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You are reading the online version of the LimitState:GEO manual, which is also available as a pdf.
Specifying the Problem Geometry
Terminology
Problem geometries are built up using Geometry objects. There are three types of geometry object:
- Solid
- This is a 2D polygon defining a body of soil or other material. Its extent is defined by the surrounding Boundary objects.
- Boundary
- This is a straight line that links two vertices and defines the edge or boundary of a Solid. If a Boundary object separates two Solids then it is shared by those solids and forms an interface.
- Vertex
- This is a point where two or more Boundary objects meet. A vertex may lie on a straight line. A Vertex is shared by the Boundary objects that meet at that vertex.
In LimitState:GEO all geometry is defined in terms of straight lines. Curves may be represented by a series of short straight line segments. Boundary and Solid objects can both be assigned material properties.
In general most geometry will be built up using the draw Rectangle and Polygon functions. If a polygon is added adjacent to an existing polygon, LimitState:GEO automatically ensures that each polygon shares the interface Boundary object.
Setting the Units
The LimitState:GEO interface can display either Metric (kN, m) or Imperial (lb, ft) units.
To set the units used, open the Preferences... dialog in the Tools menu. Go to the Units tab and select the units that you wish to use. Changes will take effect when LimitState:GEO is next restarted (see below figure).
The default units used in each setting are given in the following table
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(pcf)
(kPa)
(psf)
(pf) 
(kPa)
(psf)Starting with an Empty Project
While problem geometries may be rapidly built for common scenarios using the Wizards.
To start with an empty project, select Create a new project in the Welcome to LimitState:GEO dialog and click OK. The New Project dialog (Figure ) will then start. Select the Empty Project and click OK to display the Empty Project dialog.
Project Details
The first tab on the Empty Project dialog is for the input of project data. None of the fields are mandatory, but a default name of New Project will be displayed and used unless changed by the user. Once all the necessary fields have been filled in, clicking Next will advance on to the next stage of problem definition.
Draw Settings
The second tab in the Empty Project dialog displays the options for setting up the drawing environment. It is concerned with establishing a suitable grid geometry and snap settings to enable a model to be produced in a quick and efficient manner. The grid is a useful aid to modelling and, when set up correctly in conjunction with the snaps, can save the user much time and effort.
Grid Settings
To display the grid from the start of modelling, simply ensure that the Grid check box is ticked in the Draw Settings tab (this is done by default). There are then several variables that may be altered in order to tailor the size, spacing and behaviour of the grid:
- Auto-calculate extents - Ticking this check box causes LimitState:GEO to automatically adjust the extents of the grid during modelling so that it is always slightly bigger than the objects being drawn. When enabling this feature in the wizard, the initial size of the grid is determined by the values entered for the Minimum and Maximum project extents and Major gridline spacing.
- Minimum - Sets the initial minimum x and y coordinates of the grid (note that this value will be overridden if Auto-calculate extents is enabled).
- Maximum - Sets the initial maximum x and y coordinates of the grid (note that this value will be overridden if Auto-calculate extents is enabled).
- Major gridline spacing - Sets the initial distances (in metres) between the major x and y gridlines.
- Minor gridlines - Determines the initial number of minor intervals between the major gridlines.
Snap Settings
Object snap in LimitState:GEO is very similar to that found in many CAD and drawing packages - it causes the cursor to become attracted to certain features of the model, such as points or intersections between lines.
To use the snap functionality from the start of modelling, simply ensure that the Snap check box is ticked in the Draw Settings tab (this is done by default). You can then fine-tune the size of the snap markers and select the type of snaps that you wish to use:
- Snap Marker Size - Use the slider to make the snap marker larger (+) or smaller (-).
- Snap Sensitivity - This determines how close the cursor needs to be to a particular feature in order to snap to it. Use the slider to make the distance smaller (+) or larger (-).
- Point - Causes the cursor to snap to Points / Vertices .
- Perpendicular - When drawing a line or polygon, this will cause the cursor to snap to a point on an existing line that forms a right angle between that and the line being drawn.
- Line - Causes the cursor to snap to Line Objects.
- Grid - Causes the cursor to snap to points of intersection between the gridlines.
