I live in a first generation suburb. The retaining wall behind me is starting to crack, but it is not mine to permanently fix. After I concrete over any cracks in the retaining wall, I would like to do whatever I can prevent rats from burrowing next to that wall. I'm not a rat fan.
I would like to lay down some material along the retention wall and make it difficult for the rats to burrow. How wide and deep should I go? Fill it with big stones that are hard to move by a rat? fill with smaller stones that won't leave enough room for a rat to wiggle through? Bury some wire mesh under the grass along the wall?