You are reading the online version of the LimitState:GEO manual, which is also available as a pdf.

Quick Start Tutorial

Introduction

This chapter gives a flavour of the capabilities of LimitState:GEO. It is recommended reading for new users and is designed to give users the confidence to subsequently make use of some of the more sophisticated features of the program.

However, for the sake of brevity some important issues are not discussed in this chapter and the reader is referred to the User Guide for fuller information.

Note that for sake of simplicity all examples in this tutorial involve undrained (cohesion only) problems.

Getting Started

It is assumed that the user is starting from the Welcome to LimitState:GEO dialog.

Select Create a new project and click OK to bring up the New Project dialog.

If Cancel is selected, the user is free to define their own problem geometry. However it is easier to learn how the program works by initially using one of the predefined problem wizards. The wizards permit rapid definition of common problem geometries. The geometry can be easily amended subsequently.

Select Simple Footing Project and click OK.

The wizard then appears. Project data is entered in five stages as follows (the icons in the navigation bar on the left hand side of the wizard will be highlighted during each stage of the problem definition):

Project

Background details to the project may be entered here. Click Next to accept defaults.

Geometry

The problem geometry may be amended here. Click Next to accept the defaults.

Materials

Material properties may be entered here. Note the default value of undrained shear strength () of 60.0 kN/m. Click Next twice to accept defaults.

Loads

Loads may be entered here. Note the default value, Q (the applied load on the foundation) is set to a unit value of 1.0 kN/m. Change this to 200.0 kN/m. Click Next to accept this.

Partial Factors

The partial factors may be modified for different design codes. Click Finish to accept the default User factors which are by default all set to unity.

The problem geometry will be displayed in the main viewer (see Footing geometry).

Solving a Problem

With the default values set in the wizard, the defined problem for a short term (undrained) analysis is equivalent to the simple problem of a footing of width 1.0m on a semi-infinite layer of weightless cohesive soil of undrained shear strength = 60.0 kN/m.

While it is not possible to model a semi-infinite layer of soil, it is sufficient to model a finite size layer providing it is large enough.

To determine whether the specified load of 200.0 kN/m can be carried by the foundation,

either use the menu: Analysis / Solve, click on the icon or press F5.

The program will first display a series of nodes, superimposed on the geometry objects in the problem, and will then rapidly try out all possible combinations of slip-lines interconnecting the nodes to find the optimum solution. The program will gradually refine the failure mechanism until an optimal slip-line mechanism is found (see Footing mechanism), together with an associated Adequacy factor (margin of safety) on the specified load.

With the specified parameters, an Adequacy factor of 1.566 should be obtained (displayed in the Output window at the bottom of the screen). This means that the foundation is safe against collapse by a factor of 1.566. The actual load that would cause failure is 200 1.566 = 313.2 kN/m.

In general:

• Adequacy factor 1.0, the problem is safe against collapse
• Adequacy factor 1.0, the problem is unsafe against collapse.

In order to analyse the ultimate limit state, at least one load in the problem must be increased until the collapse state is reached. In order to indicate to LimitState:GEO which load this is to be, the Adequacy property must be set for at least one load. For the Simple Footing Project, the wizard automatically sets the Adequacy property for the load applied to the footing. The returned Adequacy Factor is the factor by which that load must be increased in order to cause collapse. For further information on the Adequacy factor and its usage, see here.

This example problem is the well known Prandtl punch solution for which the true analytical collapse load is given by:

(1)

where is the width of the footing, is the undrained shear strength, and is the surcharge load on the adjacent soil surface. Note that the solution is unaffected by the soil self weight.
The adequacy factor is given by:

(2)

where is the specified design load.
In this example, with = 0, the theoretical adequacy factor is 1.542. The overestimate by LimitState:GEO is thus 1.56%, utilizing the default Coarse nodal refinement.

