4 FOUNDATION IN OVERCONSOLIDATED CLAY - Plaxis · PLAXIS 3D - FOUNDATION IN OVERCONSOLIDATED CLAY 4 FOUNDATION IN OVERCONSOLIDATED CLAY In this chapter a ﬁrst application of PLAXIS

PLAXIS 3D - FOUNDATION IN OVERCONSOLIDATED CLAY

4 FOUNDATION IN OVERCONSOLIDATED CLAY

In this chapter a first application of PLAXIS 3D is considered, namely thesettlement of a foundation in clay. This is the first step in becoming familiarwith the practical use of the PLAXIS 3D.

The general procedures for the creation of a geometry, the generation of afinite element mesh, the execution of a finite element calculation and theevaluation of the output results are described here in detail. The informationprovided in this tutorial will be utilised in the following tutorials. Therefore, it isimportant to complete this first tutorial before attempting any further tutorialexamples.

18.0 m

75.0 m

75.0 mBuilding

Clay

x

x

y

z

z = 0z = -2

z = -40

40.0 m

Figure 4.1 Geometry of a square building on a raft foundation

PLAXIS Introductory 2014 | Tutorial Manual 65

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GEOMETRY

This exercise deals with the construction and loading of a foundation of asquare building in a lightly overconsolidated lacustrine clay. Below the claylayer there is a stiff rock layer that forms a natural boundary for theconsidered geometry. The rock layer is not included in the geometry; insteadan appropriate boundary condition is applied at the bottom of the clay layer.The purpose of the exercise is to find the settlement of the foundation.

The building consists of a basement level and 5 floors above the ground level(Figure 4.1). To reduce calculation time, only one-quarter of the building ismodelled, using symmetry boundary conditions along the lines of symmetry.To enable any possible mechanism in the clay and to avoid any influence ofthe outer boundary, the model is extended in both horizontal directions to atotal width of 75 m.

The model is considered in three different cases:

Case A: The building is considered very stiff and rough. The basement issimulated by means of a stiff plate.

Case B: The structural forces are modelled as loads on a raft foundation.

Case C: Embedded piles are included in the model to reduce settlements.

Hint: In the Professional version, the building can be simulated bylinear elastic volume elements.

4.1 CASE A: RIGID FOUNDATION

In this case, the building is considered to be very stiff. The basement issimulated by very stiff plate elements. The total weight of the basementcorresponds to the total permanent and variable load of the building. Thisapproach leads to a very simple model and is therefore used as a firstexercise, but it has some disadvantages. For example it does not give anyinformation about the structural forces in the foundation.

66 Tutorial Manual | PLAXIS Introductory 2014


Objectives:

• Starting a new project

• Creation of soil stratigraphy using a single borehole

• Creation of material data sets

• Assigning material

• Modelling of plates and defining material data set for plates

• Generation of mesh

• Generating initial stresses using the K0 procedure

• Defining a Plastic calculation

• Activation and deactivation of structural elements

4.1.1 GEOMETRY INPUT

• Start the PLAXIS 3D program. The Quick select dialog box will appearin which you can select an existing project or create a new one (Figure4.2).

Figure 4.2 Quick select dialog box

• Click Start a new project. The Project properties window appears,consisting of Project and Model tabsheets.


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Project properties

The first step in every analysis is to set the basic parameters of the finiteelement model. This is done in the Project properties window. Theseproperties include the description of the problem, the basic units and the sizeof the draw area.

To enter the appropriate properties for the foundation calculation follow thesesteps:

• In the Project tabsheet, enter "Tutorial 1" as the Title of the project andtype "Settlements of a foundation" in the Comments box (Figure 4.3).

Figure 4.3 Project tabsheet of the Project properties window

• Proceed to the Model tabsheet by clicking either the Next button or theModel tab (Figure 4.4).

• Keep the default units in the Units box (Length = m; Force = kN; Time =day ).

