TUTORIAL MANUALITABLE OF CONTENTS1 Introduction.........................................................................................................1 - 12 Getting started....................................................................................................2 - 12.1Installation......................................................................................................2 - 12.2General modelling aspects...............................................................................2 - 12.3Input procedures............................................................................................2 - 32.3.1Input of geometry objects.............................................................2 - 32.3.2Input of text and values.................................................................2 - 32.3.3Input of selections.........................................................................2 - 42.3.4Structured input ............................................................................2 - 52.4Starting the program.......................................................................................2 - 62.4.1General settings ............................................................................2 - 62.4.2Creating a geometry model...........................................................2 - 83 Settlement of circular footing on sand(Lesson 1) ...........................................3 - 13.1Geometry.......................................................................................................3 - 13.2Rigid footing...................................................................................................3 - 23.2.1Creating the input .........................................................................3 - 23.2.2Performing calculations ................................................................3 -143.2.3Viewing output results..................................................................3 -183.3Flexible footing..............................................................................................3 -214 Submerged construction of an excavation(Lesson 2) .....................................4 - 14.1Geometry.......................................................................................................4 - 24.2Calculations...................................................................................................4 -114.3Viewing output results....................................................................................4 -145 Undrained river embankment(Lesson 3).........................................................5 - 15.1Geometry model.............................................................................................5 - 15.2Calculations....................................................................................................5 - 45.3Output ...........................................................................................................5 - 96 Dry excavation using a tie back wall(Lesson 4) .............................................6 - 16.1Input ..............................................................................................................6 - 16.2Calculations....................................................................................................6 - 56.3Output ...........................................................................................................6 - 97 Construction of a road embankment(Lesson 5)..............................................7 - 17.1Input ..............................................................................................................7 - 17.2Calculations....................................................................................................7 - 47.3Output ...........................................................................................................7 - 57.4Safety analysis............................................................................................... 7 7Back to Main IndexPLAXISII8 Construction of a shield tunnel(Lesson 4) .......................................................8 - 18.1Geometry.......................................................................................................8 - 28.2Calculations....................................................................................................8 - 68.3Output ...........................................................................................................8 - 7A Appendix A - Menu tree ...................................................................................A - 1A.1Input menu ...................................................................................................A - 1A.2Calculations menu.........................................................................................A - 2A.3Output menu.................................................................................................A - 3A.4Curves menu ................................................................................................A - 4B Appendix B - Calculation scheme for initial stressesdue to soil weight............................................................................................. B - 1TUTORIAL MANUAL5-15UNDRAINED RIVER EMBANKMENT (LESSON 3)River embankments may be subjected to varying water levels. The change in water level and theresulting change in the pore pressure distribution influences the stability of the embankment. PLAXISmay be used to analyse the influence of pore pressure changes on the deformation and stability ofgeotechnical structures. This feature is used here to study the behaviour of the river embankmentduringtheincreaseofthewaterlevel,asshowninFig.5.1.Aspecialproblemrelatedtosuchasituation is the possible uplift of the