Viewing Mechanism Deformation

By default the software will automatically animate the solution after solve (see Deformed footing mechanism), by magnifying the instantaneous displacements at failure. Note that though the solution is strictly only valid for infinitesimal displacements, large displacements are displayed to assist visualization of the collapse mode. For direct control over magnification of the mechanism displacements, the slider bar can also be used. The Play Animation button can be clicked to replay the animation.

Viewing Shear and Normal Stresses

After the software has solved a problem, the user is able to select any of the solids identified in the failure mechanism (by clicking with the mouse). This will display a diagram of the normal (default) or shear stresses acting upon the selected solid. Hovering over each part of the diagram will display the magnitude of these stresses next to the mouse cursor (see Shear and normal stress figures). For further information on displaying boundary stresses, see here.

Zooming In and Out

To zoom in and out, use the magnifying glass toolbar buttons (Zoom In , Zoom Out , Zoom All ), or, if a scroll wheel mouse is being used, use the wheel to zoom in and out. Note that with the mouse, zooming takes place centred on the current position of the mouse pointer (the Select button must be on for this feature to work). Zoom All resizes the image to display optimally in the viewer. This is useful if the image has become too large or small.

Trying Different Problems and Parameter Sets

Perhaps the simplest way to vary problem parameters is to simply re-run the wizard (File>New or ) and enter different parameters; alternatively a different wizard can be tried.

However, the current problem may be modified in any way required after a wizard has been run. Making use of this capability will give the user a good feel for what the software is capable of, and how problems can be created without using the wizards.

Note that if there is a previously solved problem, the Unlock icon must be clicked to allow modification of any of the parameters (this is to prevent inadvertent alterations being made once a solution has been obtained).

The Property Editor

The Property Editor (PE), displayed on the right hand side of the display, allows the user to quickly read and / or modify the attributes of one or more objects within the current project. The following figure shows the typical parameters displayed in the Property Editor when a Material is selected.

The Property Editor is visible by default when LimitState:GEO is started for the first time, but can be hidden and shown using the View menu.

Some of the functions in LimitState:GEO are only accessible via the Property Editor. These are described in more detail in this section. Other functions and attributes can be accessed and modified elsewhere, but are shown for convenience in the Property Editor.

Generally when an object is selected on the screen it will be highlighted and its properties will be displayed in the PE - where they can be viewed or edited as required. Single clicking on any item in the Property column of the PE gives an expanded explanation of the parameter in the window at the base of the PE. A sign next to an item in the PE indicates that there are additional sub-parameters relating to that item that may be viewed. Click on the sign to access these. Clicking on a value in the PE allows it to be modified by typing or selecting the required choice, unless it is a read only value or the project is Locked.

Changing Material Properties

By default the Materials Explorer is located on the left hand side of the screen. This contains a list of available material types. Click on any icon to view the properties of the material in the Property Editor. The soil material used in the Simple Footing Project is called ‘Footing Soil’. When selected, its properties are displayed as follows:

• shear strength parameters:
  • Drained cohesion, = 0 kN/m
  • Drained phi, = 0
  • Undrained shear strength, = 60 kN/m
• unit weights:
  • Unit weight (dry), = 20.0 kN/m
  • Unit weight (saturated), = 20.0 kN/m.

Change the value of the Undrained cohesion, to 75.0 kN/m (click on the box with the value 60.0 in it, enter the new value then press Enter or click elsewhere in the PE to accept the value) and then Solve again. A new solution of Adequacy factor = 1.958 (1.25 times the previous solution discussed previously) should be obtained.

Now try changing the saturated unit weight to 10 kN/m and click Solve (remember to click Unlock first). The solution should remain as 1.958. Self weight does not affect bearing capacity problems involving cohesion only when the soil surface is horizontal.

Changing the Material Used in a Soil Layer (Solid Object)

Click anywhere in the soil mass below the footing in the viewer frame (to use single selection, ensure that toolbar buttons and are set to on).

