Geometry and boundaries

A model is a two-dimensional section built from boundaries: one external boundary that encloses the ground, material boundaries that divide it into regions, and lines that carry water, a tension crack, a weak layer or the orientation of bedding.

2.2.1Boundary types#

The Boundaries menu has one command per type, with the shortcuts Ctrl+1 to Ctrl+8. Figure 2.2.1 shows the main ones on a slope section and Table 2.2.1 lists all of them.

The commands that would break a rule are greyed out, with a tooltip that says why, as with Add Water Table once the model has one.

2.2.2Drawing and typing boundaries#

Draw the external boundary first, then the material boundaries, then the water and the other lines. Every drawing follows the conventions of the canvas (Drawing on the canvas):

1. Choose the command, for instance Boundaries → Add External Boundary (Ctrl+1).
2. Click each vertex, or type it as x, y in the Enter X,Y: field of the status bar and press Enter. Clicks are snapped to existing vertices, lines and the grid while SNAP is on, which is how a material boundary is made to start and end exactly on the external boundary.
3. Finish with Enter or with Done in the right-click menu. The external boundary is closed automatically; Esc cancels.

While drawing, Coordinate Table… in the right-click menu lists the vertices placed so far, to correct them or to type the rest of the boundary as a table. Draw the water table, the piezometric lines and the drawdown line from left to right across the whole width of the materials they serve: outside the horizontal extent of a water line the pore pressure it gives is zero.

A boundary can also be drawn on the annotation layer and turned into geometry afterwards: Tools → Draw takes typed coordinates for a line, polyline, polygon, rectangle or circle, and Tools → Convert Tool to Boundary… copies it into an external boundary, a material boundary, a water table, a piezometric line or a tension crack.

Material boundaries and regions#

The regions of the model are the pieces into which the material boundaries cut the external boundary. Each region takes one material (Section 2.3.3). For the regions to come out as intended:

• Run every material boundary from boundary to boundary, that is, from one edge of the external boundary to another, or onto another material boundary. An end that lies within about 0.8 % of the model diagonal of another boundary is welded onto it, and a node is inserted there.
• An end that stops inside the model is extended straight along its last segment until it leaves the model, cutting every region in its way. Extend the line on purpose instead of relying on this.
• A lens (a pocket of one material inside another) is a closed material boundary. It becomes a region of its own, and a hole in the region around it.

A region to which no material has been assigned takes the first material of the project (Regions without a material).

Water lines#

The water table, the piezometric lines and the drawdown line are open polylines that never divide regions. They only act on the materials that point at them, which is why finishing a water table or a piezometric line opens the Assign Water Surface dialog straight away when the project has materials. How they become pore pressures, ponded water included, is described in Section 2.4.

Tension crack#

Draw the tension crack as a line below the crest at the depth to which cracks may open; the ground between that line and the surface is the crack zone. A slip surface whose crest end lies inside the crack zone is cut where it reaches the line and closed by a vertical wall up to the ground surface: the soil above the cut keeps its weight but offers no shear resistance (Duncan & Wright, 2005). Only the crest end is cut; the toe end keeps its strength. A surface whose crest is in the crack zone and that never reaches the line is discarded.

The water in the crack is set with Properties → Define Tension Crack…, or from the right-click menu of the crack. The water pushes horizontally on the wall with the hydrostatic thrust of its depth, using the pore-fluid unit weight of the project.

Weak layers and anisotropic surfaces#

A weak layer is a joint, a shear band or a thin layer along which a slip surface can travel. It does not divide the ground into regions; its strength is that of a material. When the line is finished and the project has materials, the Assign Material dialog asks which one: a weak layer without a material has no strength of its own. A trial surface that touches a weak layer is clipped to follow it; how a surface that touches several is treated is chosen in Surfaces → Surface Options… (Section 2.7), and the steepest slice base allowed on such a surface in Section 2.1.9.

