Effect of Shaft Roughness on the Bearing Capacity of Rock-Socketed Friction Piles

Rock-socketed piles are a common type of end-bearing pile, but when there is deep sediment or holes at the pile bottom, the load is primarily supported by side resistance. In this study, based on such conditions and considering the influence of pile shaft roughness, model tests were conducted to investigate the bearing characteristics of rock-socketed friction piles. The results show that the failure mode of rock-socketed friction piles is the formation of a penetrating cylinder in the rock layer, with the cylinder diameter directly approximating the pile diameter. The load–displacement curves of the test piles are steeply variable. After reaching the ultimate bearing capacity, the residual bearing capacity of rough test pile is approximately 60% of the ultimate bearing capacity, while that of smooth test pile is 72.4%. The maximum side resistance of the test pile is located within a depth range of 25 mm below the soil–rock interface, and the upper load of 41.0% to 48.9% on the test piles was born by the pile side resistance within this depth range. As the roughness factor (RF) increases gradually from 0.0 to 0.3, the ultimate bearing capacity of the test pile shows nearly linear growth, the ultimate displacement increases sharply first and then decreases slowly, and both the axial force attenuation and the percentage of side resistance within the depth range of 25 mm below the soil–rock interface gradually increase slightly. In this paper, two existing methods are employed to calculate the ultimate bearing capacity of friction piles under the conditions of this study. Based on a comparison of the results, the applicable conditions for each method are proposed. The findings of this study can serve as a reference for the design of rock-socketed piles in similar geological formations.