Generalized Shear Function
Define shear strength as a custom function of normal stress.
The Generalized Shear Function lets you define the shear-strength envelope
directly as a table of (normal stress, shear strength) point pairs. The solver
interpolates this curve to find the strength at each slice's stress. Use it for
non-linear envelopes that do not fit Mohr-Coulomb or Hoek-Brown, such as
laboratory-derived strength curves.
Defining the function
Select Generalized Shear Function as the failure criterion on the LEM tab.
A point table appears; enter at least two (sigma_n, tau) rows:
| Column | Unit | Description |
|---|---|---|
| Normal stress (sigma_n) | kPa | Stress value at which the strength is defined. |
| Shear stress (tau) | kPa | Available shear strength at that normal stress. |
Points are automatically sorted by ascending normal stress, and duplicate normal-stress entries are collapsed (the last value wins). At least two distinct points are required; if fewer are valid, the model falls back to a two-point line generated from a cohesion of 1 kPa and friction angle of 35 degrees.
Stress basis
A stress basis setting controls which stress the table is indexed against:
| Basis | Meaning |
|---|---|
| Effective normal (default) | The curve is read at each slice base's effective normal stress. The local tangent of the curve becomes the slice friction angle, and the intercept becomes the slice cohesion. |
| Effective vertical | The curve is read at each slice's effective vertical stress (vertical total stress minus pore pressure). The interpolated shear value is used directly as the available strength, with an equivalent friction angle of zero. |
Interpolation and range behavior
Between defined points, the strength is linearly interpolated.
Outside the defined range the function does not extrapolate along a new slope:
- Below the lowest defined normal stress, the curve uses the slope of the first segment (first two points).
- Above the highest defined normal stress, the curve uses the slope of the last segment (last two points).
In other words, the end segments are extended at their own slope rather than the curve being clamped flat or shooting off at an arbitrary angle. Define points that span the full range of normal stresses expected on the slip surface so the solver operates within your data.
Tip
Because the equivalent friction angle is the local tangent of the curve, keep the function monotonically increasing. A decreasing segment produces a negative tangent (negative apparent friction angle) at that stress.