Surplus sliding force
Surplus sliding force Fn is introduced as a force acting in the bottom point of the slip surface - see picture. With this force, the limit equilibrium can be reached for the desired value of design safety factor Fsd and its value can be useful for the design of a retaining wall. The surplus sliding force has a reasonable value if the design safety factor is higher than the actual one. When the actual safety factor is high enough (the slope stability is acceptable), surplus sliding force value is equal to zero.
Surplus sliding force Fn
The surplus sliding force can also be used as a criterion of optimization. In that case, the optimization process searches the slip surface with a maximum value of surplus sliding force. This kind of optimization process can be used, if the slope stability for actual slip surface is not acceptable, because in case of an acceptable slip surface, the surplus sliding force is not calculated (equals zero). For acceptable slip surface stability, the optimization process uses as a criterion the factor of stability FS only.