To reveal the failure mechanism during punch-through , bearing capacity and the formation of cavity for layered soil were simulated by finite element model. The concept of limiting safety distance for spudcan was presented. and the effects of the strength ratio between the upper and lower soil layers, the relative thickness of the top soil layer, the normalized shear strength and non-homogeneity factor on bearing capacity of spudcan were described in detail. Then the engineering formulation of bearing capacity of spudcan in stratified soil profile with a relatively thin layer of stiff clay overlying a weaker layer wasgiven. The results show that the punch-through phenomena are related to the relative thickness and strength of the top soillayer and the strength ratio between the upper and lower soil layers. The contribution of the normalized shear strength and non-homogeneity factor to the limit bearing capacity of soil is no more than 5％ . The end bearing capacity of foundation soil has nothing to do with the top soil, depending on the strength of the lower soft soil.%分析多层土条件下,桩靴承载力和孔穴的变化规律,揭示桩靴发生穿刺现象的实质;进一步提出桩靴极限安全深度的概念,系统描述上、下土层强度比、上层土相对厚度、下层土标准化抗剪强度和土体强度非均匀系数等参数变化对桩靴承载力的影响;给出适用于工程使用的上硬下软猫土中桩靴承载力系数的计算方法.研究结果表明:桩靴发生穿刺的深度主要受到上层土相对厚度、上层土强度和上下土层强度比的影响;标准化抗剪强度系数和土体强度不均匀性系数对于桩靴承载力系数极限值的影响都不超过5%,地基土最终承载力与上层土无关,取决于下层软土的强度.
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