I am wanting to fill in the area where my lawn has sunken. I assume it's settling around the sewer line? It's a 30 ft. x 5 ft. x .25 ft. area needing leveled out. By my calculations that's about 1.4 yards needed. I have pretty heavy clay soil so I'll be using a compost/topsoil mix. MY question is... Is 10% extra soil mix enough to factor in "settling"? I'd hate to have it sink in again.