Combined Hydrology And Stability Model (CHASM)

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CHASM is an integrated slope hydrology/slope stability numerical model. The software has been developed by the Hydrology group to assist in the estimation of controls on slope stability for a scope of applications ranging from pre-site engineering investigations to evaluations concerned with the impact of bioengieering on slope stability. A commercial version of CHASM comprising a graphical user interface is distributed by the University.

Within CHASM, infiltration during rainfall is calculated using Darcy's law and vertical flow in the unsaturated zone is computed using Richards' equation solved in explicit form inside vertical columns. In the saturated zone water moves horizontaly at a speed given by Darcy's law according to the slope of the water table. Within the integrated model structure the hydrology scheme represents slope plan curvature (convexity and concavity) by varying the breadth of the columns. The effect of the 3-dimensional topography on water fluxes can thus be investigated and its impact on stability estimated.

The generated pore pressure field is then used as input to standard 2-dimensional stability analyses where the slip surface is located within the mid-plane of the 3-dimensional structure. CHASM uses limit equilibrium methods for the estimation of the slope factor of safety with an automated search procedure. Pore pressures, both negative and positive, are incorporated directly into the effective stress determination of the Mohr-Coulomb equation for soil shear strength. This allows derivation of the minimum factor of safety of the slope, with temporal variations arising from hydrodynamic responses and changes in the position of the critical slip surface.

The numerical scheme includes a surface cover model, which allows comprehensive investigation of the hydrological and geotechnical effects of vegetation on slope stability. Vegetation affects slope stability through the processes of rainfall interception, evapo-transpiration, change in hydraulic conductivity, root reinforcement and surface loading, all of which are included in the model.

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