Washington, DC contracted folks who had no knowledge of how to replace city sidewalk at base of mortared stone retaining walls and put concrete above the bottom 2 inch bottom smooth border when they didn't dig deep enough for 4 inch depth sidewalk and have created major trapping places for dirt and subsequent crabgrass, and water that freezes making stone faces to split off. What's the best way to correct this error? Or what's the best "stuff" to fill these error-created pockets? Pulling out crab grass with needle-nose pliers is a real pain for something I never had to bother with until DC hired the unknowing incompetent sidewalk contractor. Really bad part was that the sidewalk didn't need to be replaced as it had no cracks, no tree root heaving, etc., other than maybe the shade of concrete, and it's now showing the gravel, and I don't use salt to melt ice, so the entire block of sidewalk is all different colors anyway.