Soil-structure interaction - Zace Services Ltd.

Soil-structure interactionc○ZACE Services LtdAugust 20111 / 602 / 60

Examples of soil-structure interactionSheet-pile wallsPrestressed anchorsDiapraghm wallsNailingFoundation rafts with pilesBuilding-foundationIn all the above problems strong displacement/pressurediscontinuity may appear → soil-structure interfaces play animportant role Preface 2D problems.4 HOW TO RUN SHEET-PILE WALL PROBLEMData file: tutorials/sheet-pile-wall.INPDescriptionGeneration of a complex geotechnical model of installation of an anchored sheet pile wall,followed then Sheet-pile by an excavation wall: example is the goal of this tutorial. The geometry of the modelwill evolve in time and some model components like wall, anchors or excavated soil layerswill appear or disappear according to the assumed scenario. The geometry of the model isshown in the figure below.12 m 18 m6m3 / 60Excavation zone-16m323Excavation zone-2Anchors3Medium sand3Sheet pile wall8mClay5Sequence of all steps is shown in the following table.4 / 60

Sheet-pile wall: Modeling issues2D/3D model?Ultimate limit state analysis (ULS) (M-C model is enough)Serviceability limit state (SLS) is (or not) main concern ? (ifyes then M-C model is too poor and HS small strain modelshall be used)Each construction step must be reproduced (if SLS is majorconcern and to avoid numerical problems witch convergence)Contact interfaces must be presentSheet-pile wall can be modeled using beam/shell elements orspecial continuum elements (Continuum for structures)(only elastic behavior is possible)Fixed anchor zones can be activated possibly with adhesiveinterfacePrestress can be controlled in time5 / 60Sheet-pile wall: Constitutive aspectsG - current secant shear modulusG o - shear modulus for very small strainsAtkinson 1991If serviceability limit state is our major concern then weshould use more sophisticated model for soils (HS with smallstrain)6 / 60

Linear / nonlinear beams in plane strainSheet-pile wall: 2D/3D model ?Plane strain continuum shellInterval between beams a = 1Plane strain discrete ribb systemInterval between beams a = LL2D domain:Interval between beams a = 1Beam elementsUser data: A, Iz ( data per beam )(a)Program computes automatically Interval A’=A/a between I’=Iz/abeams a = L(A’, I’ – values per unit length ) Results are given per beam (!)L2D domain:2D domain:Beam elementsBeam elementsUser data: A, Iz ( data per beam )Linear / nonlinear beams in plane strainProgram computes automatically A’=A/a (b) I’=Iz/a(A’, I’ – Here values 2D per (a) unit orlength axisymmetric ) Results (b) are model given per is beam good (!) enoughPlane strain continuum shellPlane strain discrete ribb systemear beams in plane strainllSheet-pile wall: 2D/3D model ?Plane strain discrete ribb systemInterval between beams a = 17 / 60Interval between beams a = LL2D domain:= 1 Interval between beams a = LLBeam elementsUser data: A, Iz ( data per beam )Program computes automatically A’=A/a I’=Iz/a(A’, I’ – values per unit length ) Results are given per beam (!)ain:Lmentsm )1ally A’=A/a I’=Iz/a Here 2D model may not be good enough.....2) Results are given Results per inbeam beams (!) and in anchors (same definition for L) will beoutput per beam/anchor (!)8 / 60

the Problem type list. The predefined systemClayof units for both data preparation andvisualization of results can be verified in menu Control/Units.8mSheet-pile wall: Construction steps5Sequence of all steps is shown in the following table.Initial state (t = 0) Installation of sheet pile wall (t = 1)Excavation zone-1Installation of first anchor (t = 2) Excavation of 1 sand layer (t = 3)Excavation zone-1Excavation zone-1DriversInstallation of second anchor (t = 4) Excavation of 2 sand layer (t = 5)Use existence/unloading functions associated with elementsJune 16, 2007QuickHelp DataPrep Theory Benchmarks(continuum, Z Soil R -3D-2PHASE beam, v.7 etc..) plus time dependent drivers (drivenTU–37load/consolidation)The whole computational process will consist of three drivers i.e. the Initial statewhich will yield the initial stress distribution (including user defined coefficient of in situlateral pressure Sheet-pile K o = 0.8 wall: in clay Drivers layer), Time dependent/Driven load to analyze allconstruction and excavation steps and at the end Stability (using c − tan(φ) reductionalgorithm) Accessible will be carried fromtomenu: assessControl/Analysis the global safety & factor. Drivers The complete set of drivers isgiven in the following figure.9 / 60To learn on how to set up the drivers list watch the videoSet drivers10 / 60