- Intersection - Causes the cursor to snap to points of intersection between the gridlines and the Line Objects of the model.
Clicking Finish will end the setup process and display the modelling environment. The grid can
subsequently be adjusted and toggled on or off at any time during modelling via the Settings dialog of
the Draw menu and the 
icon respectively. Similarly, the snap functionality can be quickly toggled
by clicking the SNAP button on the status bar.
Starting with a DXF Imported Geometry
The user may often find it useful to import a problem geometry that has already been pre-defined using a CAD package. This can be accomplished in LimitState:GEO by using the Import from DXF... function, found under the File menu.
Importing a DXF
To start a new project with a DXF defined geometry, simply begin with the Empty Project wizard, click Finish then navigate to Import from DXF... clicking this will bring up a dialog that allows you to browse for a pre-saved DXF file.
Once a DXF file has been selected, the DXF Import dialog will appear. This offers the user several options:
- HOW TO INTERPRET CLOSED LOOPS
LimitState:GEO deals with solid bodies and the interfaces between them; therefore, by default, the DXF import function recognizes lines that form a closed loop and interprets them as being a solid object (Figure ). Lines that do not form a closed loop will be imported as construction lines so that the user is able to still use them when defining the model geometry. Alternatively, there is the option to interpret all lines within the DXF file as being construction lines. If this option is selected, LimitState:GEO solids and boundaries will not be generated and the user must draw them.
In all cases, imported DXF construction lines appear as solid grey lines in order to easily differentiate them from those defined within the program.
- THE CHOICE OF LAYERS TO IMPORT
DXF files can sometimes be composed of several layers and the user may not always wish to import all of them into their LimitState:GEO model. All layers present in a file are listed in the DXF Import dialog; to prevent one or more from being imported, simply un-check the box next to the name of the appropriate layer (N.B. where a layer has not been given a name, ‘Unnamed Layer’ will appear in the import dialog).
Permitted DXF Commands
The following commands can be used to create a DXF file that is readable by LimitState:GEO (other commands, including those used to generate 3D objects, will not be recognized):
- Point
- LWPolyline
- Line
- Polyline
- XLine
- Spline
- Vertex
Closed loops of lines forming ‘islands’ within other closed loops will be recognized and imported without difficulty. However, additional lines will be automatically generated to connect these ‘islands’ to the surrounding geometry (to allow LimitState:GEO to analyse the problem properly).
Tips for Modifying a DXF in an External CAD Package
Whilst working on a DXF file for import into LimitState:GEO it is useful to bear in mind the following points:
- ENSURE LOOPS ARE PROPERLY CLOSED
In most CAD packages it is possible to work to a very fine precision. This can sometimes lead to situations where lines that appear to intersect do not actually meet in reality. Whilst not necessarily causing a problem in the CAD model, if two lines of a loop do not intersect when importing a DXF into LimitState:GEO, a solid object will not be formed. Care should therefore be taken to ensure that, where objects are expected to be converted to solids, these are properly drawn in the CAD model before exporting to DXF (working with a grid or object snap turned on can help with this).
- REMOVE UNNECESSARY OBJECTS
Only those types of drawing object listed in Permitted DXF Commands will be recognized and imported into LimitState:GEO. If a CAD file is being worked on specifically for import into LimitState:GEO then text, hatching and other similar objects should be removed before the DXF file is generated. Failure to do this may lead to unwanted objects being created or to a loss of program efficiency as LimitState:GEO processes the superfluous data.
- KEEP THE DRAWING AS SIMPLE AS POSSIBLE
When importing from a DXF file, every intersection of two or more lines will be converted to a LimitState:GEO vertex. These vertices are, in turn, regarded as nodes along the boundaries of solid elements at the time of analysis. As LimitState:GEO normally works with a fixed number of nodes, excessive detail (leading to large numbers of boundary nodes) will reduce the number of nodes inside solid elements and potentially reduce the accuracy of the solution obtained. Wherever possible, the user is advised to appropriately simplify the ground profile of any problem before importing and try to achieve a suitable balance of boundary and internal nodes in the LimitState:GEO model. This advice is particularly relevant when working with DXF files containing multiple layers.
