Different approaches based on soil dynamics could be used to evaluate the hydrodynamic thrust developed by the sediments. As a first approximation CADAM3D uses a two-layer fluid model along the u/s face. It is thus assumed that there is liquefaction of the sediment during the earthquake. The sediment is considered as a liquid with a density larger than water. The Westergaard formulation is then used to compute the added mass (FERC 1991). The use of Westergaard solution for the sediment is an approximation to more rigorous solutions considering the two-layer fluid model, as presented by Chen and Hung (1993).

In that context, the active earth pressure for the static thrust component is questionable. If the assumption of a two-layer fluid model is retained, it would be appropriate to use K =1 (silt=fluid) for the static condition. The oscillatory motion of the u/s face is thus assumed to “liquefy” the sediment layer in contact with the dam.

As for the reservoirs, the dynamic sediment pressure is influenced by an inclination of the upstream face of the dam. CADAM3D applies the same rules for slope correction to dynamic sediment pressure distribution as for reservoirs.