Abstract: 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.

Key words: pile foundation, karst, limit analysis, work-energy equation, failure mode, safe thickness