- WORK AT THE CORRECT SCALE
LimitState:GEO is set to work in either metres or feet (dependant upon the choice of units in the Preferences menu). It should therefore be ensured that all imported DXF files are drawn to an appropriate scale.
- BE AWARE THAT LAYERS ARE FLATTENED
Since LimitState:GEO works with planar models, when multi-layered DXF files are imported, all layers are first flattened. Any overlapping lines that were distinct in the original file will no longer be separate, which may result in undesired problem geometries. It is recommended that such DXF files are simplified to a single layer representing the problem the user wishes to model.
- WORK IN THE XY PLANE
Care should be taken to ensure that the correct viewpoint is being used when preparing a DXF file for import. LimitState:GEO works in the xy plane, which corresponds to the ‘Plan’ or ‘Top’ 3D viewpoint in most CAD packages. Objects that lie predominantly in the xz or yz planes may be imported in a way that is not desirable. It is recommended that DXF files containing 3D components are simplified to a single (2D) layer representing the problem the user wishes to model.
Permitted DXF Types
DXF files can be produced in either ASCII or binary format. LimitState:GEO will only recognize those files that have ASCII encoding. It is therefore recommended that the user checks the type of any DXF file before attempting an import.
Following DXF Import
Following a successful DXF import, it will be necessary to apply the requisite material properties, loading and support conditions to the model before an analysis can be attempted.
Export to DXF
The Export to DXF command (also found in the File menu) will save the geometry of any LimitState:GEO project to a suitably formatted DXF file. If the project is in a solved state, it is possible also to save the entities generated during the solution process, such as nodes and slip-lines.
Construction Lines
In addition to the Grid, construction lines can provide a useful tool when setting up a problem geometry. These lines appear as dotted grey lines in the viewer pane and possess the same snap functionality as the standard geometry objects but, like the grid, construction lines are not counted as part of the problem during analysis.
Construction lines can be dragged and deleted, just like standard drawing objects. In addition, to clear all of the construction lines at the same time, simply choose Clear all from the Construction lines section of the Draw menu.
There are three types of construction line available: Horizontal, Vertical and Custom (angled):
Horizontal Construction Lines
To draw a horizontal construction line, click the 
button in the toolbar. When hovering over the
Viewing Pane, the cursor will be accompanied by a solid, horizontal black line that denotes the current
position of the construction line; to place this in an approximate position, simply use the cursor position
to choose an appropriate y coordinate and left-click the mouse. For a more accurate positioning,
right-click the mouse and choose the Set y coordinate option. A dialog will then appear that allows a
precise y coordinate to be entered. The option to continue placing construction lines will remain until
the user presses the ESC button on the keyboard or right-clicks the mouse and chooses the Cancel
option.
Vertical Construction Lines
To draw a vertical construction line, click the 
button in the toolbar. The procedures for adding
vertical construction lines are the same as for adding horizontal construction lines except that the x
coordinate is now set instead of the y.
Custom Construction Lines
To draw a custom (angled) construction line, click the 
button in the toolbar. The next left click of
the mouse will then set the first point of the construction line - this will be taken as the origin of
rotation. Alternately, right-click the mouse and choose Add point to set the origin more
precisely.
A solid black line will now appear that rotates around the origin point with any movement of the cursor. To set the angle by specifying a second point, simply left-click the mouse in the Viewing Pane or right-click and choose Add point to set the destination more precisely. Should it be necessary to enter an angle of rotation instead of a second point, right-click the mouse and choose the Set angle option. A dialog will then appear that allows an angle (in degrees) to be entered. This is measured around the origin point, from the horizontal axis in an anti-clockwise direction.
Drawing Functions
Rectangle
Click on the toolbar button 
, or from the menu select Draw, Rectangle. Left click with the mouse
anywhere on the geometry window and then with the mouse button held down, drag a rectangle. This
function is used to draw horizontal soil layers and structural elements.
Polygon
Click on the toolbar button 
, or from the menu, select Draw, Polygon. Left click with the mouse
anywhere on the geometry window to draw the first vertex of the polygon. Then move the mouse to the
position of the next vertex and left click to draw this. Continue until all vertices of the polygon have
been drawn. To complete the polygon, right-click to display the context menu and select Finish. This
will connect the last vertex drawn to the first vertex (there is no need to click on the first vertex
again).
