Iss26 Art2 - Local Ice Chrushing Analysis of Pile Wall.pdf - Plaxis

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Geometry and geotechnical InformationThe proposed OPEN CELL SHEET PILE retainingwall extends from an elevation of +12 feet abovethe mean sea level to elevation of natural seabed-40 feet. The assumed geotechnical profileconsists of an assumed sandy gravel fill aboveelevation -40 feet. A layer of very stiff interbeddedand sometimes intermixed clayey silt and finesilty sand was utilized for depths between -40feet to -45 feet. Below the silt, sands with varyingamounts of silt and fine gravel were assumedfrom elevations -45 to -50 feet. The sand layer wasthen underlain by well graded sandy gravel. Thefill material was first assumed to be placed in asomewhat loose condition. However, subsequentanalyses indicated that compaction of the soilbehind the sheet piles would be necessary for wallstability and additional ice resistance. Figure 2depicts the assumed geometry and soil profile.Soil parameters utilized in the analyses are listedin Table 1.Three-dimensional analysesPLAXIS 3D finite element modelingA soil model section with a dimension of 310feet x 95 feet x107 feet (LxBxH) was developedin 3DFoundation. Horizontal displacementswere fi xed at the boundaries along the soil blockperimeter. The length of 107 feet was chosen toenable modeling three cells of the OPEN CELLSHEET PILE structure to appropriately evaluatethe local ice crushing resistance of the system byloading the center cell. The face sheet piles ofthese structures are typically constructed withnineteen 19.69-inch-wide flat sheet piles arrangedin the curvilinear pattern, which results in cellwidths of approximately 30 feet width. For thisanalysis, the face sheet piles were connectedstructurally to tail wall sheets that extended about45 feet into compacted granular fill. A plan layout ofOPEN CELL SHEET PILE wall is shown in Figure 3.Varied mesh size were applied to soil blocksand OPEN CELL SHEET PILE structure to savecomputation time. A very fine mesh was assignedto the middle cell and the enclosed granular fillright behind it where the ice force was directlyapplied. The rest of the soil blocks, as well as theadjacent two cell structures, share a medium,fine mesh. The strategic mesh refinement led toa mesh of approximately 12,700 elements andapproximately 63,000 nodes. The sensitivitystudies indicate that deformation discrepancyof sheet pile wall system with very fine mesh andmedium mesh is within an acceptable range of5%. A fully meshed PLAXIS 3D model is presentedin Figure 4. The elastic-plastic Mohr-Coulombconstitutive soil model for the soil layers in thedrained conditions was applied.The most complex portion of modeling of OPENCELL SHEET PILE wall for this project was toFigure 2. OPEN CELL SHEET PILE Wall Geometry andSoil ProfileParameterDry UnitWeight(kcf)Elasticmodulus (ksf)Poisson’sratioCohesion(ksf)FrictionAngle (deg)Sandy Gravel Fill 0.12 3100 0.3 0.02 34Clayey Silt 0.1 544 0.35 0.8 25Silty Sand 0.12 450 0.33 0.05 33Deep Sandy Gravel 0.12 4000 0.3 0.02 34Table 1. Soil Properties Used in PLAXIS 3DFigure 3. Plan Layout of OPEN CELL SHEET PILE Wallwww.plaxis.nl l Autumn issue 2009 l Plaxis bulletin 13
Plaxis practice: Practice: Mohr-Coulomb Local ice crushing parameters analyses of for OPEN modelling CELL of SHEET concrete PILE® structures Wall by 3DFoundationappropriately model the steel wall, especiallythe contiguous sheet piles vertically attachedvia interlocks. During an ice event, an interlockconnection will first freely rotate around thevertical axis when an inward out-of-plane load isapplied. Under an increasing load, the interlockswill first straighten, followed by continued inwarddeflection. Eventually, the pile interlocks willbind and begin to resist load until, ultimately, theinterlock pries apart.The inappropriate application of isotropicsteel properties could result in an extremelyunconservative behavior whereby, in the mostextreme condition of a solid steel plate, mostof the ice loads could be carried by the OPENCELL ® structure rather than the enclosed soilbody. Consequently, a series of fully decouplednonlinear force strain (i.e. axial force-axial strain)or moment-curvature (i.e. bending momentrotationcurvature) relations were developed.A bilinear force-strain relation (large in-planehorizontal tension vs. small in-plane horizontalcompression) was also applied to appropriatelyestimate the axial hoop tension in steel wall. Thebending moment-rotation curvature to define ahinge property caused by interlock rotation wasderived and applied to the model based on thedestructive sheet pile testing performed by theauthor. Full sheet pile section properties havebeen assigned to the sides of model (boundary).Considering the large dimension betweenadjacent OPEN CELL ® structures and the localice pressure that is only applied to the center ofmiddle cell, the effects of the boundary cell (sidesof the model) is neglected.Loading conditionsThe localized ice crushing force was set up asstatic ice pressure in Plaxis following the localpressure-area curves identified in Table 2. Fromelevation +2 feet to -10 feet, ISO 19906 criteriawere utilized considering this area could beexposed to confined ice with large compressivepressure. Above elevation +2, the ice wouldtypically be rubbled and ice pressure is thereforegreatly reduced. Similarly, ice located belowelevation -10 would generally be weaker andwould not experience the confinement necessaryfor large local loads. Therefore, lower compressivepressures were utilized for the upper and lower icecrushing zones.For modeling purposes, the ice load was appliedat the highest elevation within each boundaryconsidering that this location would have theminimum soil mass behind the sheet pile to resistthe crushing force.Failure criteriaDevelopment of failure criteria for the analyzedstructure presented a significant challenge to theproject. Considering the extreme nature of theice loads, plastic soil movement behind the sheetpiles was acceptable, as long as the sheet pilesdid not come apart. The bilinear behavior of theinterlock hinges (as well as modeling limitations)also made it difficult to utilize structural stresseswithin the steel as an evaluation method.Ultimately, it was determined that the bestmeasure of structure limit state was to examinethe amount of rotation at the interlocks. The anglebetween adjacent sheet piles right before failurewas defined as ultimate deflection angle. Thisdeflection angle was derived from destructivesheet pile testing which indicated a 20 degreerelative angle (between adjacent sheet piles) couldbe utilized as the failure criteria when evaluatingthe Plaxis results. A depiction of this criteria andthe associated angle is presented in Figure 5.It should be noted that there is likely much morereserve capacity in the sheet piles than that whichwas utilized in the modeling effort. The ultimatetesting proved to be difficult in that plasticbending of the sheet piles generally occurredsooner than ultimate failure of the knuckles. Suchresults indicate that the sheet pile wall would likelybend and form plastic hinges prior to knucklefailure, which would allow for much greaterdeflections and load absorption/distribution thanthat utilized in this analysis.Figure 5. Sheet Pile Failure Criteria DiagramFigure 4. Meshed 3D PLAXIS Model14 Plaxis bulletin l Autumn issue 2009 l www.plaxis.nl
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Iss26 Art2 - Local Ice Chrushing Analysis of Pile Wall.pdf - Plaxis

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