Bearing Capacity of Piles Driven into Rock

An experimental study is presented on the point bearing capacity of piles in rock. The bearing capacity for granite, limestone, and sandstone was found to be approximately four to six times the unconfined compressive strength of the rock material when the load was applied perpendicular to the rock surface. The ultimate bearing capacity was affected by the inclination of the applied load and by the penetration depth. The ultimate strength decreased rapidly when the inclination exceeded 45°. The strength increased with increasing penetration depth. The increase for granite and sandstone was 25 to 50% when the depth increased from 0 to 1.0 times the point diameter. The corresponding increase for limestone was 60 to 70%.The point bearing capacity was calculated by the Coulomb–Mohr and by the Griffith failure theories. The test results indicated that the effect of the inclination of the rock surface and of the penetration depth for granite and sandstone was less than calculated. Satisfactory agreement was found for the limestone between measured and calculated values. Comparisons with the Griffith failure theory indicated that the measured failure loads were approximately twice the calculated failure loads.