfailure surface
The Hardening Soil model with small strain stiffnessRafal Obrzud, GeoMod SA26.08.2013, Lausanne, Switzerland
- HS Standard
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HSs model: stiffness representation
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HSs model: calibration
(S)CPTU field test
(S)DMT field test
Triaxial test (CD) including shear wave velocitymeasurement as a calibration test for CPTU/DMTcorrelation formuli
CPTU/DMT serve us stress history parameter OCRand Ko in situ
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Micropile-subsoil interaction:fully conforming discretization (A)
Micropile (3D continuum)+interface
Subsoil
Resulting FE models are huge and extremely timeconsumingEach redesign of piles requires new mesh for wholesystem
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Fully conforming discretization: interfacetreatment
Interface thickness is zero
Contact stress computationσn,N+1 = kn gn,N+1
τN+1 = τN + ks ∆gs and |τN+1| ¬ σ′n tg(φ) + c′
kn and ks are penalty factors for rigid plastic interface
kn and ks can be related to the shear band thicknesst and its quasi-elastic stiffnesskn = E/t while ks = G/t
Rigid plastic interface leads to overstiffening of themicropile response
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Micropile-subsoil interaction:overlaid mesh approach (B)
Resulting FE models are smaller than for conformingmodelRelatively coarse mesh for subsoil is used whilemesh for micropile+interface+small part of subsoil isdense
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Micropile-subsoil interaction:micropiles as 1D members embedded in 3Dcontinuum (C)
Resulting FE models are smallSpecial interface must be implementedRedesign of micropile system is very easy
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Micropiles as 1D members embedded in 3Dcontinuum: interface treatment
Beam elementsMaster segment „m”
Master node of pile tipInterface „B”
Top pile node „T”
Plate/shell element „p”
3D continuum element „c”
Slave segment „s”
NB. Effect of micropile installation will be discussed later18 / 29
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Interface micropile-subsoil in simplifiedapproach
τ=σn tan φ+c
ft=0, fc< fcult
xL
yL
zL
m1
m2s2
s1
segment mastersegment slave
In simplified approach there is no way to recover σn
from the interfaceHence we have to recover it from the adjacentcontinuum
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Interface micropile-subsoil in simplifiedapproach
Recovering σn from adjacent continuumxL
yL
zLR
R = SQRT (A/π)
ΔLiPi
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Effect of installation: K-pressure method
K-pressure method (PhD by Syawal Satibi, Stuttgart,2009)
In situ Ko state „excavation” unloading is delayed
Add (K‐Ko) hAdd micropile/unload exc. forces/remove pressure
But how to define K value ?21 / 29
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Effect of micropile installation
Micropile diameter is relatively small→ effect onincrease of radial stress due to installation islocalized in a relatively narrow zone
This effect can be analyzed in an analytical mannerusing known solutions for cavity expansion problem
In methods (A) and (B) we can use K-pressuremethod (PhD by Syawal Satibi, Stuttgart, 2009)
In method (C) K-pressure method is applicable butmesh size must be carefully choosen
Back analysis of load test may yield→ K value andinterface stiffness
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Effect of micropile installation in method (C):possible solution
Stress variation due to installation is neglected insubsoil
Equivalent interface friction angle tan (φ∗) has to beused to reproduce skin friction
This may lead to overestimation of micropilesettlements near the limit state
K-pressure is recommended (adding axisymmetricstress field)→ not available so far
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Effect of dilatancy in the interface zone
In methods (A) and (B) effect of dilatancy is present
In simplified approach (C) this effect is missing (sofar)
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An example: loading test on single micropileD = 18cm
‐0.025
‐0.02
‐0.015
‐0.01
‐0.005
00 100 200 300 400
Settlemen
t
Force [kN]
Simplified model
Experiment
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An example: micropile foundation system
3x3 and 5x5 micropile foundation system (D =20cm)
Micro-pile foundation system
3x3 and 5x3 D=0.2
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An example: micropile foundation systemComparison of:
3D FEM whith fully conformed mesh3D FEM with simplified ZSoil® method of pile modelling
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An example: micropile foundation systemComparison of displacements:3D FEM whith fully conformed mesh3D FEM with simplified ZSoil® method of pile modelling
UySimpl = 2.5mm
slighty stiffer (~15%) response
Uy=3mm s = 3 mm s = 2.5 mm (stiffer response)
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Conclusions
Proposed simplified approach is a very useful tool forsolving problems with large number ofmicropiles/piles
Standard discretization technique (A) is inefficient forcomplex 3D problems
Both approaches (A)/(B) and/or (C) require carefulcalibration of strength and stiffness parameters (byback analysis)
Combined standard design methods (for micropile)and numerical modeling of whole system seem to bethe most appropriate approach
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