Tension Cracks
Add tension cracks, optionally water-filled, at the slope crest.
Add tension cracks, optionally water-filled, at the slope crest.
A tension crack is a closed polygon, typically near the crest, that truncates the slip surface. Soil cannot sustain tension, so where a trial surface would pass through the crack the surface is cut and replaced by a vertical face. If the crack holds water, the resulting hydrostatic thrust is applied to that face, reducing the factor of safety.
Drawing a tension crack
- Click Add Tension Crack in the Geometry panel.
- Place the polygon vertices on the canvas.
- Click Done Tension Crack.
A tension crack requires at least 3 points. Use Clear Tension Cracks to remove all cracks.
Water-fill modes
Set how the crack is filled with water from the properties pane. The available modes are:
| Mode | Description |
|---|---|
| Dry | No water in the crack; no hydrostatic thrust. |
| Fully Saturated | The crack is full to the top. |
| Partially Saturated | Filled to a user-set percentage of depth (0–100%). |
| Follow Water Table | Water level in the crack tracks the model water table. |
| FEA Pore Pressure | Crack water pressure comes from the FEA pore-pressure field. |
The default mode is Dry. For Partially Saturated, the fill percentage defaults to 50%.
The FEA Pore Pressure mode reads from the finite-element seepage solver, whatever the Pore Pressure Source. With Defined Phreatic Surface the slices keep the phreatic pore pressures and only the crack water uses the FEA field.
In a transient groundwater scenario, an FEA Pore Pressure crack makes LEM Compute calculate the factor of safety per stage: the initial state and every stage ticked Calculate FOS, each with that stage's crack water. The stage player only chooses which stage result is shown.
How a tension crack clips the slip surface
Where a trial surface intersects the crack, the surface is truncated and a vertical face is inserted from the crack to the surface. Which side of the crack is removed depends on the failure direction of the analysis: for a right-to-left failure the right portion is cut off, and for a left-to-right failure the left portion is cut off. This keeps the retained sliding mass on the correct side of the crack.