3D terrain integration

I have the DEM's of pre and the post event. But to slice the pre dem stl from the post dem stl, is it necessary that the DEM's should have thickness like the one given in GeoXPM tutorial. Can i cut the dem's directly in the GeoXPM without having the thicknesses in the stl file. Or is there any other way, please help me out. 
Also, if we obtained the sliding mass and the stationary object then how we are going to flow that sliding mass over the terrain, because the tutorial present in the site doesn't mention anything about this.
Please help me.......