Experimental studies on the behaviour of single pile under combined vertical-torsional loads in layered soil

Abstract Pile foundations may experience torsional loads induced by eccentric horizontal loads arising from wind, wave or ship collisions in offshore environment, in addition to axial loading from the supported superstructures. 1g model experiments were conducted in the laboratory to investigate the behaviour of single piles under independent vertical (V) and torsional load (T), and combined vertical-torsional (V-T) loads. Four types of layered soil profile of SC (sand-over-clay), CS (clay-over-sand), SCR (sand-over-clay-over-rock) and CSC (clay-over-sand-over-clay) were considered. The vertical load-displacement response, the torsion-rotation response, and the load transfer under two loading sequences (i.e., V→T and T→V) were observed. For the piles embedded in SC and CS, the bearing capacity envelopes in V-T plane and their simplified calculation formula were obtained as a design reference for engineers. The results indicate that the response of piles subjected to independent loading is substantially different from combined loading, and the soil profile and the load sequence have a significant effect on the bearing behaviour of piles. Compared with pure torsional pile, the ultimate torsional bearing capacity of the piles in SC under the combined V→T loading decreases by about 13-25%. However, the ultimate torsional bearing capacity of the piles in CS and SCR under the combined V→T loading increases by about 39-69% and 2%, respectively. In comparison to independent vertically loaded pile, the ultimate vertical bearing capacity of the piles in SC, CS and CSC under the combined T→V loading decreases by about 3% to 16%. This study shows that a complex interaction of vertical and torsional loads in various layered soils needs to be considered in a practical design.