[SWHouston]

I used not to have (a Fortress) one, but I just kept on "nibbling" on the weak spots, and eventually....

One easy "fix" for a low place or one where something is digging under a Fence is...
Just get a regular bag of Concrete, pour it directly into the hole/low spot and wet it with water. It will harden up, level out the hole, and be nearly impervious to something digging under. Putting it in dry, allows it to seep down through the ground, and really make a tough deep barrier.