• The General box indicates a fixed gravity of 1.0 G, in the verticaldirection downward (-z). The value of the acceleration of gravity (1.0 G)can be specified in the Earth gravity box. This should be kept to thedefault value of 9.810 m/s2 for this exercise. In the γwater box the unitweight of water can be defined. Keep this to the default value of 10



kN/m3.

• Define the limits for the soil contour as xmin = 0, xmax = 75, ymin = 0 andymax = 75 in the Contour group box.

Figure 4.4 Model tabsheet of the Project properties window

• Click the OK button to confirm the settings.

Hint: In case of a mistake or for any other reason that the projectproperties need to be changed, you can access the Projectproperties window by selecting the corresponding option in theFile menu.

Definition of soil stratigraphy

When you click the OK button the Project properties window will close andthe Soil mode view will be shown. Information on the soil layers is entered inboreholes.

Boreholes are locations in the draw area at which the information on theposition of soil layers and the water table is given. If multiple boreholes are


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defined, PLAXIS 3D will automatically interpolate between the boreholes, andderive the position of the soil layers from the borehole information.

Hint: PLAXIS 3D can also deal with layers that are discontinuous, i.e.only locally present in the model area. See the ReferenceManual for more information.

In the current example, only one soil layer is present, and only a singleborehole is needed to define the soil stratigraphy. In order to define theborehole, follow these steps:

Click the Create borehole button in the side toolbar to start defining thesoil stratigraphy. Click on position (0 0 0) in the geometry. A borehole willbe located at (x , y ) = (0 0). The Modify soil layers window will appear.

• In the Modify soil layers window add a soil layer by clicking on the Addbutton. Keep the top boundary of the soil layer at z = 0 and set thebottom boundary to z = −40 m.

• Set the Head value in the borehole column to −2 m (Figure 4.5). Thecreation of material data sets and their assignment to soil layers isdescribed in the following section.

4.1.2 MATERIAL DATA SETS

In order to simulate the behaviour of the soil, a suitable material model andappropriate material parameters must be assigned to the geometry. InPLAXIS soil properties are collected in material data sets and the variousdata sets are stored in a material database. From the database, a data setcan be assigned to one or more clusters. For structures (like beams, plates,etc.) the system is similar, but different types of structures have differentparameters and therefore different types of data sets.

PLAXIS 3D distinguishes between material data sets for Soils and interfaces,Plates, Geogrids, Beams, Embedded Piles and Anchors. Before a calculationin Staged construction can be made, material data sets have to be assignedto all soil volumes and structures.



Figure 4.5 Modify soil layers window

Open the Material sets window by clicking the Materials button.

• Click the New button on the lower side of the Material sets window. TheSoil window will appear. It contains five tabsheets: General, Parameters,Flow parameters, Interfaces and Initial.

• In the Material set box of the General tabsheet (Figure 4.6), write"Lacustrine Clay" in the Identification box.

• Select Mohr-Coulomb as the material model from the Material modeldrop-down menu and Drained from the Drainage type drop-down menu.

• Enter the unit weights in the General properties box according to thematerial data as listed in Table 4.1. Keep the unmentioned Advancedparameters as their default values.

• Click the Next button or click the Parameters tab to proceed with the


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Figure 4.6 General tabsheet of the Soil and interfaces data set window

Table 4.1 Material properties

Parameter Name Lacustrine clay Unit

General

Material model Model Mohr-Coulomb −Drainage type Type Drained −Unit weight above phreatic level γunsat 17.0 kN/m3

Unit weight below phreatic level γsat 18.0 kN/m3

Parameters

Young's modulus (constant) E ' 1 · 104 kN/m2

Poisson's ratio ν ' 0.3 −Cohesion (constant) c'ref 10 kN/m2

Friction angle ϕ' 30.0 ◦

Dilatancy angle ψ 0.0 ◦

Initial

K0 determination − Automatic −Lateral earth pressure coefficient K0 0.5000 −



input of model parameters. The parameters appearing on theParameters tabsheet depend on the selected material model (in thiscase the Mohr-Coulomb model). The Mohr-Coulomb model involvesonly five basic parameters (E ', ν ', c',ϕ',ψ'). See the Material ModelsManual for a detailed description of the different soil models and theircorresponding parameters.

