Weak Layer Handling
How the search snaps slip surfaces to weak layers.
Weak layers are thin, low-strength seams that often control the failure mechanism: bedding planes, liquefiable layers, foundation contacts, or construction interfaces. Because a slip surface tends to follow such a seam, JW Slope can clip candidate surfaces onto the weak-layer geometry so the search captures the controlling mechanism.
A weak layer is defined by a polyline path through the model, optionally carrying its own strength properties. When a candidate slip surface touches a weak layer, the segment of the surface that overlaps the seam is replaced by the weak-layer path, so the surface runs along the seam where it is geometrically valid.
Handling modes
The weak-layer handling mode controls what happens when a surface touches one or more weak layers:
| Mode | Description |
|---|---|
| Snap to highest (default) | The surface is clipped onto all touched weak layers, snapping the surface up onto the seams. A single clipped surface is produced. |
| Consider all combinations | For the touched weak layers, every subset (every combination of which seams to follow) is generated and evaluated as a separate surface. The most thorough but most expensive option. |
| Random case generation | One touched weak layer is selected at random per surface, producing a single clipped variant. A sampling compromise between the other two modes. |
How clipping works
For each weak layer the surface touches, the overlapping surface segment is replaced by the weak-layer segment between the surface/seam intersection points. A replacement is accepted only if it raises the surface (lifts it toward the seam) and yields a valid surface — strictly progressing horizontally with at least two points. Clipping is applied across passes so a surface can be routed along several seams.
Automatic weak-layer identification
An advanced option, off by default, can identify weak layers automatically from FEA elastic stress results rather than requiring you to draw them. It scans the mesh for material interfaces where the local shear factor of safety is low and the materials contrast in strength, then traces connected low-FS paths into candidate weak-layer surfaces. The identified layers are combined with any user-drawn weak layers and made available to the search.
Note Automatic identification depends on FEA elastic stress results and is intended for advanced workflows. For most analyses, draw the weak layers explicitly and use Snap to highest, switching to Consider all combinations when multiple seams could each control the failure.