Gophers or moles? I am probably mistaken, but thought that gophers dug primarily to build nests (dens, whatever). Moles, on the other hand, dig in pursuit of food, and chew through almost anything in their paths.
Around here, it's a problem (moles & underground wires, etc), so we bury everything at least 12" deep, and that solves it. Moles only work near the surface.
LLoyd