Surface-Altering Optimization
Refine a found surface with surface-altering optimization.
Surface-altering optimization is an optional post-search step that refines the critical slip surface to find a lower factor of safety. After a search returns its best surface, this step perturbs the surface's control points — moving its endpoints and reshaping its interior — seeking a lower-FS geometry that an ordinary search grid may have missed.
The optimization uses the BOBYQA derivative-free algorithm (from the NLopt library, compiled to WebAssembly) to adjust control-point positions. Because it turns the surface non-circular, even circular search results can be improved.
Enabling
Surface-altering optimization is off by default. Enable it in the slip-surface settings to apply it after the search completes.
How it works
- The critical surface is resampled to a set of control points.
- Over several stages, the algorithm moves the endpoints and reshapes the surface curvature, evaluating the factor of safety with BOBYQA at each step.
- Stages repeat until the configured stage count is reached, or until the improvement in factor of safety falls below the minimum-change threshold.
Candidate evaluations are spread across background workers, with a controlled amount of starting-point variation between workers to broaden the search.
Parameters
| Parameter | Default | Description |
|---|---|---|
| Enabled | off | Toggles the optimization on. |
| Stages (iterations) | 3 | Number of optimization passes over the surface. |
| Circular control points | 48 | Number of points used to sample an incoming circular surface before optimizing. |
| Endpoint move fraction | 0.35 | Maximum endpoint movement, as a fraction of the surface width. |
| Curvature move factor | 1.5 | Scale governing how far interior points may move when reshaping curvature. |
| BOBYQA max evaluations | 20 | Maximum factor-of-safety evaluations per BOBYQA optimization. |
| Worker count | 0 (auto) | Number of background workers. 0 auto-detects an appropriate count. |
| Worker variation | 25% | Variation applied to worker starting points to diversify the search. |
| Minimum FS change | 0.0001 | Stops early when the factor-of-safety improvement between stages falls below this value. The early-stop check is enabled by default. |
The stage count, endpoint move fraction, curvature move factor, and BOBYQA evaluation limit are also adjusted together by the surface-optimization complexity slider.
Methods optimized
Surface-altering optimization runs per analysis method. By default it applies to Bishop Simplified, Janbu Simplified, Spencer, and Morgenstern-Price; you can restrict it to a subset of methods.
Note Optimization adds compute time, especially with many stages or methods selected. Enable it for final, critical-surface refinement rather than during exploratory runs.