Volume from mesh

I'm trying to figure the volume between two meshes (excavation for bridge pier).  I've created one mesh for the excavation limits which extends (upward) to the existing ground surface (meeting the Level: Terrain_Ex_Calc_Triangles_GS), the other mesh is the existing ground surface.

I'm stuck on how to get the volume between the two.

I've attached the 3D file.

Using MS SS3 C 08.11.09.608

Thanks.[View:/cfs-file/__key/communityserver-discussions-components-files/5922/3D_5F00_Excavation.dgn:940:0]