Spliting mesh

I spent several hours trying to split that mesh and already found several topics about splitting meshes in Rhino but I couldn’t find any solution. Unfortunately, my mesh is pretty big and every try takes quite a bit of time, so I decided to ask if any of you have any idea of how can I force mesh split to work in my case? Extrusion is just a cutter which should leave me with a cutout of a road from the mesh - instead of, it leaves me with some strange results.

I tried both Rhino 6 and WIP version.

File:
https://1drv.ms/u/s!Ara31Y5vLih0gas-kMv4epDgTglTeg

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Hello - whether or not it has to do with the split problem, the first thing I’d do is vastly simplify the surfaces in the polysurface.

-Pascal

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Thank you Pascal,

I sucessfully splitted the mesh after I simplified cutting geometry.