Looking at the picture you attached I assume that you actually restrained the vertical displacements of the model defining fixed supports at the top edges of the solid. My proposal would be to apply some elastic support to the nodes generated at the bottom of the solid or define elastic surface support for the bottom side of the solid instead.
I'm sorry but the value of the elasticity (or the decision to just apply rigid supports) for the bottom of the soil is going to be your choice. You are the person who knows what kind of the structure is modeled and what existing boundary conditions are in reality. I can only try to help you in understanding how to use the available functionalities in Robot.