We have a SPA tub and the front panel started leaking a fine sand. Later my wife noticed the scratching of a critter in the area. I removed the front panel and the entire front side of the tub was filled in with dirt.
It's a tight space, but with a mirror and flashlight I can tell the tub is sitting on a concrete "pillow" but the drain hole to the sewer pipe was not well constructed with about a 12 square inch hole around the drain pipe. I imagine that was for easy adjustment when connecting the plumbing.
I vacuumed as much of the dirt up as I could to expose the entire hole in the concrete but don't see an obvious gopher hole. I'll wait a little to see if he makes any repairs to give me a clue of where the tunnel might be, just so I have that information... and maybe "reward" him with a bit of poison.
I plan to put some concrete to cover the entire hole in the concrete. As someone else suggested I won't make it too thick. However, I'm wondering how thick the concrete should be. They are strong and persistent little buggers. Also, I'm wondering if some other material would be better, such as plaster.