# Surface Altering Optimization

The **Surface Altering Optimization** option in the Slip Surface Options dialog is a local search method which uses the results of the primary Search Method (e.g. sphere or ellipse from Cuckoo Search or Particle Swarm), converts this to a spline surface, and searches for a spline surface with a lower safety factor.

To enable this option:

- Select the
**Surface Altering Optimization**checkbox in the**Slip Surface Options**dialog. - Select the
**Options**button to customize the optimization search parameters (Conversion Resolution, Iteration Tolerance, Max Iterations, Max Concavity Angle). Default options are in effect.

By default the Surface Altering Optimization checkbox is ON. It is recommended for most analyses.

## Surface Altering Optimization

*Slide3* employs an efficient slip surface search method known as Surface Altering (SA) optimization. SA is a powerful tool to yield lower factors of safety by modifying geometry of a given slip surface. SA is a novel approach based on a derivative-free constrained nonlinear optimization*.

Unlike the metaheuristic method, SA is a **local** optimization method.

- In SA, the input surface (e.g. sphere or ellipsoid) is first converted to a spline approximation.
- The converted spline surface can be formed based on a 5x5, 7x7 or 9x9 grid of control points. This can be set via the
**Conversion Resolution**option. - After conversion, control points defining the surface are modified in a way that minimizes the factor of safety.
- The optimization process is stopped when either the maximum number of iterations is reached or when the optimization does not make any further progress.

*Bound Optimization BY Quadratic Approximations (BOBYQA) developed by Powell in 2009.

## Surface Altering with User Defined Surfaces

If the Surface Generation Method = **User Defined Surfaces**, you can apply the SA Optimization to the User Defined Surfaces. The SA Optimization uses a User Defined Surface as the starting point for the optimization, in exactly the same way as it is applied to the global minimum surface obtained by a primary search method (e.g. Cuckoo Search or Particle Swarm).

## Surface Type = Multi-Planar

If the Surface Type = **Multi-Planar**, the Surface Altering Optimization is NOT available, since the Multi-Planar surface option assumes that you wish to search for wedge-type failure surfaces formed by the intersection of planar surfaces.

**Anisotropic Surface Altering**

Surface altering has been improved to clip the slip surfaces to conform to the shape of anisotropic surfaces and planes. For instance, regions of the minimum slip surface with an anisotropic surface colored in blue as shown below will be improve as shown in the figure below: