考虑嵌岩段侧摩阻力的溶洞顶板稳定性研究

Study on Stability of Karst Cave Roof Considering Side Friction Resistance of Rock-socketed Section

为进一步完善岩溶区桩基荷载作用下的溶洞顶板稳定性评价方法,根据嵌岩桩荷载传递机理及溶洞顶板承载特点,建立了考虑嵌岩段侧摩阻力及溶洞顶板自重的简化计算模型。从抗冲切、抗剪切和抗弯拉3方面对溶洞顶板稳定性进行了分析。假定溶洞顶板发生冲切破坏时,会产生一个冲切圆锥台,取该圆锥台作为分析对象,并考虑冲切体自重的影响,对冲切破坏面上的最大、最小主应力进行了求解,同时引入格里菲斯准则,对溶洞顶板的抗冲切进行了验算。基于莫尔判据,对两种剪切破坏模式进行了验算:(1)桩端岩层发生剪切破坏,其破坏面由桩侧截面延伸至顶板底端;(2)溶洞顶板边缘岩层发生剪切破坏,其破坏面由岩层表面贯穿顶板边缘。根据弹性力学的变分法对溶洞顶板底面应力进行了求解。考虑到溶洞顶板底面中心处拉应力最大,则在抗弯拉验算中将该点作为验算点。将本研究所提方法用到某公路大桥的溶洞顶板稳定性验算中,并将验算结果与未考虑嵌岩段侧摩阻力的计算方法进行了对比。分析表明:嵌岩段岩层侧阻力占嵌岩桩设计荷载比值可达11.5%,这说明嵌岩段侧摩阻力占承载力比例较大,且随着嵌岩深度的增长,该比例将继续增大。因此,当嵌岩深度较长,岩石质量较好时,在实际工程中应适当考虑嵌岩段侧摩阻力的发挥。

In order to further improve the stability evaluation method of karst cave roof under the load of pile foundation in karst area, according to the load transfer mechanism of rock-socketed pile and the bearing characteristics of roof, a simplified calculation model considering the lateral frictional resistance of rock- socketed section and the self-weight of roof is established. The stability of karst cave roof is analyzed from the aspects of punching shear resistance, shearing resistance and bending resistance. Assuming that the roof is punched, a punching truncated cone will be produced, taking the truncated cone as the analysis object and considering the influence of the weight of the punched body, the maximum and minimum principal stresses on the failure surface of the punched failure surface are solved, at the same time, Griffiths criterion is introduced to check the punching of the roof. Based on Mohr criterion, 2 shear failure modes are checked ： （ 1 ） shear failure occurs on the pile end rock, and its failure surface extends from the pile side section to the bottom of the roof; （2） shear failure occurs on karst roof edge, and its failure surface extends from the rock surface to the roof edge. According to the elastic mechanics variational method, the stress on the bottom surface of the cave roof is solved. Considering the maximum tensile stress at the center of the bottom of the cave roof, this point is taken as the checking point in the bending resistance test. The proposed method is used to check the stability of the cave roof of a highway bridge, and the calculation result is compared with the result obtained by the calculation method without considering the side friction resistance of rock-socketed sections. The analysis shows that the ratio of the side frictional resistance of rock-socketed section to the design load of rock- socketed pile can reach 11.5%, this shows that the frictional resistance of rock-socketed section occupies a larger proportion of bearing capacity, and the proportion will continue to increase as the embedded depth in the rock increases. Therefore, when the embedded depth is long and the rock quality is good, the side frictional resistance of rock-socketed section should be properly considered in practical engineering.