The active backfill pressure on the dam is determined using the forces F, R and WR.

Figure 213

The force F acts at an angle ϕ (internal friction angle of the backfill) with respect to the normal of the assumed sliding surface (ab). The resultant R is inclined by δ on the downstream face (ae) of the dam. The weight WR (abcde) of the backfill acts at the centre of gravity of this section. The active earth pressure is maximized by choosing different sliding planes (ab) passing through the downstream foot of the dam. CADAM3D can consider several slide planes to maximize the stabilizing moment from F. The resultant R is assumed to act at the intersection of the line parallel to the sliding plane (ab) and passing through the center of gravity of the section (abcde) and the downstream face of the dam (ae).

By using active pressure instead of pressure at rest, the method is conservative. Its advantage is the possibility to calculate the slope of the failure plane (ab) of the backfill (passing through the downstream foot) instead of just assuming it. Calculations have shown (Carter 1970) as well as CADAM3D that this plane of failure is almost vertical.