岩溶区桩基桩端岩层破坏模式及安全厚度计算方法研究

The safe thickness calculation of pile end rock strata in karst region is one of the key technologies in the bridge construction. The problems of pile in karst area is more complex than the rock-socketed pile in common area.The pile end rock strata deform under pile tip load, and then destroy due to the rock strength reach limit in the process of load increases. So，how to determine the failure mode of pile end rock strata and the corresponding safe thickness calculation method is to be desired. This study that based on the existing research results will proceed system indoor experiments for failure mode of pile end rock. Considering the main factors of failure mode，the test design is orthogonal. The deformation development process and the failure specific pattern of pile end of rock, the pile load transfer character in bearing failure process, skin friction and tip resistance load sharing percent and so on basic subject under various conditions will to be analysised. Combining with the numerical simulation method the mechanical properties and failure mechanism of pile end rock will be clarified.Based on the energy theory, through the investigating deformation process and corresponding potential energy incremental change of the pile ，surrounding rock soil and pile end strata in three of the load at all levels, the pile -pile end rock system energy balance equation will be established.Using the principle of minimum potential energy, the safe thickness calculation method that is appropriate for the pile foundation in karst area on various working conditions will be proposed.Thus the method can used to guidance the safe thickness design for pile foundation in the karst area.

岩溶区桩基桩端岩层的安全厚度计算是岩溶区桥梁建设的关键技术之一。岩溶区桩基问题较普通地区嵌岩桩更为复杂，桩端岩层因在桩端荷载作用下产生变形，在荷载增大的过程中由于岩层强度达到极限值从而导致破坏。如何确定桩端岩层的破坏模式及相应的安全厚度计算方法是急待解决的问题。本研究拟结合目前已有研究成果，进行系统的室内桩端岩层破坏模式的试验。考虑影响破坏模式的主要因素进行正交设计，分析各种工况下桩端岩层的受力变形发展过程、破坏的具体模式及桩基承载破坏过程中荷载传递性状、桩侧桩端荷载承担比等基本课题，结合数值模拟分析方法阐明桩端岩层受力性状和破坏机理。基于能量理论，通过考察桩基、桩周岩土体、桩端岩层三者在各级荷载下的受力变形过程及相应势能变化增量，建立桩基-桩端岩层体系的能量平衡方程，采用最小势能原理，提出适用于岩溶区基桩各种工况下岩层安全厚度的计算方法，由此指导岩溶区桩基安全厚度的方案设计。

【摘要】岩溶区桩基桩端岩层的安全厚度计算是岩溶区桥梁建设的关键技术之一。岩溶区桩基问题较普通地区嵌岩桩更为复杂，桩端岩层因在桩端荷载作用下产生变形，在荷载增大的过程中由于岩层强度达到极限值从而导致破坏。如何确定桩端岩层的破坏模式及相应的安全厚度计算方法是急待解决的问题。研究结合目前已有研究成果，进行了1组不同桩端岩溶顶板的桩基的承载破坏试验和5组岩溶顶板承载力破坏试验，结合数值模拟分析方法阐明桩端岩层受力性状和破坏机理。基于试验结果，采用极限分析上限法原理，确定冲切破坏体的功能方程，并通过变分求极值原理获得冲切破坏体的母线方程，进一步通过微分得到桩端岩层抗冲切安全厚度计算公式。结合岩溶区桩基实践，分析抗冲切安全厚度影响因素的取值范围并进行参数分析，结果表明：(1) 冲切破坏体上下直径比 d1/d随岩体地质力学分类指标 RMR 值的增大呈非线性减小，说明岩体质量好的岩溶顶板其冲切所形成的锥形旋转体底部直径小，反之，旋转体底部直径大；(2) 根据不同的 d1/d 及 RMR 值，初始冲切角 为24° ～42°；(3) 厚径比 h/d 随 RMR 的增大呈非线性减小，表明岩体质量越好，所需安全厚度也越小；在岩体质量较好(RMR＞65)的情况下，h/d基本在 2.5 以下。提出适用于岩溶区基桩各种工况下岩层安全厚度的计算方法，由此指导岩溶区桩基安全厚度的方案设计。