The static failure plane used to calculate the backfill thrust depends on the simplified method of analysis selected by the user. However, the seismic failure plane of the backfill differs from the static failure plane. In fact, CADAM3D suggests using the internal friction angle value of the backfill as the seismic failure plane. However, the user can determine a different value than the one suggested by the software.

Once the seismic failure plane has been established, the effective seismic mass is the volume of backfill above the seismic failure plane. The seismic force of the embankment corresponds to the effective mass multiplied by the ground acceleration. The point of application of the backfill seismic load is at the centre of gravity of the effective mass. When areas of a backfill are submerged, the overburden weight (porous and permeable backfill) is used; for a saturated backfill, the total weight is used for the calculation of the seismic forces (in which case the Westergaard thrust is adjusted according to the porosity and permeability of the backfill).