Best way to break down large terrain into smaller, faster-processing sections?

I am new to ORD and have a very large Terrain model for a multi-mile freeway project.  What is the best way to break the Terrain down into smaller Zones of the project?

Is there a way to copy a portion of an existing Terrain by Fence?, Or to copy the whole large one and delete everything outside a fence or element?  I am open to any suggestions and would really appreciate the advice from more experienced users.



Using release if it matters