反拱曲面异形桩异形效应理论分析及透明土模型试验研究

The abnormal pile with inverted arch surface changes the traditional concept which means that the pile shape is circular, otherwise square. There are inward-concaved and inverted arch surfaces along the pile circumference. Two popular abnormal piles with inverted arch surface are the Y-section and X-section piles, both of which have been widely applied in the soft soil treatment in the highways of Zhejiang and Jiangsu and have been incorporated in the relevant national and local standards. The engineering practices show that the economy and bearing capacity of abnormal piles with inverted arch surface are significantly better than those of equal-area circular piles, and its bearing characteristics have abnormal effect. Currently, most studies are focused on the macro aspects which demonstrate that the abnormal pile with inverted arch surface has its own superiorities in the bearing capacity and settlement controlling when compared with circular pile. However, the mobilized mechanism of abnormal effect is not fully understood. Therefore, the abnormal effect has been the scientific and technical issue to be solved firstly for establishing the analytical theory of abnormal piles with inverted arch surface. According to the usual and transparent soil model tests, through the way of combining the numerical simulation and theoretical analysis, taking the Y-section pile as the research carrier, considering the three dimensional stress state, and based on soil arching and cavity expansion theories, the methods with abnormal effect taken into account are established for the analysis of the skin friction, ultimate pile base resistance and settlement of Y-section pile. These methods for Y-section pile are expanded and applied to other abnormal piles with inverted arch surface, such as X-section and H-section piles etc., which can reveal the differences in abnormity effect and bearing mechanism between abnormal pile and traditional circular pile. This project is of great theoretical and practical significance for the unsolved scientific and technical issues of abnormal piles with inverted arch surface.

反拱曲面异形桩改变了桩"非圆即方"的传统理念，桩周具有向里凹的反拱曲面，以Y、X形桩应用最为广泛，已在浙江、江苏公路软基处理中较大规模应用，并被列入国家及地方相关规范；其经济性及承载力明显优于等面积圆形桩的特点已被工程实践所证明，同时表明其承载特性具有异形效应。目前对其研究多侧重于证明承载力和沉降与圆形桩相比具有优越性的宏观方面，其异形效应发挥机理尚未明确，异形效应已成为建立其分析理论首先要解决的科学技术问题。本项目通过常规及透明土模型试验、数值模拟和理论分析相结合的手段，以Y形桩为载体，考虑桩周土体的三向应力状态，基于土拱效应和小孔扩张理论，建立考虑异形效应的Y形桩桩侧摩阻力、桩端极限承载力及沉降分析理论，并将其拓展用于X形桩、H形桩等反拱曲面异形桩研究，揭示反拱曲面异形桩区别于传统圆形桩的的异形效应及受力机理。本项目针对反拱曲面异形桩亟待解决的科学技术问题，具有较大的理论及现实意义。

反拱曲面异形桩以Y、X形桩应用最为广泛，已在浙江、江苏等公路软基处理中较大规模应用，其经济性及承载力明显优于等面积圆形桩的特点已被工程实践所证明，同时表明其承载特性具有异形效应。目前对其研究多侧重于证明承载力和沉降与圆形桩相比具有优越性的宏观方面，其异形效应发挥机理尚未明确，异形效应已成为建立其分析理论首先要解决的科学技术问题。本项目通过常规及透明土模型试验、数值模拟和理论分析相结合的手段，以Y形桩为载体，在与传统圆形桩对比分析的基础上，揭示了反拱曲面异形桩区别于传统圆形桩的异形效应及受力机理。主要研究内容：（1）反拱曲面Y形桩桩周侧土压力及侧摩阻力异形效应分析理论研究。（2）考虑异形效应的Y形桩桩端极限承载力分析理论研究。（3）考虑异形效应的反拱曲面异形桩分析理论及受力机理研究。主要研究成果及关键数据：（1）考虑桩周土体的三向应力状态，建立了Y形桩桩周土拱效应分析理论，基于小孔扩张理论， 建立了Y形桩桩周侧土压力分布的简化分析模型，进而建立考虑异形效应的侧摩阻力分析理论。（2）通过桩端透明土模型试验，揭示了Y形桩桩端土体破坏模式基础上，建立了其桩端极限承载力分析理论，进而建立了Y形桩竖向承载力分析理论。（3）建立了考虑侧摩阻力异形效应的Y形桩沉降分析理论基础上，集成考虑异形效应的Y形桩分析理论，并将其拓展应用于X形桩、H形桩等反拱曲面异形桩的分析理论建立、异形效应分析及机理研究，揭示了反拱曲面异形桩区别于传统圆形桩的异形效应、受力及荷载传递机理。（4）基于Y形桩与圆形桩承载机理对比模型试验，关键数据如下：（1）与等截面圆形桩相比，Y形桩极限承载力是圆形桩的1.60倍，桩侧表面积是圆形桩的1.56倍，极限承载力的提高与桩侧表面积增大效果基本一致，在各级荷载作用下圆形桩的端阻力均小于Y形桩的端阻力，异形桩端截面增大了端阻力；（2）与等周长圆形桩相比，Y形桩极限承载力是圆形桩的0.84倍，但混凝土用量为圆形桩的0.41倍，单位体积混凝土极限承载力为圆形桩的2.04倍，在各级荷载作用下Y形桩侧摩阻力均大于圆形。本项目研究内容针对反拱曲面异形桩分析理论建立首先要研究解决的科学技术问题，可解决目前工程亟需的同时，还有助于丰富和发展刚性桩的试验和分析理论，具有一定的理论及现实意义。