How stratify a topographic surface?

Hi,
I have to layered a topographic model construct from an XYZ file . I import the file as an interpolation function and then construct the parametric surface. Besides I construct a block (block1) of the same depth and width of parametric surface. I convert to solid the block with the parametric surface,and obtain csol1; so the block result divided into two domains.

Now I need to stratify this object betwen the plane z=0 and the parametric surfacesurface . Because the surface is accident and rough, the process can create many subdomain, also isolated in the inner part of the surface (for example one or more mountains in the middle of the surface).
I try at first only with a layer, at 66 m of elevation (z), but the problem is that I obtain only the subdomains adiacent to lateral border of the xy domain (of the surface) and no the inner subdomains.
I proceeded in this way: I create a block with the same base of block1 and that arrive at 66m of altitude. I convert to solid csol1 with this second block. But I don't obtain some subdomain, and also the respective inner edge don't form.
Do you know what I miss and how can I do?
I attach the model file.
Thank you,
best regards
Patrizia Canepa