Is it possible to make a terrain model that covers several unconnected areas?

Hi,

Is there any way to make one terrain model that covers several unconnected areas? 

I need to model and calculate volumes for all the ditches beside a rail track. There are about 30 and they are separate from each other.

Whenever I try to make a single terrain model, either from element or alternate surface from corridor, they triangulate between and I can't delete the triangles. This is obviously incorrect and will lead to errors in the volume calculation.

As a work-around I thought about making a complex terrain by adding the individual ditch surfaces to the existing terrain but that is way too slow and creates a heavy file. The fastest solution I have is to make the 30 separate volume calcs and add them up in a spreadsheet.