Cut a mesh, or convert multiple meshes into a Brep

Hello everyone ! 

First I will explain you what I'm trying to do : 

I have a set of multiple points which correspond to my topography site points. I did a Delaunay mesh of this points in order to see how the terrain of the project looks like. Then I choose a medium flat level, and I am basically trying to know how much quantity of earth (volume of earth) is upper and how much is under this level. So I tryed to make a volume out of this mesh and then to split it with a plane surface (which would correspond to my flat level for the project). 

And I have a bug with the fact to either extrude the mesh and split it with the plane surface (my reference level for the project), either making a Brep out ouf 4 meshes and then split it, either intersect the delaunay mesh directly with a plane surface; either (last solution) : intersect a Brep with a delaunay mesh. Is this even possible ? :)

So basically my questions are : Is it possible to extrude a delaunay mesh and how ? Is it possible to split a brep with a delaunay mesh and how ? Is it possible to make a Brep out ouf 4 meshes (including one delaunay mesh) and how ? 

Thanks a ton in advance, and all the best to all of you !

Best regards,

José