To exit from drawing the polygon and clear the currently drawn vertices, press ESC.
When drawing a Polygon, it is permissible to utilize existing Vertex objects as part of the polygon. LimitState:GEO can be set to snap to existing geometry objects to facilitate this. This is the normal way in which polygons are added adjacent to existing polygons.
Line
Lines (Boundary objects) are not allowed to exist in isolation in LimitState:GEO and are used to split
existing Solid objects. To draw a line, click on the toolbar button 
, or from the menu, select Draw,
Line. Now left-click on any existing Line or Vertex and then using the mouse connect it to
another Line or Vertex by clicking on it. The Line so drawn must pass across an existing
Solid object. If this is not the case then a warning message is displayed. If the line is
legitimate then a message appears in the output pane stating that the Solid object has been
split.
When a Line is drawn from a point on one Line to a point on another across an existing Solid object, additional Vertices will automatically be created where required.
Vertex
Vertex objects are not allowed to exist in isolation in LimitState:GEO but may be used to split existing
Boundaries. To add a Vertex, click on the toolbar button 
, or from the menu, select Draw, Vertex.
and left-click on any existing Boundary. If the vertex is legitimate then a message appears in the
output pane stating that the boundary has been split.
Selecting Objects
Single Click Selection
Single selection mode may be activated by using the toolbar buttons ‘Select’ 
and ‘Click’ 
. In
this mode any object may be selected by clicking on it with the mouse. As soon as it is selected it will
be highlighted and its properties will appear in the Property Editor. If it is a Geometry Object, then
its coordinates will also appear in the Geometry Editor.
Rectangle Selection
Rectangle selection mode may be activated by activating the toolbar buttons ‘Select’ and
‘Rectangle’ 
. In this mode any object or set of objects may be selected by dragging a rectangle
around them with the mouse. The rectangle must fully enclose the required items. As soon as they are
selected they will be highlighted and common properties shared by all the selected items will appear in
the Property Editor. To select only specific types of objects, use the Selection Filter dialog from the
main menu (Select - Filters...).
Multiple Selection
Once a selection has been made using ‘Rectangle’ or ‘Click’, additional objects may be added to the selection by holding the CTRL key down while making additional selections using either ‘Rectangle’ or ‘Click’. Already selected objects may also be removed from the selection by re-selecting them using either ‘Rectangle’ or ‘Click’.
Snapping to Other Objects
It is possible to snap to most other geometrical objects on the screen. Extensive control is provided via the Snap tab in the Preferences dialog accessible via the Draw, Settings menu as shown below.
Global control over snap is also provided by the SNAP button on the Status bar.
Modifying the Geometry
Using the Mouse
Any geometry object (Solid, Boundary, Vertex) may be selected and then dragged using the mouse (hold down the left button while moving the mouse). This provides a quick and flexible way to modify the geometry. If the modification should distort the geometry into an invalid form (e.g. turn a polygon inside out) then a warning message is displayed and the original geometry is restored.
The following actions are possible:
- Dragging a Solid object will drag the solid and its adjacent Boundary objects and Vertex objects.
- Dragging a Boundary object will drag the boundary and its end point Vertex objects.
- Dragging a Vertex object will drag the vertex and the ends of the adjacent Boundary objects.
Dragging is enabled by clicking on the DRAG button on the Status bar. Dragging may be constrained to orthogonal directions only by clicking the ORTHO button on the Status bar or by pressing F8.
To Delete, the following methods are recommended for the listed object types:
- Vertex and Boundary
- drag the Vertex at one end point of the Boundary onto the Vertex at the other end point of the Boundary. The Vertex objects will be merged and the Boundary object deleted.
- Vertex
- Select the Vertex and click Delete
. The Vertex and one of the adjacent Boundary
objects will be deleted. It is not defined which of the two Boundary Objects will be deleted.
If this would result in an ambiguous or illegal geometry this action is not permitted. To
ensure a specific Boundary Object is deleted, use the method for deleting a Vertex and
Boundary.
- Boundary
- Select the Boundary and click Delete . The outcome for this action depends on
the location of the Boundary:
- If the Boundary object is an internal interface between two Solids, then the Boundary object is deleted and the Solids are merged.