Contact interface: mesh discontinuity1 Here at the interface 3 nodal points are created to modelstrong discontinuoes motion of the neighbouring domains2 In the initial state we want all these nodes to be compatible15 / 60Contact interface: Setting continuity/real contact modeABRemarks1 The ⊠ Continuity for all inactive periods check-box set to ON will enforceautomatic generation of contact elements with full continuity attribute in allinactive periods of true contact behavior2 The ⊠ Automatic generation of continuity prior to first contact apparitionoption enforces automatic generation of contact elements with full continuityattribute only in the first inactive period for true contact behavior16 / 60

z’Contact interface: Flow through...Fully permeable contact with compatible pressures on both facesz’ x’x’Permeable contact k ′ x = k x hk ′ z = k z /h (h is a thin layer thickness)17 / 60Contact interface: Effective vs total stressFor permeable interfaces effective stress mode is enforcedFor impermeable interfaces effective/total stress mode can beselected NB. Effective stress mode makes sense only when at leastto one side of the interface a permeable continuum is adjacent18 / 60

Contact interface: General remarkssingular pointInterface elements are treated as any other elementsIf we do not deactivate the interface during excavation theprogram will do it automaticallyIf we do not assume an unloading function (0 index) then theinterface will inherit it from the excavated adjacent continuum19 / 60Contact interface: Setting existence/unloading functionssingular pointInterface elements are treated as any other elementsIf we do not deactivate the interface during excavation theprogram will do it automaticallyIf we do not assume an unloading function (0 index) then theinterface will inherit it from the excavated adjacent continuum20 / 60

Contact interface: Augmented Lagrangian approachAccessible from menu: Control/Contact algorithmThis algorithm in nonlinear applications must be used withcareExcessive overpenetration leads to underestimation ofinternal forces in contacting bodies25 / 60Modeling elastic structures with aid of continuumelementsStandard continuum finite elements representing structureslike beams/shells yield very poor results unless very fine meshis usedTo remedy the problem a family of robust continuum elementswas implemented to enhance bending/shear behaviorThese elements are generated exactly in the same manner asstandard continuum but at the material level Continuum forstructures instead of Continuum must be selectedElastic model is the only one allowed by Continuum forstructures formulationMinimum 2 elements per thickness must be generated torecover properly bending moment26 / 60

Modeling elastic structures with aid of continuum ele...Example of cantilever beam (recovering of sectional forces)q=1 kN/m24mMz=7.33 kNm/mStandard Q4 elementsMz=7.95 kNm/mEnhanced elementsNB. New stress recovery technique is used for postprocessinghence only results from the central point are stored27 / 60Prestressed anchors: General remarksanchor fixedzoneanchorAnchor consists of two parts: active and fixed partStiffness of both parts is assumed to be the sameThe active part joins the anchor head and fixed partAdhesive interface can be generated between soil and fixedpart28 / 60

Prestressed anchors: prestressingPrestress markerLink markerThe anchor endpoint may be attached to the backgroundcontinuum at any pointPrestress can be controled via existence function and loadtime function29 / 60Prestressed anchors: fixed zoneanchorAnchorfixed zoneThe fixed part may can be created but exlusively in thedirection indicated by the local X axis of the truss elementThe split value should be compatible with the meshdensity of the background continuumGeneration of fixed anchor zone interface is optional30 / 60

Prestressed anchors: fixed zone interfacedSame option option applies to nails31 / 60Diapraghm walls: Modeling issues2D/3D model?Serviceability limit state (SLS) is the main concern → smallstrain stiffness must be consideredEach construction step must be reproduced (if SLS is majorconcern and to avoid numerical problems witch convergence)Contact interfaces must be presentDiapraghm wall can be modeled using beam/shell elements orspecial continuum elements (Continuum for structures)(only elastic behavior is possible)Fixed anchor zones can be activated possibly with adhesiveinterfacePrestress in anchors can be controlled in time32 / 60

Diapraghm walls: Constitutive aspectsG - current secant shear modulusG o - shear modulus for very small strainsAtkinson 1991Here we are in the range of small strains in major part of thecomputational domain35 / 60Diapraghm walls: example of excavation in Berlin sand(after Schweiger...)36 / 60

