Triangulate Solid and control its points

Hi all,

Newbie here, I have difficulties to modify the ground for my architectural project.
The ground I imported in rhino is triangulated, but after I use boolean difference to implant the shape of my building, the ground is no more triangulated. Therefore, modifying the control points of the solid makes “ruled surfaces”, which I don’t want.
I’m looking for a way to triangulate my solid, going through meshes seems to heavy ?

rhino file (70Mo)

Hello - I would work with it as a mesh and not surfaces at all if you want all triangles. But is that actually what you want? My guess is you’d like the terrain ‘simplified’ to a surface, just judging from your image.