How to control crabs that come beneath a the deep wall and impose holes surrounded by grass? ...
Answers: Don't know where you are but it sounds resembling you have a specie of environment crab.
If possible, you should sink some hardware cloth either surrounded by front or behind your the deep wall--with a guage small enough to hang on to the crabs out.
In our area, we aren't allowed to alter ocean walls in any road without a authorization. I don't know if you have any sort of regulation similar to that in your nouns.
My thinking is that you should put the wire between the seawall and your property.