Following the study by Conte, Troncone and Vena (2017), the passive resistance is fully mobilized only down to a depth X form the excavation level (see here), with X that is less or equal to the embedment depth, D. As the net pressure changes during the earthquake, the portion of the soil where the passive resistance is fully mobilized progressively increases. Consequently the depth X increases as well.
An example of the evolution of the depth X during the earthquake provided by DReW Seismic is shown in the figure below.
Evolution of the depth X during the considered earthquake
The user must be careful that the depth X provided by DReW Seismic is never greater than the embedment depth. In addition, it is necessary to check that X takes positive values.