循环荷载作用下单桩动力模型试验与桩−土界面特性研究

通过开展红黏土中单桩轴向循环振动模型试验，研究不同循环荷载比和加载频率对桩长期动力特性的影响，从桩侧土剪切刚度和侧阻退化两方面出发，对循环荷载作用下桩顶累积沉降机制进行分析。在FLAC3D中，实现能够反映剪切刚度疲劳退化的修正Hardin-Drnevich（H-D）模型，并对常法向刚度（CNS）循环剪切下侧阻退化进行数值模拟。试验发现，循环荷载幅值是桩顶累积沉降变化的重要影响因素；桩顶动刚度在加载初期要先经历一个迅速降低的短暂过渡阶段，之后则不随振次的增加而改变；桩身振动在桩周土中引起的超孔压较小，有效应力的降低不足于使侧阻力发生较大程度的退化；随着加载速度的增大，桩顶动刚度和加速度均随之增大。采用修正H-D模型得到的理论滞回曲线与数值结果基本吻合，验证了程序编制的正确性。

Study of dynamic model testing of single pile and behaviors of pile-soil interface under cyclic load

The performance of small-scale model piles embedded into red clay and tested under cyclic axial loadings is presented in this paper. The influences of cyclic loading levels and rates on long-term dynamic behaviors of pile are observed, and the mechanics of accumulated settlement is analyzed from the views of shear stiffness softening and skin friction degradation. Furthermore, the modified Hardin-Drnevich (H-D) model, being capable of reflecting the fatigue degradation of shear stiffness, is achieved in the FLAC3D, and the behavior of shear resistance degradation during quasi-static cyclic shearing under constant normal stiffness (CNS) condition is numerically investigated. Results indicate that the growth pattern of the permanent deformation is significantly affected by the cyclic loading amplitude; and the pile dynamic stiffness experiences a significant decrease in a short transition period, after which it remains invariable with cycles. The excess pore pressure caused by the shaft vibration is rather small; and the induced effective stress reduction can not sufficiently result in large friction degradation. The dynamic stiffness and acceleration of pile top increase simultaneously with the loading rates. With the modified H-D model re-developed in FLAC3D, the hysteresis curve of an easy case is well consistent with the theoretical result, which verifies the credibility of the program.