岩溶区桩基冲切破坏模式及安全厚度研究

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 ， 值越小，冲切破坏体底部直径 越小；反之，冲切破坏体底部直径 越大。一般情况下， 的值在2.0～4.0之间。岩石抗压与抗拉强度的比 也是抗冲切安全厚度确定的主要影响因素，随 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。

Study of punching failure mode and safe thickness of pile foundation in karst region

Contraposing the deficiencies in studying the punching failure mode for pile foundation in karst region,combining the load-carrying capability characteristics of the concrete slab,the punching failure mechanism of cave roof at pile tip is established by using limit analysis principle which accords with practical engineering in karst area.Introducing Griffith nonlinear rock strength criterion,based on work-energy principle,the ultimate load of rock strata at pile tip anti-punching failure is derived.Then,the generatrix equation of the punching failure body is obtained by the variational principle.Furthermore,the calculation formula of safe thickness for the rock strata anti-punching failure is obtained by differential equation.Parameter analysis shows that the punching failure mode mainly depends on the ratio n of compressive and tensile strength,the smaller the value of n is,the smaller the bottom diameter of the punching failure body,and on the contrary,the bigger it would be.Under normal circumstances,the value of d1 /d is in 2.0 to 4.0.The ratio n is also a main influencing factor determining the safe thickness of anti-punching failure.As increasing of the value of n,the safe thickness of security can be increased appropriately.For limestone region,the safe thickness generally taking 2 to 3 times the diameter is reasonable.Comparative analysis result of an engineering example shows that the calculated values by the presented method agree well with the measured data.