基于冲切破坏模式的嵌岩桩桩端溶洞顶板临界厚度确定方法研究

An approach for determining the critical thickness of the karst cave roof at the bottom of socketed pile based on punch failure mode

桩端下伏溶洞顶板厚度是影响嵌岩桩竖向承载力的一个重要影响因素,在工程实践中,我国相关规范要求桩端平面以下的顶板厚度不小于3倍桩径。根据规范确定溶洞顶板厚度具有一定的经验性和笼统性,没有考虑溶洞顶板岩体质量,亦忽略了溶洞自身尺寸的影响。基于桩端岩体的冲切破坏模式,并结合广义Hoek-Brown准则和极限定理上限法建立了溶洞顶板临界厚径比(h/d,即顶板厚度h/基桩直径d)的计算方法,给出了不同岩体基本质量级别的临界厚径比建议值。结果表明:极限端阻力、岩石坚硬程度和岩体质量均对临界厚径比产生影响,极限端阻力越小、岩体质量越好、岩石越坚硬,则溶洞顶板的临界厚径比越小;以《工程岩体分级标准》(GB/T50218-2014)中岩体基本质量分级为参考依据,Ⅰ~Ⅳ级岩体的临界厚径比建议值分别是Ⅰ级岩体h/d≤2.41、Ⅱ级岩体2.41≤h/d≤2.96、Ⅲ级岩体2.96≤h/d≤3.73、Ⅳ级岩体3.73≤h/d≤4.91。

The thickness of karst cave roof at the bottom of socketed pile is a significant influence factor on the vertical bearing capacity of the pile. In practice, related engineering standardizations in China request that the thickness of karst cave roof should not be less than 3 times of the pile diameter. Obviously, this method determining the thickness of karst cave roof based on engineering standardizations is empirical and non-specific without consideration of rock-mass quality and karst cave’s size. In this paper, based on punch failure mode of rock mass around the tip of the pile, we develop an approach to determine the critical thickness-diameter ratio of karst cave roof( h/d, the ratio of karst cave roof’s thickness to pile’s diameter) through generalized Hoek-Brown failure criterion and limit analysis method. In addition, the suggested values of critical thickness-diameter ratio for different classifies quality of engineering rock mass are also supplied. The results show that the ultimate tip resistance, hardness degree of rock and rock-mass quality have significant influence on the critical thickness-diameter ratio. The smaller the ultimate resistance, the better the rockmass quality and the harder the rock, the smaller critical thickness-diameter ratio. Besides, this paper suggests that the critical thickness-diameter ratio for the rock-mass with Ⅰ~Ⅳ quality levels are: h/d ≤2.41 for Ⅰ level, 2.41≤h/d ≤2.96 for Ⅱ level, 2.96≤h/d ≤3.73 for Ⅲ level and 3.73≤h/d ≤4.91 for Ⅳ level.