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针对岩溶区桩基冲切破坏模式研究的不足,结合混凝土平板受力特性,运用极限分析原理确立了符合工程实际的岩溶区桩端溶洞顶板冲切破坏机制。引进格里菲斯非线性岩石强度准则,基于功能原理导出了桩端岩层抗冲切破坏的极限荷载,然后通过变分原理求得了冲切破坏体的母线方程,最后通过微分获得了岩层抗冲切安全厚度计算公式。参数分析表明:冲切破坏模式主要取决岩石抗压与抗拉强度之比n,n值越小,冲切破坏体底部直径d1越小;反之,冲切破坏体底部直径d1越大。一般情况下,d1/d的值在2.0～4.0之间。岩石抗压与抗拉强度的比n也是抗冲切安全厚度确定的主要影响因素,随n值的提高,安全厚度应适当增大。对于石灰岩地区,安全厚度一般取2～3倍桩径较为合理。工程算例对比分析表明,理论计算与实测结果吻合较好,其结果对岩溶区桩基设计有一定参考价值。
Aiming at the deficiency of punching failure mode of pile foundation in karst area and combining with the stress characteristics of concrete slab, the punching failure mechanism of roof caverns in the karst section at the end of the pile is established by using the principle of limit analysis. The Griffith nonlinear rock strength criterion is introduced to derive the ultimate load of punching failure of pile-end rockfill based on functional principle. Then the generatrix equation of punching failure body is obtained by variational principle. Finally, Cut safety thickness calculation formula. The parameter analysis shows that the failure mode of blanking mainly depends on the ratio of compressive strength and tensile strength of rock, the smaller the value of n and n is, the smaller the diameter d1 of the bottom of blanking failure body is. On the contrary, the diameter d1 of the bottom of blanking failure body increases. In general, the value of d1 / d is between 2.0 and 4.0. The ratio of rock compressive strength to tensile strength, n, is also the main factor that determines the safety thickness of punching shear. As the value of n increases, the safety thickness should be increased appropriately. For limestone areas, the safety thickness is generally taken 2 to 3 times the pile diameter is more reasonable. The engineering example comparative analysis shows that the theoretical calculation is in good agreement with the measured results, and the results have certain reference value for the design of pile foundation in karst area.