The effect of pore water pressure plays an essential role in the analysis of stability and safety of geotechnical engineering. Formulation of a perfect elastoplastic model satisfying D-P criterion is presented based on Viesca’s poroelastoplastic model considering saturated pore pressure. Under the undrained condition

the equivalent strength parameters of geomaterial shall be used

which brings out the change of yield surface shape; and the expression of exact integration algorithms is modified based on the closest point projection method. The effect of undrained pore pressure is thus considered on the elastoplastic response of pore geomaterial; and the evaluation of geotechnical structure is implied by using the unbalanced force inheriting the concept of overstress. A heterogeneous slope suffering rainfall infiltration is simulated by using nonlinear finite element method code

which proved the practicability of the model. The nonequilibrium evaluation of slope stability can be quantified by combining our procedure with the unbalanced force. It is also found that the increment of Skempton’s coefficient weakens the layer of slope suffering sustained rainfall infiltration and threatens its safety.