• Enter the model parameters E ', ν ', c'ref , ϕ' and ψ of Lacustrine clayaccording to Table 4.1 in the corresponding boxes of the Parameterstabsheet (Figure 4.7).

Figure 4.7 Parameters tabsheet of the Soil and interfaces data set window

• No consolidation will be considered in this exercise. As a result, thepermeability of the soil will not influence the results and the Flowparameters window can be skipped.

• Since the geometry model does not include interfaces, the Interfaces tabcan be skipped.


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• Click the Initial tab and check that the K0 determination is set toAutomatic. In that case K0 is determined from Jaky's formula: K0 =1− sinϕ.

• Click the OK button to confirm the input of the current material data set.The created data set appears in the tree view of the Material setswindow.

• Drag the set Lacustrine clay from the Material sets window (select it andhold down the left mouse button while moving) to the graph of the soilcolumn on the left hand side of the Modify soil layers window and drop itthere (release the left mouse button). Notice that the cursor changesshape to indicate whether or not it is possible to drop the data set.Correct assignment of the data set to the soil layer is indicated by achange in the colour of the layer.

• Click the OK button to close the Material sets window.

• Click the OK button to close the Modify soil layers window.

4.1.3 DEFINITION OF STRUCTURAL ELEMENTS

The structural elements are created in the Structures mode of the program.Click the Structures button to proceed with the input of structural elements.To model the building:

Click the Create structure button at the side tool bar.

Select the Create plate option from the additional tools displayed.

• Position the cursor at the coordinate (0 0 0). Check the cursor positiondisplayed in the cursor position indicator. As you click, the first surfacepoint of the surface is defined.

• Define three other points with coordinates (0 18 0), (18 18 0), (18 0 0)respectively. Press the right mouse button or <Esc> to finalize thedefinition of the surface. Note that the created surface is still selectedand displayed in red.

Open the Material sets window by clicking the Show materials buttonand set the Set type to Plates.

• Create a data set for the rigid foundation according to Table 4.2.



• Drag and drop the data sets to the plate indicating the rigid foundation.

• Close the Material sets window.

Table 4.2 Material properties of rigid foundation

Parameter Name Rigid foundation Unit

Thickness d 2 mWeight γ 50 kN/m3

Type of behaviour Type Linear, isotropic −Young's modulus E1 3 · 107 kN/m2

Poisson's ratio ν12 0.15 −

4.1.4 MESH GENERATION

The model is complete. In order to proceed to the Mesh mode click the Meshbutton. PLAXIS 3D allows for a fully automatic mesh generation procedure, inwhich the geometry is divided into volume elements and compatible structureelements, if applicable. The mesh generation takes full account of theposition of the geometry entities in the geometry model, so that the exactposition of layers, loads and structures is accounted for in the finite elementmesh. To generate the mesh, follow these steps:

Click the Generate mesh button in the side toolbar or select theGenerate mesh option in the Mesh menu. Change the Elementdistribution to Coarse in the Mesh options window (Figure 4.8) and clickOK to start the mesh generation.

Hint: By default, the Element distribution is set to Medium. TheElement distribution setting can be changed in the Mesh optionswindow. In addition, options are available to refine the meshglobally or locally.» The finite element mesh has to be regenerated if the geometry ismodified.» The automatically generated mesh may not be perfectly suitablefor the intended calculation. Therefore it is recommended thatthe user inspects the mesh and makes refinements if necessary.


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Figure 4.8 Mesh options window

As the mesh is generated, click the View mesh button. A new window isopened displaying the generated mesh (Figure 4.9).

Figure 4.9 Generated mesh in the Output window

Click on the Close tab to close the Output program and go back to theMesh mode of the Input program.



