考虑溶洞空间形态的岩溶桩基稳定性分析方法

Stability Analysis Method of Pile Foundation in Karst Area Base on Cavern Spatial Form

岩溶桩基的应用随岩溶地区交通工程建设的快速发展而越来越普遍,如何评价桩端岩溶顶板稳定性成为岩溶桩基设计的关键问题之一,针对目前桩端岩溶顶板稳定性分析平面假设的不完善性,考虑溶蚀作用形成的溶洞所具有的空间形态特征进行岩溶桩基稳定性分析。首先,将基桩作用下的岩溶顶板分别简化为固支梁、抛物线拱、圆拱与固支双向板等承载模型,采用结构力学与双向板分析理论建立不同模型的桩端岩溶顶板抗弯最小安全厚度计算方法;其次,通过计算结果对比分析,揭示岩溶顶板最小安全厚度随矢高的变化规律;在分析岩溶顶板冲切破坏与剪切破坏形式的基础上,探讨桩端岩溶顶板破坏模式的控制因素及其影响规律,进而获得桩端荷载、石灰岩抗拉强度、溶洞跨度与矢高等因素对桩端岩溶顶板承载特性的影响规律;然后,基于溶洞钻孔探测所得地质勘查信息构建岩溶桩基稳定性分析流程,提出考虑溶洞空间形态特征的岩溶桩基稳定性分析方法;最后,通过工程案例具体分析桩端岩溶顶板最小安全厚度及其破坏模式随矢高的变化规律。研究结果表明:桩端岩溶顶板破坏模式不仅与溶洞跨度、桩径有关,而且与溶洞形态及其矢高也密切相关,此外,石灰岩抗拉强度对岩溶顶板稳定性的影响同样较大,详细全面的工程勘察资料能使桩端岩溶顶板稳定性分析结果更接近实际情况。

Pile foundations are increasingly used in karst areas with the rapid development of traffic engineering construction. However, evaluating the stability of the karst cave roof underneath the bottom of the pile has become one of the key problems of pile foundation design in karst areas. The current stability analysis method for the karst cave roof underneath the bottom of the pile with the plane cross-section assumption is unsuitable. The stability of pile foundations in karst areas should be analyzed based on the spatial characteristics of the karst cave formed by dissolution. Firstly, the karst cave roof bearing the pressure from the pile is simplified as a clamped beam model, parabolic arch model, arch model, two-way slab model, or shell model. Then, calculation methods for the minimum safe thickness of the karst cave roof controlled by bending resistance are proposed based on structural mechanics and bidirectional plate theory. Secondly, the changing requirements for minimum safe thickness of the karst cave roof along with different arch heights are obtained from the contrastive analysis of calculation results. The factors controlling the failure mode of the karst cave roof and their effects are discussed by analyzing the punching failure, shearing failure, and bending failure caused by pressure from the pile bottom. The effects of the load underneath the pile bottom, tensile strength of limestone, and span and rise of the karst cave on the bearing capacity of the cavern roof are also discussed. Thirdly, the stability analysis process for pile foundations in karst areas is presented with the geological exploration information got from karst cave drilling. The stability analysis method for pile foundations in karst areas that considers the spatial characteristics of the cavern can then be obtained. Finally, the changing rules for minimum safe thickness of the karst cave roof and its failure mode with different arch heights are analyzed through one engineering case. The results show that the failure mode of the karst cave roof is affected by the span of the karst cave, pile diameter, spatial form, and rise of the karst cave. The influence of the tensile strength of the limestone is also significant. Detailed and comprehensive engineering survey data can make analysis results of the karst cave roof underneath the pile bottom close to the actual values.