- If the Boundary object is an external Boundary, it will be deleted together with its adjacent Vertex objects and one of the adjacent Boundary objects. The Vertex objects at the other end points of these deleted Boundary objects will be linked by the remaining Boundary. It is not defined which of the two Boundary Objects will be deleted. If this action would result in an ambiguous or illegal geometry this action is not permitted.
- Solid
- Select the Solid and click Delete . The Solid is then deleted together with all adjacent
Boundary and Vertex objects that are not shared with other Solids.
It is usually preferable to delete objects by dragging and merging so that the properties of the remaining amended objects are retained.
Connecting Geometry Objects
To connect a Solid object to another Solid object, it is best to work with the relevant vertices. Ensuring that Snap to Point is activated (see snapping to other objects) will make certain that the objects share common vertices rather than simply having vertices in very close proximity to each other.
For the example below, two initially unconnected square solid zones are to be modified so that, after the process is completed, they share a common boundary:
- a) The first vertex is selected. It is then clicked and the left mouse button held down.
- b
) The selected vertex is dragged over the target vertex (a red rectangle signifies that the
two will be merged upon release the left mouse button). The left mouse button is then
released.
- b
and c) The above steps are repeated for the second vertex.
Overlapping Geometry Objects
To overlap one Solid object with another, drag the former over the latter. This process will then split the original two Solid objects into three new Solid objects. If a Solid object is dragged over more than one other Solid object then a multiple split will be carried out.
For the example below, two initially unconnected square solid zones are to be modified so that, after the process is completed, they overlap one another:
- The first solid is selected. It is then clicked and the left mouse button held down.
- The selected solid is dragged over the target solid and the left mouse button is then released.
- A new zone is created by the overlapping areas of the two solids. Should the two original solids have different associated materials (as in this case), the new zone will contain a combination of all the materials and will be displayed with a ‘checkerboard’ pattern (for more information on assigning materials to zones see here).
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Changing the End Vertex of a Boundary
There are several ways in which the end vertex of a line can be changed. However, in situations where the geometry is perhaps rather complex, the following method may be used (see the figure below):
- Using the Line function (see snapping to other objects) draw a new boundary line in the desired position.
- Use the mouse pointer to select the original boundary line then press the Delete button to remove it.
- If the zone created in step 2 now contains a mixture of materials (c), you can alter this by setting the materials as described in this section.
- If desired / appropriate, the original vertex may now be deleted.
Easier methods in the above case would have been to edit the coordinates of the vertex in the Geometry Editor or drag and drop the original endpoint of the line using the mouse (possibly with ORTHO enabled). However, these shortcut approaches are not always suitable in practice, for example where the vertex also defined the geometry of another Solid which was to be left unchanged.
Creating One Object Inside Another
It is often useful to create a zone that lies entirely within another solid (for example to model a material
lens or large void). To achieve this, simply draw the new zone in the required position using the
rectangle () or polygon () draw functions. Additional boundary lines will be created in order to
link the new zone to the rest of the model and the new zone will automatically be assigned the material
of the surrounding solid. To create a lens of material from the new zone, change the material as
described here. To create a void, select the new zone with the mouse and press the Delete button (see
below).
Using the Geometry Editor
Instead of utilising the mouse, the coordinates of points making up any geometry object may be directly edited within the Geometry Editor. Select the required object and its coordinates will appear in the Geometry Editor. When all changes have been made, click the apply button in the Editor to commit the changes.
Undo/Redo
Many specific modifications may be undone utilising the Undo button 
or the menu Edit, Undo.
Any undone actions may be redone using the Redo button 
or the menu Edit, Redo. Redo may
only be carried out if no intervening modifications to the geometry have been made. If Undo or Redo
are not available then the toolbar buttons and menu items are disabled (appear in grey).
Hovering the mouse over the Redo or Undo button will display a tooltip describing the last
action.
Editing Geometry Object Properties
In addition to displaying the coordinates of a Geometry Object in the Geometry Editor, selecting any object displays its properties in the Property Editor and these may be edited. The properties available for a Vertex, Boundary and Solid are depicted in Vertex properties, Boundary properties and Solid properties.
