In the first option, when you apply an acceleration load to a translational joint, that is applying the acceleration to the linear translation of the joint. It is a motion input, not an inertial acceleration load that acts on all mass at the same time, but acts only to displace the joint.  If the soil acts like a very soft elastic body and is sitting on a rigid platform that suddenly accelerates upward, the bottom of the soil will move with the rigid rectangle in the first instant, while the soil at the top will have no motion.  That is very different than applying an inertial acceleration load where all the soil will experience the acceleration in the first instant.

In the second option, I forgot that you can't assign a Fixed Support to a Rigid body. Just keep the joint but change it to a Fixed joint.