Once selected with the mouse the Solid Object representing the soil layer will be highlighted in pink (as shown here), and its properties displayed in the PE. To change the allocated material three options are available:

1. If a material defined by the user (e.g. in a wizard) has been selected, its properties may be freely edited directly in the PE in the same way as described in Changing material properties. (It is necessary to click the sign next to the caption ‘Footing soil’ in the Property column of the PE to access the material properties.)
2. If it is necessary to allocate an already defined material to the Solid Object representing the soil layer, ‘drag and drop’ may be used. Using the mouse, drag a material from the Materials Explorer onto the Solid Object representing the soil layer. A dialog box will be displayed and the user asked whether the material is to be added or replaced. Click Replace and the new material will be used in place of the previous material. The colour of the solid object should change to reflect the change in material (this will not be seen while the soil mass is still selected). Try replacing the default soil with Very stiff clay. After solving, the same failure mechanism as before should be obtained, but an adequacy factor of 3.916 will be obtained (this is 2.5 times the original value obtained with 60 kN/m since the cohesion of the Very stiff clay is 150 kN/m).
3. The third option, which is an alternative to drag and drop, is to click on the Value cell in the ‘Materials’ row in the PE (this should say ‘1 Material assigned...’). A Change button will appear. Click this and the Edit Object Material(s) dialog will allow the selection of a material (or set of materials) that can be used in the soil layer.

Note that the system defined material properties (indicated by a padlock symbol on the material: ) are read only and may not be changed. User editable copies may however be made or new materials created (see Defining Materials).

Changing the Material used in an Interface (Boundary Object)

As well as being able to assign material types to a Solid Object (soil layer), it is also possible to add materials to a boundary or interface (termed a Boundary Object). With a few exceptions (e.g. foundation / soil interfaces) the wizards will assign no materials to boundaries by default.

Material properties assigned to a Boundary Object are often related to the material allocated to an adjacent Solid Object. Thus if the material allocated to the Solid Object is changed, it is usually necessary to change the material in the adjacent boundary (if set), otherwise an unexpected solution may be obtained. For example, Unlock the project and drag Very soft clay onto the bottom boundary of the soil (whilst retaining Very stiff clay in the main soil layer). The boundary should be highlighted in pink when the mouse is exactly over it, and will change colour to the material colour if the material has been successfully assigned to it. After solving, you will see that the solution changes to utilize this weak layer of soil - the mechanism is attracted to the base of the soil (see Modified boundary problem) and a lower adequacy factor of 3.572 is obtained. To remove this material from the line, click Unlock, select the boundary line and in the PE click the cell opposite ‘Materials’ to show the Change button. Click this button, uncheck Very Soft Clay in the Edit Zone Material(s) dialog and click OK.

Modifying Loads

Try selecting the upper right hand surface of the soil. View the load parameters by clicking the adjacent to the ‘Loading’ parameter label in the PE, and then click the adjacent to the ‘loads’ parameter label. The three load types permanent, variable and accidental will be displayed. Loads for each of these load types may be defined and they can attract different partial factors depending on the partial factor set chosen (recall that the default User set in which all parameters were set to be unity was chosen earlier).

Now click on the adjacent to the permanent parameter label to see the Shear and Normal applied loads. These should both be zero. Change the Normal value to 10.0, ensure Adequacy is set to ‘false’, and then repeat for the upper left hand surface. It can be seen that non-zero loads (with no adequacy factor applied to them) are displayed graphically in the viewer as green arrows. Click Solve, and a solution of 3.966 should be obtained. This is higher than the solution with zero surface load (3.916) as expected.

Note that if Adequacy had been applied to these surface surcharge loads, the program would try to find the factor by which these loads and the load on the footing would have to be simultaneously increased to cause collapse and would generate a very different factor to that which might be initially expected.

In this example, with = 10 kN/m and = 150 kN/m, the theoretical adequacy factor is given by:

(3)

The overestimate by LimitState:GEO is this case is now 1.4%, utilizing the default Coarse nodal refinement.