4.1.5 PERFORMING CALCULATIONS

Once the mesh has been generated, the finite element model is complete.Click Staged construction to proceed with the definition of calculation phases.

Initial conditions

The 'Initial phase' always involves the generation of initial conditions. Ingeneral, the initial conditions comprise the initial geometry configuration andthe initial stress state, i.e. effective stresses, pore pressures and stateparameters, if applicable. The initial water level has been entered already inthe Modify soil layers window. This level is taken into account to calculate theinitial effective stress state. It is therefore not needed to enter the Waterconditions mode.

When a new project has been defined, a first calculation phase named "Initialphase", is automatically created and selected in the Phases explorer (Figure4.10). All structural elements and loads that are present in the geometry areinitially automatically switched off; only the soil volumes are initially active.

Figure 4.10 Phases explorer

In PLAXIS 3D two methods are available to generate the initial stresses:Gravity loading and K0 procedure. The K0 procedure is the defaultcalculation type for the Initial phase.

Hint: The K0 procedure may only be used for horizontally layeredgeometries with a horizontal ground surface and, if applicable, ahorizontal phreatic level. See the Reference Manual for moreinformation on the K0 procedure.


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The Phases window (Figure 4.11) is displayed by clicking the Edit phasebutton or by double clicking on the phase in the Phases explorer.

Figure 4.11 The General tabsheet in the Phases window for Initial phase

• Click OK to close the Phases window.

• Make sure that the plate is inactive.

Construction stage

After the definition of the initial conditions, the construction of the building canbe modelled. This will be done in a separate calculation phase, which needsto be added as follows:

Click the Add button in the Phases explorer. A new phase, namedPhase_1 will be added in the Phases explorer.

• Double-click Phase_1 to open the Phases window.

• In the General tabsheet, write (optionally) an appropriate name for thenew phase in the ID box (for example "Building") and select the phasefrom which the current phase should start (in this case the calculationphase can only start from Initial phase, which contains the initial stressstate).

• The default settings are valid for this Plastic phase calculation (Figure4.12).

• The calculation parameters for phase Building have now been set. ClickOK to close the Phases window.



Figure 4.12 The Phases window for Building phase

• In the Model explorer click the square in front of the plate to activate theplates.

Hint: Calculation phases may be added, inserted or deleted using theAdd, Insert and Delete buttons in Phases explorer or in thePhases window.

Execution of calculation

All calculation phases (two phases in this case) are marked for calculation(indicated by a blue arrow). The execution order is controlled by the Startfrom phase parameter.

Click the Calculate button to start the calculation process. Ignore thewarning that no nodes and stress points have been selected for curves.

During the execution of a calculation, a window appears which givesinformation about the progress of the actual calculation phase (Figure 4.13).

The information, which is continuously updated, shows, amongst others, thecalculation progress, the current step number, the global error in the currentiteration and the number of plastic points in the current calculation step.


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Figure 4.13 Active task window displaying the calculation progress

It will take a few seconds to perform the calculation. When a calculation ends,the window is closed and focus is returned to the main window.

• The phase list in the Phases explorer is updated, showing green tickmarks to indicate that the calculations were finished successfully. Anunsuccessful calculation would be indicated with a red cross.

Before viewing results, save the project.



Viewing calculation results

Once the calculation has been completed, the results can be displayed in theOutput program. In the Output program, the displacement and stresses in thefull three dimensional model as well as in cross sections or structuralelements can be viewed. The computational results are also available intabular form. To view the current results, follow these steps:

• Select the last calculation phase (Building) in the Phases explorer tree.

Click the View calculation results button in the side toolbar to open theOutput program. The Output program will, by default, show the threedimensional deformed mesh at the end of the selected calculationphase. The deformations are scaled to ensure that they are clearlyvisible.

• Select Total Displacements→ |u| from the Deformations menu. The plotshows colour shadings of the total displacements (Figure 4.14).

Figure 4.14 Shadings of Total displacements at the end of the last phase

• A legend is presented with the displacement values at the colourboundaries. When the legend is not present, select the Legend optionfrom the View menu to display it.


