How to make very big terrain to calculate physics

i have a task to make very big terrain which need to be loaded all at once, even without vizualization, it can be only physics.

we have real world height grid. Based on this grid i build TerrainGrid.
But we need to make simulation of objects moving(vehicles) in very big distanse (like 100 km). It even can be only physics, I need to get position of every object after a certain period of time repetitively. When I try to use Terrain grid it only shows central Quad and load only with camera moving, but i need that everything have loaded independently of camera.Now vehicle can fall off the edge if camera not move that way and new tile not loaded

Sorry if the question is badly formulated, i will answer all clarifying questions.
I will appreciate any help

Thank you in advance.