Resizing a Block of Soil

Changing the geometry of a problem is straightforward. For example, rerun the Simple Footing wizard, (click New to display the New Project dialog again) check the Model as symmetrical half space box on the Geometry page and accept all the other defaults (or simply click Finish after checking the half space box). This will produce a half space model. Solve to obtain the baseline Adequacy factor of 312.1. Now click on the top right hand Vertex of the soil block and with the left hand mouse button held down, drag this Vertex downwards to form a slope (see dragging a vertex). Then Solve and a smaller value of adequacy factor should be obtained. Points may also be moved more precisely by typing in their coordinates. The Geometry Editor (located below the PE) can be used for this purpose. Again click on the same point and the and coordinates will be displayed in the Geometry Editor. Set = 1.75 and = 0.4 and Solve again. An Adequacy factor of 252.4 should be obtained.

Changing a Boundary Condition

The current boundary conditions for the problem are displayed graphically. They may also be viewed in the PE by selecting any boundary. In the PE it is possible to set the boundary type, and the permanent, variable and accidental loads independently. Try selecting the upper right hand surface of the soil. It will be seen in the PE that this has a Support Type of Free. This can be changed to Fixed (or Symmetry, though this latter setting may not be appropriate here).

Change the Support Type on this boundary (upper right hand soil surface ) to Fixed. Then select the vertical right hand edge of the soil zone and set its Support Type to Free. Solve again to obtain a radically altered failure mechanism in which the soil can only flow out of the right hand side free edge.

Solving Collapse Problems Driven by Soil Self Weight

The final example in this quick start tutorial involves a slope analysis. Slope failure driven by the soil self weight can be examined as follows. Run the slope stability wizard (click File>New or and choose Slope Stability Project and click OK). Accept the defaults and click Finish. In this problem no external loads (applied to a boundary) are present. To solve a problem, the Adequacy Factor must be applied to a parameter. Normally this may be applied to an external load, however in the case of slope stability the parameter driving the problem to failure is the soil self weight. This is automatically applied to the self weight by the wizard. To see how this may be modified, click anywhere on the body of soil constituting the slope. In the Property Editor, click the symbol adjacent to the ‘Self weight loading’ entry to see two entries: Loading type and Adequacy factor. The Adequacy factor has been set to true and Loading type to unfavourable. This latter term determines the application of partial factors and is discussed further under Scenario Manager.

Click on the icon to solve. The failure mechanism depicted here and a solution of Adequacy factor = 2.477 should be obtained. This means that the self weight needs to be 2.477 times as great as it is to cause failure, or alternatively that the undrained strength would need to be 2.477 times lower to cause failure.

This solution may be compared with that derived by Taylor (1948). For a 68 slope the Taylor stability chart gives the stability number 0.2. For this problem with = 0.5, or 2.5.

Modelling Layered Soils

Frequently it is required to model several layers of soil in a problem. Layers may be built up individually or existing bodies of soil split into layers. The latter will be covered in this example, for further information on the former, see Drawing Functions.

Using the geometry of the slope from the previous section, click on the Line icon on the left hand tool bar. Then click on the vertex at the toe of the slope (A in the below figure) and then drag the line that appears across to the far right hand vertical boundary (B in the below figure) and click here. This adds a new Boundary object and splits the existing Solid into two layers. By default these new layers receive the same properties as the original one, however these may be modified as required.

For example, change the material in the lower layer to a Soft Clay (e.g. by using drag and drop as described earlier). Clicking solve should generate a significantly changed failure mechanism similar to that depicted here. An important point to note here is that the critical mechanism intersects the boundaries of the problem domain (in this case in four places). It is thus unlikely to be the correct solution for a problem where the soil in reality extends much further to the left, right and below. In general it is necessary to move the boundaries outwards until the mechanism lies fully within the problem domain. However note that for some classes of problem, the mechanism is in theory semi-infinite and will always touch one or more boundaries.

Conclusion

This brief quick start tutorial has been designed to familiarize users with the basic functionality of LimitState:GEO. It is recommended that users experiment with the various wizards, and modify the various parameters involved, before constructing problems from scratch.