I'm endlessly revising my perimeter footing insulation project and need to know: how far below grade do subterranean termites tunnel? These would be normal subterraneans, not formosans. For some reason, my search-fu is failing and I cannot seem to find this information anywhere online.
According to this paper from the University of Nebraska, they can go up to 20 feet in the ground:
Subterranean termites are ground-dwelling social insects living in colonies. The two species found in Nebraska have similar habitats. These termites have the ability to adjust the depth of their colony (nest) in soil depending on temperature and moisture requirements. The colony may be 18-20 feet deep in the ground.
The paper goes on to say that the species they observed are local to Nebraska, however I think it demonstrates these things can dig deep, probably deeper than you can make a barrier. Other less-reliable sources cite depths up to 40 feet.