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In the Output window click the Iso surfaces button to display the areashaving the same displacement.

Hint: In addition to the Total displacements, the Deformations menuallows for the presentation of Incremental displacements andPhase displacements.» The incremental displacements are the displacements thatoccurred in one calculation step (in this case the final step).Incremental displacements may be helpfull in visualising failuremechanisms.» Phase displacements are the displacements that occurred in onecalculation phase (in this case the last phase). Phasedisplacements can be used to inspect the impact of a singleconstruction phase, without the need to reset displacements tozero before starting the phase.



4.2 CASE B: RAFT FOUNDATION

In this case, the model is modified so that the basement consists of structuralelements. This allows for the calculation of structural forces in the foundation.

The raft foundation as well as the walls of the basement consist of 50 cmthick concrete. The loads of the upper floors are transferred to the floor slabby a column and by the basement walls. The column bears a point load of11650 kN and the walls carry a line load of 385 kN/m, as sketched in Figure4.15. The column itself, however, will not be modelled.

12.0 m12.0 m

6.0 m6.0 m

385 kN/m385 kN/m

11650 kN

5.3 kN/m2

Figure 4.15 Geometry of the basement

In addition, the floor slab is loaded by a distributed load of 5.3 kN/m2. Theproperties of the clay layer will be modified such that stiffness of the clay willincrease with depth.

Objectives:

• Saving project under a different name

• Modifying existing data sets

• Defining a soil stiffness that increases with depth

• Moving structures

• Modelling of beams and defining material data set for beams

• Assigning point loads


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• Assigning line loads

• Assigning distributed loads to surfaces

• Deleting phases

• Activation and deactivation of soil volumes

• Activation and deactivation of structural elements

• Activation of loads

• Zooming in Output

• Drawing cross sections in Output

• Viewing structural output

Geometry input

The geometry used in this exercise is the same as the previous one, exceptthat additional elements are used to model the foundation. It is not necessaryto create a new model; you can start from the previous model, store it under adifferent name and modify it. To perform this, follow these steps:

Start the PLAXIS 3D program. The Quick select dialog box will appearin which the project of case A should be selected.

• Select the Save project as option in the File menu to save the projectunder a different name (e.g. "Tutorial 1b").

The material set for the clay layer has already been defined. To modify thismaterial set to take into account the stiffness of the soil increasing with depth,follow these steps:

Open the Material sets window by clicking the Show materials button.

• Make sure that the option Soil and interfaces is selected as Set type.

• Select the Lacustrine clay material set and click the Edit button.

• In the Parameters tabsheet, change the stiffness of the soil E ' to 5000kN/m2.

• Enter a value of 500 in the E 'inc box in the Advanced parameters. Keepthe default value of 0.0 m for zref . Now the stiffness of the soil is defined



as 5000 kN/m2 at z = 0.0 m and increases with 500 kN/m2 per meterdepth.

• Click OK to close the Soil window.

• Click OK to close the Material sets window.

Definition of structural elements

Proceed to the Structures mode to model the building. First the plate has tobe moved 2 meters downwards:

Click the Selection button and select the plate at the top of the soil.

Click the Move button and while holding the <Shift> key pressed, movethe plate to z = −2.

Hint: By default, the cursor is located at z=0. To move in the verticaldirection, keep the <Shift> key pressed while moving the mouse.

Second step is the creation of the shape of the building:

Select the Create plate option from the tools displayed after clicking theCreate structure button.

• Position the cursor at the coordinate (0 18 0) and click to define the firstpoint of one of the basement walls.

• Move to position (18 18 0) and click to define the second point of thisbasement wall.

• While holding the <Shift> key, move the cursor to coordinate (18 18 -2)and click.

• Move to position (0 18 -2) and click. Press the right mouse button tofinalize the definition of this surface.

• In the same way, define the second basement wall as a plate with pointsat (18 18 -2), (18 0 -2), (18 0 0) and (18 18 0).


