# Particle Swarm Search

The **Particle Swarm Search** is one of the Search Method options in **Slide3** for locating critical slip surfaces. The Particle Swarm Search is also known as Particle Swarm Optimization (**PSO**).

The Particle Swarm Search is a metaheuristic search method in which randomness is introduced in some steps of the algorithm.

To enable a Particle Swarm Search:

- In the
**Slip Surface Options**dialog, set the**Search Method**=**Particle Swarm Search**. - Set the
**number of Mins**to**One**or**Multiple.**

The Particle Swarm Search requires no user-defined search objects, the algorithm will run automatically when you select Compute.

**One Min or Multiple Mins**

It is often the case that a slope will have multiple critical regions that may fail, instead of a single critical region. However, slope stability analyses generally focus on the search for the single *global* minimum surface.

With the **Multiple** min option, the user can now let the algorithm search for multiple *local* mins instead of the single critical one – this is called multimodal optimization (MMO). The parameter of note is the “radius span of search space.” The number of local mins found will be equivalent to the number of particles in the search. As such many of these will be very similar. The radius option filters out the surfaces that are similar and only takes the most critical local minima in each region. The default value of 10% is recommended. The user can also set a maximum value for mins to view as well as a maximum FS on the viewed mins.

The Multiple option considers both the global minimum computed from the One option as well as the local minima computed from the Multiple option and provides the user with the three most critical minima. This means the global minimum of the “Multiple” option will always be at least as critical as or more critical than the one from the “One” option. As with all non-circular search methods, optimization is strongly recommended with PSO. It is worth noting that since optimization is being performed on several mins instead of the single global min, computation time tends to be longer with the MMO or multiple min option.

## Particle Swarm Search Method

- Search starts with a random population of failure surfaces in the search scope (known as particles). Each particle contains values of the parameters which define the shape of the slip surface. For example, a spherical slip surface can be defined using an entrance point, an exit point, and a radius. As these parameters are varied, different slip surfaces are obtained.
- Particle Swarm Search can use either spherical, ellipsoidal or spline surfaces
- The search proceeds by updating each particle for the next iteration:
**One min case (unimodal)**: Updating parameters in Vi are selected randomly such that the new particle is inspired by the best-found solution among all particles (SG) and best-found solution by that particle in the previous iterations (SB)**Multiple min case (multimodal):**Updating parameters in Vi are selected randomly such that the new particle is inspired by the*two closest*neighbouring particles (N1, N2):

- Search process is stopped after a finite number of iterations