Ultimate Capacity of Earth‐Covered Slabs

Three tests were conducted on identical one‐way reinforced concrete slabs. The tests were conducted using water over a waterproof membrane to apply a uniform surface pressure under three conditions: (1) With the test slab surface flush; (2) with the slab surface one foot (30.5 cm) deep in a clay soil backfill; and (3) with the slab surface one foot (30.5 cm) deep in a sand backfill. The reaction structure supporting the slabs was rigid enough to effectively prevent translation or support rotation at the clamped edges and to prevent in‐plane thrust in the slab generated by lateral earth pressures. The surface flush slab failed at about 174 psi (1.22 MPa), failure in the clay backfill occurred at about 171 psi (1.20 MPa), and failure in the sand backfill occurred at about 835 psi (5.83 MPa), overpressure. The almost fivefold increase in capacity in the sand backfill was due primarily to soil arching in the high shear strength sand backfill. These test results indicate that soil arching is much more important than current calculations indicate at this very shallow burial depth.