膨胀土地基中单桩受扭非线性分析

膨胀土地基中桩-土相互作用较为复杂，膨胀土浸水对单桩扭矩承载特性的影响机理尚未明确。该文基于荷载传递法建立桩身位移控制方程，考虑膨胀土浸水隆起对桩侧摩阻力及桩端边界条件的影响，利用有限差分法求解出桩侧竖向摩阻力的分布。在此基础上，结合边界元法提出膨胀土地基浸水后单桩受扭的非线性分析方法，揭示了桩侧环向摩阻力的分布规律，并通过模型试验验证了该文方法的正确性。研究表明:膨胀土地基浸水膨胀引起的地表隆起会使单桩的扭矩承载力大幅度降低，常规的计算方法高估了基桩的极限扭矩和桩-土体系加载刚度，偏于不安全；浸水前单桩的桩身扭矩沿桩身近似线性减小，浸水后桩-土界面“滑移段”的扭矩接近于桩顶扭矩，“未滑移段”桩身扭矩近似线性减小；随着膨胀土膨胀率的增加，单桩的抗扭能力降低，且桩-土界面“滑移段”的长度也在增加。

NONLINEAR TORSIONAL ANALYSIS OF A SINGLE PILE IN AN EXPANSIVE SOIL FOUNDATION

The pile-soil interaction in an expansive soil foundation is complicated, and the influence of the inundation swelling deformation on the torsional bearing characteristics of the pile foundation remains unknown. In this paper, the displacement governing equations for the pile shaft were proposed considering the load transfer method. The distribution of the vertical resistance of the shaft was calculated by the finite difference method considering the influence of the swelling deformation of the expansive soil on the shaft resistance and the boundary condition of the pile tip. A new nonlinear analytical method for the torsion of a single pile due to the inundation swelling deformation was proposed incorporating the boundary element method. The distribution law of the circumferential frictional resistance of piles was revealed and the effectiveness of the proposed method was verified by model tests. It was shown that the ground heave induced by the swelling of the expansive soil foundation would significantly reduce the torsional bearing capacity of the single pile. In addition, the safety is threatened because the conventional calculation method overestimates the ultimate torque of the pile and the loading stiffness of the pile-soil system. The torque of a single pile approximately decreases linearly along the pile shaft prior to the inundation swelling deformation. However, the torque of the pile shaft is close to that of the pile top and the torque of the pile shaft decreases rapidly in the “unslip section”. The length of the interface “slap section” increases and torsional force capacity of the single pile decreases with the increase of the swelling ratio.