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The shape of the building has now been created. Next step will be thedefinition of the material data set for the basement walls and floor.

Figure 4.16 Location of plates in the project

Open the material data base and set the Set type to Plates.

• Select the Rigid foundation material and delete it by clicking the Deletebutton.

• Create a data set for the basement slabs according to Table 4.3.

• Drag and drop the data set to the basement floor and the basementwalls accordingly. It may be needed to move the Material sets window byclicking at its header and dragging it.


Table 4.3 Material properties of the basement slabs

Parameter Name Basement slabs Unit

Thickness d 0.5 mWeight γ 15 kN/m3

Type of behaviour Type Linear, isotropic −Young's modulus E1 3 · 107 kN/m2

Poisson's ratio ν12 0.15 −

Furthermore, the loads and beams have to be defined.

• Right-click the bottom of the surface of the building volume and selectthe Create surface load option from the appearing menu. The actualvalue of the load can be assigned in the Structures mode as well aswhen the calculation phases will be defined (Staged construction mode).



Hint: When specifying a unit weight, please consider the fact that theelement itself does not occupy any volume and overlaps with thesoil elements. Hence, it might be considered to subtract the unitsoil weight from the real unit weight of the plate, beam orembedded pile material in order to compensate for the overlap.For partially overlapping plates, beams or embedded piles thereduction of the unit weight should be proportional.

In this example, the value will be assigned in the Staged consturctionmodes.

Click the Create line button in the side toolbar.

Select the Create line load option from the additional tools displayed.

• Click the command input area, type "0 18 0 18 18 0 18 0 0 " and press<Enter>. Line loads will now be defined on the basement walls. Thedefined values are the coordinates of the three points of the lines. Clickthe right mouse button to stop drawing line loads.

Click the Create line button in the side toolbar.

Select the Create beam option from the additional tools displayed.

• Click on (0 6 -2) to create the first point of the first horizontal beam. Clickon (18 6 -2) to define the second point of the beam. To stop drawingclick the right mouse button.

• Create the second horizontal beam from (6 0 -2) to (6 18 -2).

Hint: By default, the cursor is located at z=0. To move in the verticaldirection, keep the <Shift> key pressed while moving the mouse.

Open the material data base and set the Set type to Beams.

• Create a data set for the horizontal beams according to Table 4.4.Assign the data set to the corresponding beam elements by drag and


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drop.

Table 4.4 Material properties of the basement beams

Parameter Name Basement beam Unit

Cross section area A 0.7 m2

Volumetric weight γ 6.0 kN/m3

Type of behaviour Type Linear −Young's modulus E 3 · 107 kN/m2

Moment of Inertia I3 0.058 m4

I2 0.029 m4

Click the Create load button in the side toolbar.

Select the Create point load option from the additional tools displayed.Click at (6 6 -2) to add a point load indicating the load from the column.

Proceed to the Mesh tabsheet to generate the mesh.

Mesh generation

Click the Generate mesh. Keep the Element distribution as Coarse.

Inspect the generated mesh.

• Click on the Close tab to close the Output program and go back to theMesh mode of the Input program.

As the geometry has changed, all calculation phases have to be redefined.

4.2.1 PERFORMING CALCULATIONS

Proceed to the Staged construction mode.

Initial conditions

As in the previous example, the K0 procedure will be used to generatethe initial conditions.

• All the structural elements should be inactive in the Initial Phase.

• No excavation is performed in the initial phase. So, the basementvolume should be active.



Construction stages

Instead of constructing the building in one calculation stage, separatecalculation phases will be used. In Phase_1, the construction of the walls andthe excavation is modelled. In Phase_2, the construction of the floor andbeams is modelled. The activation of the loads is modelled in the last phase(Phase_3).

The calculation type for the phases representing the construction stagesis set by default to Plastic.

• In the Phases window rename Phase_1 to "Excavation".