lowlands behind the embankment. This is due to the fact that thelightsoftsoillayerscannotsustainthehighporepressuresthatariseinthepermeablesandlayerbelow. This effect may reduce the stability of the embankment.The embankment in Fig. 5.1 is 5 m high and consists of relative impervious clay. The upper 6 m of thesubsoil consists of soft soil layers, of which the top 3 m is modelled as a clay layer and the lower 3m as a peat layer. The soft soil layers are nearly impermeable, so a short term variation in the riverwater level hardly influences the pore pressure distribution in these layers. Below the soft soil layersthere is a deep permeable sand layer, of which the upper 4 m are included in the finite element model.It is assumed that the water in the sand layer is in contact with the river, which means that the hydraulichead in the sand layer follows the river water level variation closely.Figure 5.1 Geometry of the river embankment subjected to a changing water level5.1GEOMETRY MODELThe geometry of Fig. 5.1 is modelled with a plane strain geometry model. The finite element mesh isbased on the 6-node elements. The units used in this example are meters for length, kiloNewton forforce and day for time. The dimensions of the geometry are 65 m in horizontal direction and 15 m invertical direction. The full geometry can be created using the Geometry line option. The Standardfixities option is used to define the boundary conditions. The suggested geometry model is shown inFig.5.2.PLAXIS5-2Figure 5.2Geometry model of the river embankment projectMaterial setsThree material layers are adopted for the soil. The properties are given in Table 5.1.Table 5.1.Material properties of the river embankment and subsoilParameter Name Clay Peat Sand UnitMaterial modelType of behaviourDry soil weightWet soil weightHorizontal permeabilityVertical permeabilityYoung's modulusPoisson's ratioCohesionFriction angleDilatancy angleModelTypedrywetkxkyErefcrefMCUndr.16180.0010.00120000.352.0240.0MCUndr.811.50.010.0015000.355.0200.0MCDrained17201.01.0200000.31.0300.0--kN/m3kN/m3m/daym/daykN/m2-kN/m2TUTORIAL MANUAL5-3Open the material data base and create three data sets named 'Clay', 'Peat' and 'Sand' with the modelparameters as listed above. The interface properties are not relevant in this example. Note that theMaterial type of the clay and peat layers is Undrained, whereas the sand layer is Drained. Drag thedata sets to the respective layers in the geometry model (see Fig. 5.1).Mesh generationThis example, in which an uplift situation is modelled, is sensitive to the degree of refinement of themesh. Therefore theGlobal coarseness is set toMedium in theMeshmenu.Inaddition,largerdisplacement gradients may be expected at the right hand embankment toe. In order to model thatpart of the geometry more accurately, select the geometry point of the embankment toe and selectRefine around point from the Mesh menu. As a result, the element size around the embankment toeis modified to half the average element size. The generated mesh is shown in Fig. 5.3.Figure 5.3Finite element mesh of river embankment projectI nitial conditionsThe geometry contains a non-horizontal soil surface. Therefore the K0-procedure cannot be usedto calculate the initial stress field. Instead the initial stresses must be calculated by means of 'Gravityloading'. This is a calculation option which will be explained in section 5.2. The activation of waterpressures is always done together with the soil weight, but the generation of water pressures may bedone in advance. In order to generate the proper initial water pressures, follow these steps:Click on the button.Accept the default value of the water weight (10 kN/m3).Enter a general phreatic line from point (0.0; 10.0) to point (65.0; 10.0).Generate the pore pressures from the phreatic line by clicking on theGeneratewaterpressures button and subsequently clicking the button.In the Output window, check the pore pressure distribution and click on the button.PLAXIS5-4Back in the Input window, click directly on the button. Do not generate theinitial stresses according to the K0-procedure.Save the input under an appropriate name.5.2CALCULATIONSThe calculation consists of two phases. First the initial stress field has to be calculated since this hasnot been done during the input of the initial conditions. The calculation of the initial stresses can bedone in a plastic calculation where the multiplier for the soil weight is increased from 0.0 to 1.0. Acalculation of this sort is calledGravity loading.Thisprocedureisrecommendedwhenthesoilsurface,thelayeringorthephreaticlineisnon-horizontal.Gravityloadingalwaysresultsinanequilibrium stress state, whereas the K0-procedure does not in the case of a non-horizontally layeredsubsoil. During Gravity loading both the soil weight and the pore pressures (that were generatedpreviously) are activated.The second calculation phase is the increase of the river water level, and the pore pressure, in the sandlayer. This is done in the Staged construction mode. In order to define the two calculation phasescorrectly, follow this procedure:For the first calculation phase, accept all default values of the General tabsheet and advanceto the next tabsheet.IntheParameterstabsheet,selectIgnoreundrainedbehaviourintheControlparameters box. SelectTotal multipliers in theLoadinginputboxandclickonthe button.In the Multipliers tabsheet, enter a value of 1.0 for Mweight (the multiplier for the soil weight).Hints: Sincetheinitialstressesarenotsubjecttoundrainedbehaviour,itisimportantthatundrained behaviour is disabled during gravity loading. This can be done by selectingIgnore undrained behaviour in the Parameters tabsheet of the Calculations window.