It is not so suitable to use 2D (plain strain) f.e. Plaxis. This is because the approach will not be able to model one of essential features of d-wall installation procedure, e.g. the utilisation the benefits of arching by division of panel excavation in segments. It is more appropriate to use 3D f.e. packages in this case. You can modify the density and specify internal pore pressure (within the trench) during trench excavation stage.
In 2D, perhaps you may try use axisymetric model in 2D f.e. But be mindful that the arching effect may be overestimated. It is better for you to calibrate with actual site instrumentation data.