It is extremely important to check the terrain for gaps and close any gaps. Even tiny gaps can cause issues with the physics engine. Below is a quick and easy method to highlight the issues, and allow you to quickly fix them in 3ds Max.
XView in Max can allow you to see overlapping verts for debugging gaps.
- Top left corner of viewport, click and on xView Tools, pick overlapping vertices.
- You can get it to select them too. You may wish to weld these together. It works on a mesh by mesh basis.
- You can customize the tolerance
- Overlapping show in green.