An anisotropic surface carries no strength and divides nothing: it says which way the bedding runs at each point, for a material whose strength model is Anisotropic Linear or Snowden Modified Anisotropic Linear and that selects it in Define Materials (Strength type). The orientation is read at the closest point of the polyline, not at the point vertically above, and the order in which the vertices are clicked decides the angle at a kink, so click them in order along the bedding.

Block Search objects#

With Surface Type = Non-Circular and Search Method = Block Search in Surface Options, four commands of the Surfaces menu draw the objects the search builds its trial surfaces from: a Window (a closed polygon), a Line (two clicks), a Point (one click) and a Polyline (clicked from one end to the other, and advancing in $x$). Right-clicking an object opens Block Search Object…, which shows its kind, the point options of a polyline and its Group ID. They are described with the search in Section 2.7.

2.2.3Editing boundaries#

The commands of Boundaries → Edit are tools: choose one, then click the boundary or vertex it applies to; Esc returns to the Select tool. Right-clicking a boundary or a vertex offers the most common ones directly (Drawing on the canvas). Geometry edits can be undone with Edit → Undo (Ctrl+Z).

Scaling and rotation are about the pivot typed in the dialog, (0, 0) by default, and not about the centre of the boundary.

2.2.4Reshaping the external boundary#

Expand / Shrink External#

Boundaries → Expand / Shrink External… (also in the right-click menu of the external boundary) adds ground to the model or removes it, as one undo step. It first asks for the method:

• Draw polyline (recommended): click the first point on the external boundary (the snap shows it), then the intermediate points outside the model to expand it or inside to shrink it, and the last point on the external boundary again; finish with Enter or Done. The program then asks whether the part of the old external boundary that was removed should be kept as a material boundary, which is useful to keep the original ground surface as the base of a fill or the outline of an excavation.
• Numeric offset (all edges by a fixed distance): moves every edge by the Offset distance (1.0 m by default): positive expands, negative shrinks.

Change Slope Angle#

Boundaries → Change Slope Angle… changes the overall angle of the slope face (the chord from toe to crest) and moves nothing else. Click the toe vertex and then the crest vertex of the external boundary; the dialog shows the current overall angle and asks how to change it.

Vertices of other boundaries that lie on the face move with it, so a layer that reaches the face still reaches it. Supports and loads do not move. The change is one undo step, and the status bar gives the old and new angles.

2.2.5DXF import and export#

Importing a drawing#

File → Import DXF… asks for a drawing and opens the Import DXF dialog, which lists every layer of the drawing and lets you say what each one represents.

The layer table gives each layer's entities and vertex count; layers without a proposed role are highlighted for you to decide. A layer is proposed from its name, ignoring case and separators: OGR_EXTERNAL, EXTERNAL, EXTERIOR or CONTORNO as the external boundary; OGR_MATERIAL, MATERIAL, SUELO or ESTRATO as material boundaries; OGR_WATER_TABLE, WATER_TABLE, WT or FREATICO as the water table; OGR_PIEZO or PIEZOMETRIC as piezometric lines; OGR_DRAWDOWN or DRAWDOWN as the drawdown line; OGR_CRACK or TENSION_CRACK as the tension crack; and a few more aliases in English and Spanish.

Preview runs the import without committing it and reports the vertex count before and after simplification, the number of regions found and whether the areas of the regions add up to the area of the external boundary, which is the decisive check. If they do not, a region did not close: raise the welding tolerance a little and preview again. Import is never refused: what could not be resolved is listed, and when there are problems the non-modal DXF Import — Problems panel opens after the import. It groups the problems by kind, with their severity and a suggested remedy; selecting one centres the canvas on it, double-clicking zooms in, and Go to problem and Zoom all do the same from buttons.

Exporting a drawing#

File → Export DXF… writes the model to a DXF drawing. The geometry goes to layers the importer recognises, so that a model can be exported, edited in a drawing program and brought back; everything that is a drawing of results goes to layers whose name starts with OGR_X_, which the importer ignores by default.

Options for things the model does not have (supports, loads, a mesh, results) are greyed out. DXF carries geometry only: materials, the material of a weak layer and the link from a material to an anisotropic surface are not exported. The status bar reports what was written and what was skipped.