循环加载下X形桩竖向承载特性模型试验研究

实际工程中桩基经常受到各种动荷载作用，如高铁路基中的加固桩长期承受列车行车时产生的循环动荷载作用，桩基在循环荷载作用下的承载特性研究对动荷载下的工程设计至关重要。X形桩是一种在传统圆形截面桩基础上发展而来的新型异性桩技术，其承载机制不同于传统圆形截面桩。为了深入研究X形桩在循环荷载作用下的动力特性和荷载与沉降规律，开展了砂土中X形桩竖向循环加载大比尺模型试验。试验结果表明，随着循环加载的进行，X形桩顶产生累计沉降，且循环荷载比越大，加载频率越高，桩顶沉降越快；循环加载初期，X形桩顶动刚度降低，桩身轴力响应增大，桩侧摩阻力发生弱化，之后逐渐趋于稳定，且桩身下半段侧摩阻力较大；在同等加载条件下X形桩与等截面圆形桩相比，承受动荷载能力较强，桩侧摩阻力较大，长期循环加载作用下产生的累计位移较小。研究结果可为X形桩在动荷载作用下的工程设计提供参考依据。

Model test of vertical bearing characteristics of X-section pile under cyclic loading

In practical engineering, pile foundations are often subjected to dynamic loads. For example, piles under high-speed railway are subjected to cyclic loading induced by train movement. Studying on the dynamic bearing characteristics of pile foundations under cyclic loading is of great importance in engineering design. The X-section pile is a new type of technique improved based on the traditional circular section pile. Its load-bearing mechanism is different from traditional circular section piles. The objective of this paper is to make further study on the dynamic characteristics and load-settlement relation of the X-section pile under cyclic loading; and then large-scale model tests on the performance of the X-section pile subjected to axial cyclic loading in sand are conducted. The results show that cumulative settlement of X-section pile is induced by the cyclic loading; and the settlement increases with increasing amplitude and frequency of the cyclic loading. At the initial period, the dynamic stiffness of X-section pile decreases and the axial stress of the pile increases. Furthermore, the friction resistance at the pile side decreases. After the initial period, they tend to be stable. These indicate that the dynamic performance of the X-section pile is better than the circular section pile; moreover, the friction resistance of the X-section pile is larger, the cumulative settlement of the X-section pile under long-term cyclic loading is smaller. The study results may provide some guidelines for the engineering design of the X-section pile under dynamic loading.