Water bubbles up through slab near vent pipe in wall. I can see down about 12 inches with a light. As we have a lot of ground water and the CI is about 50 years old I expect to have to cut the slab and replace any damaged CI and refill the trench. Should i use sand or rock to refill? The dirt we have is clay like and moist all the time. The CI pipes in several nearby homes have deteriorated and had to be fixed. I'm assuming the worst case and preparing for the dig.