穿越溶洞型嵌岩桩竖向承载机理及设计计算方法研究

The design calculation method for rock-socketed pile penetrating karst cave is one of the key technologies in the bridge construction. The project that based on the existing research results will proceed system model experiments，numerical analysis and theoretical study for rock-socketed pile penetrating karst cave. Firstly，the model tests of rock-socketed pile penetrating monolayer，double-layer and three-layer karst cave will be designed through the similarity principle .Through model tests the features in different karst cave occurrence condition of pile-rock interface friction resistance evolution，pile side friction resistance along the pile length，the size of the pile side resistance，end resistance load sharing ratio，radial stress diffusion and so on will be examined. The vertical bearing and failure mechanism for rock-socketed pile penetrating karst cave foundation will be comprehensively explained. Based on the FLAC3D numerical analysis technology，the influence of different cave radius，the separation distance ，cave height and the thickness of the plate for ultimate bearing capacity and failure mode for rock-socketed pile penetrating karst cave will be analysied. By investigating the deformation process and the corresponding potential energy changes under load of pile foundation，surrounding rock of pile and end bedrock，the limit state of energy balance equation of pile foundation and pile end bedrock will be build. Then the limit damage load wil be obtained by the principle of minimum potential energy. Combining with catastrophe theory analysis method，the buckling mechanism and buckling failure mode of rock-socketed pile penetrating karst cave will be researched. Based on energy theory and catastrophe theory, the calculation method of buckling stability calculation method for rock-socketed pile penetrating karst cave will be propounded.

穿越溶洞型嵌岩桩设计计算方法是岩溶区桥梁建设的关键技术之一。项目拟结合已有研究成果，进行系统的穿越溶洞型嵌岩桩的模型试验、数值分析和理论研究。首先，通过相似原理设计穿越单层溶洞、双层和三层溶洞三种情况下嵌岩桩模型试验。考查在不同溶洞赋存条件下桩-岩界面的摩阻力发挥、桩侧摩阻力沿桩长的分布、桩端阻力的大小、端阻荷载分担比及径向应力扩散等性状，全面阐释穿越溶洞型嵌岩桩的竖向承载和破坏机理。基于FLAC3D数值模拟技术，研究不同溶洞半径、溶洞竖向间距、溶洞高度及隔板厚度对穿越溶洞型嵌岩桩极限承载力及破坏模式的影响。通过考察桩基、桩周岩体、桩端基岩三者在荷载下的受力变形过程及相应势能变化，分别建立桩基、桩端基岩的极限状态能量平衡方程，由最小势能原理求得嵌岩桩极限承载力。结合突变理论分析方法，研究穿越溶洞型嵌岩桩屈曲机理与失稳模式，建立基于能量原理与突变理论的穿越型溶洞嵌岩桩屈曲稳定计算方法。

针对岩溶区穿越溶洞桩基承载机理及破坏模式相关问题，开展了穿越溶洞桩基极限承载力及破坏模式室内模型试验，揭示了桩基极限承载力随溶洞高度变化的规律，并将不同高度溶洞下桩基和溶洞顶板的破坏模式进行了分类,结合数值模拟分析方法验证了破坏模式及极限承载力随洞高的变化规律。基于试验结果，依据能量法原理建立了穿越溶洞桩基总势能方程。根据桩基屈曲的突变特性，引入尖点突变理论，建立了桩基在顶部弹嵌、底部嵌固情况下失稳的尖点突变模型，导出了该体系的分岔集方程，进而通过分析其失稳条件，提出了穿越溶洞桩基屈曲临界荷载计算方法。研究表明：(1)穿越溶洞桩基发生屈曲的最小洞高为6d。穿越小洞高溶洞桩基在单轴抗压强度小于桩端及桩侧极限阻力时发生桩身材料受压破坏，反之发生桩端基岩破坏。穿越大洞高溶洞桩基在屈曲失稳承载力小于桩端及桩侧极限阻力时发生桩身屈曲失稳破坏，反之发生桩端基岩破坏，且随着洞高增加，桩基屈曲失稳承载力大致呈线性降低；(2) 穿越溶洞桩基极限承载力取决于桩身材料强度、溶洞高度、桩周岩体强度等多种因素，桩身材料受压破坏、桩基屈曲破坏、桩端基岩破坏三者极限承载力中的最小值即为穿越溶洞桩基极限承载力；(3)理论方法适用于桩基发生屈曲的情况，且与试验结果吻合较好。研究提出的岩溶区穿越溶洞桩基承载力计算方法，可指导岩溶区桩基的设计与施工。