In a perfect world you would have an outside perimeter drain around the foundation at about the level of the basement floor. You may have had that and it failed sometime in the past when some one put the sump inside to solve that problem. Now you have water finding a path to the sump and like a river it is taking the sand with it. You want to make sure the landscaping is sloped away from the house and if the down spouts are going into pipes around the house, devert them out in the yard away from the house. There are people that can raise the slab by pumping foam or concrete under it but you would like to stop the water problem first. Stopping the water may be difficult with the slab sloping toward the house. How deep is the basement floor in relation to the slab outside? That's the question I should have asked earlier.