Elastic-plastic torsional behavior of single pile in double-layered non-homogeneous subsoil

To discuss the torsional behavior of a single pile in double-layered non-homogeneous subsoil, assuming the shear modulus of each single layer as the power function distribution and considering a nonlinear variation of the limit friction resistance along the pile-soil interface, the governing differential equation for the pile shaft under torque is established. Then, the internal forces (torque T) and deformations (torsion angle Φ) at the pile top and along the pile shaft are obtained respectively. By simplifying the solutions to single-layered non-homogeneous or double-layered homogeneous subsoil, good agreements are found between the calculated results and the available achievements. Besides, a parameter analysis of pile top T-Φ curve shows that increasing the shear modulus of pile shaft G_p helps to control the torsional angle at pile top. And a double value of pile radius r₀ will result in an increase of 4~6 times the torsional resistance at pile top. Similarly, the performance of torsional piles can be improved by greater values of the nonlinear coefficients α₁ and α₂ and the surface modulus μ₁. Besides, comparative analysis results of torsional influence factors at pile top I_Φ show that the pile shaft can be seen as a rigid body for ζL<0.2, while a value of ζL>5.0 will cause a flexible pile with the torque transferred to the pile tip small enough to be ignored. Moreover, for a combined value of ζL>5.0 and shear modulus ratio of upper to lower layers μ₁/μ₂>1.0, the main contribution to torque resistance comes from the 0.2L surface subsoil, while the 0.4L surface soil layer will determine the torsional behavior of pile shaft for ζL>10 and μ₁/μ₂<1.0.