• In the Staged construction mode deactivate the soil volume located overthe foundation by selecting it and by clicking on the checkbox in front ofit in the Selection explorer.

• In the Model explorer click the checkbox in front of the platescorresponding to the basement walls to activate them.

In the Phases explorer click the Add phase button. A new phase(Phase_2) is added. Double-click Phase_2. The Phases window popsup.

• Rename the phase by defining its ID as "Construction". Keep the defaultsettings of the phase and close the Phases window.

• In the Model explorer click the checkbox in front of the platecorresponding to the basement floor to activate it.

• In the Model explorer click the checkbox in front of the beams to activateall the beams in the project.

Add a new phase following the Construction phase. Rename it to"Loading".

• In the Model explorer click the checkbox in front of the Surface loads toactivate the surface load on the basement floor. Set the value of thez-component of the load to -5.3. This indicates a load of 5.3 kN/m2,acting in the negative z-direction.

• In the Model explorer, click the checkbox in front of Line loads to activatethe line loads on the basement walls. Set the value of the z-componentof each load to -385. This indicates a load of 385 kN/m, acting in the


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negative z-direction.

• In the Model explorer click the checkbox in front of Point loads toactivate the point load corresponding to the one on the basementcolumn. Set the value of the z-component of the load to -11650. Thisindicates a load of 11650 kN, acting in the negative z-direction.

Click the Preview phase button to check the settings for each phase.

As the calculation phases are completely defined, calculate the project.Ignore the warning that no nodes and stress points have been selectedfor curves.

Save the project after the calculation.

Viewing calculation results

• Select Construction option in the Phases explorer.

Click the View calculation results button to open the Output program.The deformed mesh at the end of this phase is shown.

• Select the last phase in the Displayed step drop-down menu to switch tothe results at the end of the last phase.

In order to evaluate stresses and deformations inside the geometry,select the Vertical cross section tool. A top view of the geometry ispresented and the Cross section points window appears. As the largestdisplacements appear under the column, a cross section here is mostinteresting.

• Enter (0.0 6.0) and (75.0 6.0) as the coordinates of the first point (A) andthe second point (A') respectively in the Cross section points window.

• Click OK. A vertical cross section is presented. The cross section can berotated in the same way as a regular PLAXIS 3D view of the geometry.

• Select Total displacements→ uz from the Deformations menu (Figure4.17). The maximum and minimum values of the vertical displacementsare shown in the caption. If the title is not visible, select this option fromthe View menu.

• Press <CTRL><+> and <CTRL><−> to move the cross section.



Figure 4.17 Cross section showing the total vertical displacement

• Return to the three dimensional view of the geometry by selecting thiswindow from the list in the Window menu.

• Double-click the floor. A separate window will appear showing thedisplacements of the floor. To look at the bending moments in the floor,select M11 from the Forces menu.

Click the Shadings button. The plot in Figure 4.18 will be displayed.

To view the bending moments in tabulated form, click the Table option inthe Tools menu. A new window is opened in which a table is presented,showing the values of bending moments in each node of the floor.


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Figure 4.18 Bending moments in the basement floor

4.3 CASE C: PILE-RAFT FOUNDATION

As the displacements of the raft foundation are rather high, embedded pileswill be used to decrease these displacements. These embedded pilesrepresent bored piles with a length of 20 m and a diameter of 1.5 m.

Objectives:

• Using embedded piles

• Defining material data set for embedded piles

• Creating multiple copies of entities

Geometry input

The geometry used in this exercise is the same as the previous one, exceptfor the pile foundation. It is not necessary to create a new model; you canstart from the previous model, store it under a different name and modify it.To perform this, follow these steps:

Start the PLAXIS 3D program. The Quick select dialog box will appearin which the project of Case B should be selected.



• Select the Save project as option in the File menu to save the projectunder a different name (e.g. "Tutorial 1c").

Definition of embedded pile

• Proceed to the Structures mode.

Click the Create line button at the side tool bar and select the Createembedded pile from the additional tools that appear.

