Earthwork volumes with Rock layer

I have a question about getting different earthwork volumes.  I am in really deep rock cut.  I'd like to get the volume between my proposed ground and the top of rock layer.  then i'd like to get the volume between the top of rock layer and top of existing ground (dirt).  (total of 3 surfaces)  My proposed surface has the property type "Design", my existing ground has the property type "Existing".   Is there a certain type that my rock surface is supposed to be?  (substratum, design...?)  I can change it to different types, but but all that does is effect what layers are looked at when doing End Area Volumes.   Is there a trick to getting the different volumes?....like turning on my 3rd surface after doing an initial EAV or somehting, and then running it again with the 3rd one on?....can't seem to get the separate volumes correctly.   I would greatly appreciate any help!