Answer is Sand.
Crab prefer sandy soil, because sand is easy to dig and remove, so it makes easier for them to make burrow. Clay is hard, so it is not easy to dig burrow. Silt collapses quickly. Crabs live in sand and burrows up to several feet deep. These burrows are close enough to the ocean to allow water to seep in for moisture, yet deep enough to protect them from any predators or the accidental human foot. Mangroves, marshes and the oceanic coastline are preferred location for crab to burrows.
