Cutting mesh surface with extrusion?

I imported a mesh surface created by photogrammetry (in *.obj) and I try to edit this surface in order to “dig” a pool: I can design the pool in 3D with Rhino but need to create a hole in my surface in order to insert this pool.
See this scren capture:

Can someone help me how to do so ?
(Please not using Mesh2Nurbs: it softens the surface too much)

Thank you in advance


use MeshSplit, it will create 2 surfaces and you can delete the inner one.