• Define a pile from (6 6 -2) to (6 6 -22).

Open the material data base and set the Set type to Embedded piles.

• Create a data set for the embedded pile according to Table 4.5. Thevalue for the cross section area A and the moments of inertia I2, I3 andI23 are automatically calculated from the diameter of the massive circularpile. Confirm the input by clicking OK.

Table 4.5 Material properties of embedded pile

Parameter Name Pile foundation Unit

Young's modulus E 3 · 107 kN/m2

Unit weight γ 6.0 kN/m3

Pile type - Predefined −Predefined pile type - Massive circular pile −Diameter Diameter 1.5 mSkin resistance Type Linear −Maximum traction allowed at the top of theembedded pile

Ttop,max 200 kN/m

Maximum traction allowed at the bottom ofthe embedded pile

Tbot ,max 500 kN/m

Base resistance Fmax 1 · 104 kN

• Drag and drop the Pile data to the embedded pile in the draw area. Theembedded pile will change colour to indicate that the material set hasbeen assigned successfully.


Click the Select button and select the embedded pile.

Click the Create array button.


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Hint: A material set can also be assigned to an embedded pile byright-clicking it either in the draw area or in the Selection explorerand the Model explorer and selecting the material from the Setmaterial option in the displayed menu.

• In the Create array window, select the 2D, in xy plane option for shape.

• Keep the number of columns as 2. Set the distance between thecolumns to x = 12 and y = 0.

• Keep the number of rows as 2. Set the distance between the rows tox = 0 and y = 12 (Figure 4.19).

• Press OK to create the array. A total of 2x2 = 4 piles will be created.

Figure 4.19 Create array window



Mesh generation

As the geometry model is complete now, the mesh can be generated.

Create the mesh. Keep the Element distribution as Coarse.

View the mesh.

• Click the eye button in front of the Soil subtree in the Model explorer tohide the soil. The embedded piles can be seen (Figure 4.20).

• Click on the Close tab to close the Output program and go back to theMesh mode of the Input program.

Figure 4.20 Partial geometry of the model in the Output

Performing calculations

After generation of the mesh, all construction stages must be redefined. Eventhough in practice the piles will be constructed in another construction stagethan construction of the walls, for simplicity both actions will be done in thesame construction stage in this tutorial. To redefine all construction stages,follow these steps:


TUTORIAL MANUAL

• Switch to the Staged construction mode.

• Check if the K0 procedure is selected as Calculation type for the initialphase. Make sure that all the structural elements are inactive and all soilvolumes are active.

• Select the Excavation phase in the Phases explorer.

• Make sure that the basement soil is excavated and the basement wallsare active.

• Activate all the embedded piles.

• In the Phases explorer select the Construction phase. Make sure that allthe structural elements are active.

• In the Phases explorer select the Loading phase. Make sure that all thestructural elements and loads are active.

Calculate the project.

Save the project after the calculation.

Click the View calculation results button to open the Output program.

• Select the Loading phase and view the calculation results.

• Double-click the basement floor. Select the M11 option from the Forcesmenu. The results are shown in Figure 4.21.

• Select the view corresponding to the deformed mesh in the Windowmenu.

Click the Hide soil button in the side toolbar.

• To view the embedded piles press <Shift> and keep it pressed whileclicking on the soil volume in order to hide it.

Click the Select structures button. To view all the embedded piles, press<Ctrl>+<Shift> keys and double click on one of the piles.

• Select the option N in the Forces menu to view the axial loads in theembedded piles. The plot is shown in Figure 4.22.



Figure 4.21 Bending moments in the basement floor

Figure 4.22 Resulting axial forces (N) in the embedded piles


TUTORIAL MANUAL


4 FOUNDATION IN OVERCONSOLIDATED CLAY - Plaxis · PLAXIS 3D - FOUNDATION IN OVERCONSOLIDATED CLAY 4 FOUNDATION IN OVERCONSOLIDATED CLAY In this chapter a ﬁrst application of PLAXIS

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