Unwanted Zig-zag lines in the model

I have a model with the main domain with few tunnel cross sections. I have modelled tunnel segments and ground (main domain) separately and positioned at one place.
The main domain and others have been partitioned (Split into blocks) but later have seen each block has zig-zag stripe lines over it which are causing poor meshing.
How to remove those zig-zag lines and make a better m

Please help.

These lines are found to be Surface edges and surface iso-curves. How to remove unwanted surcface edges in Rhino?

try _MergeAllFaces for polysurfaces.