Diapraghm walls: excavation in Berlin, sand FE model(after Schweiger...)37 / 60Diapraghm walls: excavation in Berlin, results-600 -400 -200 0 200 4000-5-0.04 -0.03 -0.02 -0.01 00-5Y [m[]-10-15-20-25HSHS-smallMCY [m]-10-15-20-25HS-smallHSMCMeasurement-30-30M [kNm/m]-35Ux [m]-35Y [m]0 0.01 0.02 0.03 0.040-10-20-30-40-50-60-70-80-90-100Uy [m]HSHS-smallMCUY [m]0 20 40 60 80 100 120 1400.010.0050-0.005-0.01-0.015-0.02X [m]HSHS-smallMC38 / 60

Nailing: general remarks120 ft 120 ft40ftExcavated layers1234567815 oL=30 ft40ftContrary to some simple limit equilibrium methods finiteelement model requires a multi-step excavation and nailinstallation procedure to eliminate spurious forces in nails andpotential numerical divergence problemsNails can be attached to the facing wall at any point notnecesarilly at the node (important mainly in 3D)Nail core is modeled as beam element39 / 60Nailing: general remarksdNail core=beamNail injection zoneNail interfaceNail consists of two material zones: core + interfaceStiffness of the injection zone is neglectedAdhesive interface can be generated between soil and injectionzone40 / 60

Nailing: generating nails43 / 60Nailing: generating nailsNail interface is optional (if not created then full displacementcompatibility is enforced)Mesh density for the nail (defined as split parameter) shouldcorrespond to the one in the background continuumThe material data for the interface is the same as for fixedanchor zones (see next slide)During stability analysis both soil and soil-nail interfacestrength parameters are reduced (unless it is redefined at thematerial level)44 / 60

Foundation rafts: discretization problemIf we have lot of piles it is almost impossible to1 Create 3D compatible FE mesh for plate-piles-interfaces system2 Compute the problem on a PC platform3 Each redesign of piles generates new complex 3D meshConclusion: we need absolutely a simplified treatment47 / 60Foundation raft: real FE vs simplified FE modelplate-pile connectionShell Q4beam elements48 / 60

Foundation rafts: Z Soil implementationPiles are modeled with aid of beam elementsBeams are embedded in continuum without any restrictionput on FE meshesBeam nodes can be connected to other elements likeshells/beams/membranes/continuum not necessarily atelement verticesBeam nodes can be connected to other elements via selectedset of degrees of freedomThe sliding interface between beam and continuum is createdautomaticallyThe additional interface between bottom of the pile andsubsoil can be optionally addedNodal forces can be applied at any point on the raftPenalty approach is not accepted (except for the frictionalcontact)49 / 60Nodal link option43A1 2Constraint equation(s): u A = ∑ 4i=1 N i u iHence: stiffness, force vector from node A of a beam elementis dispatched on shell degree of freedomDOF’s of node A are dependent on other DOF’sAttention: constraints cannot be nested (!)50 / 60

Nodal link: example of beam-beam connectionLink node to theselected elementDeformation51 / 60Nodal link: example of beam-shell connection52 / 60

Foundation rafts: pile frictional interfaceτ=σ n tan φ+cf t =0, f c < f cultHow to estimate σ n ?NB. We can leave φ = 0 and make contact purely adhesive like incodes for pile design53 / 60Foundation rafts: σ n estimation in pile interfacesxLR = SQRT (A/π)ΔL iP iRzLyL∫Lσ n =min (σ ni, 0)dl∫L dlσ ni is computed by effective stress transformation from thecontinuum elements in which interface and beam is embedded54 / 60

Foundation rafts: generating piles55 / 60Foundation rafts: generating piles1 Split parameter controls mesh density in pile macro-element;it is recommended to avoid too big differences in meshdensities between continuum and piles; such modeling maylead to axial force oscillations in the pile for high strengthparameters of subsoil2 To avoid instabilities due to rigid rotation of the pile along itsaxis the rotation along local pile axis is fixed internally by thepreprocessor3 Mesh refinement near the zone of the pile foot isrecommended to avoid underestimation of settlements56 / 60

Foundation rafts: real life example57 / 60Building-subsoil interaction: ExamplesExample 1At the material level in groupMainFrame structure(static/pushover/dynamic analysisNext slideRemark: Each member is discretized by one element (notnecessarily for reinforced concrete because of differentamount and position of the reinforcement)58 / 60

Building-subsoil interaction: ExamplesExample 2Frame structure resting on subsoilAt the material level in groupMainNext slideRemark: Each member is discretized by one element (notnecessarily for reinforced concrete because of differentamount and position of the reinforcement)59 / 60Flexibility vs displacement based beam formulationq=1.0 kN/m1.33Mz for 1 „flexibility based” beam0.6671.331.25Mz for 4 „displ. based” beam0.751.25Gauss pointsNodal pointsNote that result for Flexibility based beam (one per member)is exact !60 / 60