Numerical Solution of Single Piles Subjected to Pure Torsion

Torsional load on pile foundations is induced by the action of eccentric horizontal forces on the supporting structures. Such loading not only initiates twist at the pile head, but also reduces its axial capacity significantly associated with increased settlement of the foundation. Inadequate design of piles under torsion may lead to disastrous consequences. Although several theoretical and experimental investigations have already been carried out on piles under torsional load as evidenced from the available literature, there is ample scope for further contributions on the influence of torsional load on pile foundation. This paper presents a novel numerical model (boundary element method) to analyze the response of a single, vertical, floating pile subjected to pure torsional load. The nonlinear stress-strain response of soil has been incorporated in the model by hyperbolic correlation, whereas the pile material has been idealized as elastic–perfectly plastic. The effect of progressive pile-soil slippage at the interface is considered. Apart from predicting the load-displacement response with incremental angle of twist of pile under pure torsion, the profiles for shear stress and angle of twist and the depth of interface slippage are some of the salient features captured by the proposed model. Validation of the model developed by comparing with existing models and experimental data indicate the suitability and accuracy of the solutions developed. However, minor discrepancies in the solution appeared owing to the simplified assumptions of perfectly plastic postpeak soil behavior and the absence of residual stress in the soil owing to pile installation. The proposed model is also applied successfully to selected case studies on concrete piles ($L/D$ ranged from 10–40) in medium-soft clay, medium-dense sand, and layered soil, and a set of design curves have been constructed.