The stability assessment techniques used in CHASM is Bishop’s simplified circular method (Bishop, 1955). At each time step of the simulation, pore pressures, both negative and positive, are incorporated directly into the effective stress determination of the Mohr–Coulomb equation for soil shear strength. The Fredlund et al. (1978) criterion is used for the unsaturated portion. This provides input into the limit equilibrium technique for derivation of the minimum factor of safety (the ratio of restoring to destabilising forces), with temporal variations arising from hydrodynamic responses and changes in the position of the critical slip surface (Wilkinson et al., 2002). This analysis involves a numerical search for the slip surface that minimises the factor of safety.CHASM uses the Bishop method of slices to perform slope stability analysis.
The automatic grid search specification is included which can be specified in terms of grid location, size of grid and search radius. The slip circle is then itself specified in terms of the initial radius and radius increment. The search procedure for the minimum factor of safety is then carried out by successive steps in the slip grid and changes to the radius of the slip circle. This search is carried out every hour during the simulation run time.
On completion of the simulation, a graphical output shows the grid slip surface and associated factor of safety at each hour during the total model runtime