> In contrast to the K0-procedure, the calculation of initial stresses by means of gravityloadingresultsindisplacements.Thesedisplacementsarenotrealistic,becausetheembankment is modelled as it appears in reality and the calculation of the initial stressesshould not influence the displacements computed later in the analysis. These unrealisticdisplacements can be reset to zero at the start of the next calculation phase by selectingReset displacements to zero in the next phase.TUTORIAL MANUAL5-5Click on the button to create the next calculation phase. In this phase the 'Ultimate'situation as indicated in Fig. 5.1 will be defined.In the General tabsheet, accept all default values. The default setting is such that the currentphase starts from the results obtained from the previous phase.IntheParameterstabsheet,selectResetdisplacementstozerointheControlparameters box. This will eliminate the non-physical displacements resulting from the firstcalculation phase. This operation, however, does not affect the stresses.Select Staged construction in the Loading input box and click on the button.In the Geometry configuration window, click on the left button of the 'Switch' to arrivein the water pressure mode.Enter a general phreatic line through the points (0.0; 15.0), (20.0; 15.0), (45.0; 10.0). Thisgeneral phreatic line is only meant to generate the external water pressures on the left sideof the embankment (see also Fig 5.4). Individual water conditions will now be assigned tothe different layers.Figure 5.4General phreatic line for generation of external water pressuresPLAXIS5-6ClickontheSelectionbuttonandselecttheclusteroftheclaylayer(includingtheembankment).While the clay layer cluster is marked, click on the Phreatic line button and draw a phreaticlinethroughthepoints(0.0;10.0),(65.0;10.0).This'Userdefined'phreaticlineonlyapplies to the indicated cluster. For reasons of clarity the screen shot has been Fig 5.5 isedited. Fig 5.5 only shows the phreatic line for the clay layer. The general phreatic line isnot indicated.Figure 5.5Phreatic line for clay layerClick on the Selection button and select the cluster of the sand layer.While the sand layer is marked, click on the Phreatic line button and draw a phreatic linethrough the points (0.0; 15.0), (65.0; 13.0). This phreatic line only applies to the sand layercluster. For reasons of clarity the screen shot of Fig 5.6 has been edited. Fig 5.6 only showsthe phreatic line for the sand layer, other phreatic lines are not indicated. TUTORIAL MANUAL5-7 Figure 5.6Phreatic line for sand layerClick on the Selection button and double-click, or click using the right mouse button, theintermediate peat layer. As a result, a Pore pressures window appears. In the Cluster box,there are three radio buttons. By default the General phreatic line is selected. The otheroptions are User defined phreatic line and Interpolate from adjacent clusters or lines.The former option is automatically selected if a 'User defined' phreatic line is entered, asdescribedabove.Forthecurrentcluster(thepeatlayer)youshouldselecttheoptionInterpolate from adjacent clusters or lines (see also Fig 5.7). This will result in a lineardistribution from the pressure at the bottom of the upper clay layer to the pressure at the topof the sand layer. Click the button to close the window.Hint: The phreatic line corresponding to a particular cluster is indicated in red as soon as theclusterisselected.Clickingoutsidethegeometryresultsinanindicationofthegeneralphreaticline.InaclusterwheretheInterpolate...optionapplies,nophreaticlineisindicated.PLAXIS5-8Figure 5.7Definition of pore pressures for peat layerClick on the Generate water pressures button to generate the water pressures accordingto the ultimate situation.The pore pressure distribution is presented as principal stresses (by means of crosses) inthe Output window. Click on the Cross section button and draw a vertical line through thetopoftheembankmenttothebottomofthegeometry.Asaresult,theporepressuredistributionoverallthreelayersisdisplayedinaseparatewindow.Inadditiontothehydrostatic part of the pore pressure distribution in the clay and sand layers, the plot showsthe linear increase in pore pressure through the peat layer.Click on the button to return to the geometry configuration.In the geometry configuration, click on the window to return to the Calculationswindow.Click on the Select points for curves button. In the Output window, select suitable pointsfor load-displacement curves (for example the toe and crest points of the embankment) andclick on the button.In the Calculations window, click on the button to start the calculations.TUTORIAL MANUAL5-95.3OUTPUTAfter the calculation has finished, click on the button to view the results of the secondcalculation phase. The Output program will now display the deformations of the embankment due tothechangeofthewaterlevel.Theplotclearlyshowstheupliftofthesoftsoillayersbehindtheembankment and the movement of the embankment itself. This becomes even clearer if you selectTotal increments from the Deformations menu (see Fig. 5.8).Figure 5.8Displacement increments due to the change in water levelOn selecting Effective stresses from the Stresses menu, it can be seen that at the top of the sand layerat the right hand side of the model the effective stresses are nearly zero (see Fig. 5.9). This is due tothe increase in pore pressures in the sand layer. From the stress plot it can also be seen that themovement of the embankment causes a passive stress state in the clay layer behind the embankment.Figure 5.9Effective stresses in embankment after the increase of the water levelThe undrained behaviour in the clay and peat layers causes excess pore pressures to develop. Theexcess pore pressures can be viewed by selecting Excess pore pressures from theStresses menu(see Fig. 5.10).Figure 5.10Excess pore pressures after the increase of the water levelPLAXIS5-10