Ensuring a safe foundation design in soft clay is always a challenging task to engineers. In the present study, the effectiveness of under-reamed piles in soft clay underlaid by stiff clay is numerically studied using the lower-bound finite element limit analysis (LB FELA). The bearing and uplift capacities of under-reamed piles are estimated through non-dimensional factors N_cul and F_cul, respectively. These factors increased remarkably and marginally compared to N_cul and F_cul of the piles without bulbs when the bulb is placed in stiff and soft clay, respectively. For a given ratio of undrained cohesion of stiff to soft clay (c₂/c₁), the factors N_cul and F_cul moderately increased with the increase in the length-to-shaft-diameter ratio (L_u/D) and adhesion factors in soft clay (α_s1) and stiff clay (α_s2). The variation of radial stress along the pile-soil interface, distribution of axial force in the under-reamed piles, and state of plastic shear failure in the soil are also studied under axial compression and tension. The results of this study are expected to be useful for the estimation of the bearing and uplift capacities of under-reamed piles in uniform clay and soft clay underlaid by stiff clay.

确保软粘土中安全的基础设计对工程师而言始终是一项艰巨的任务。在本研究中，使用下界有限元极限分析（LB FELA）数值研究了刚性粘土在软粘土中欠扩孔桩的有效性。欠扩桩的承载力和抗拔能力分别通过无量纲因子N _cul和F _cul估算。当将球茎分别放置在硬质和软质粘土中时，与没有球茎的N _cul和F _cul相比，这些因素分别显着增加和略有增加。对于给定比例的硬土与软土的不排水内聚力（c ₂ / c₁），所述因子Ñ _CUL和˚F _CUL适度与所述长度与轴直径比（的增加而增加大号_Ù / d）和软土粘附因子（α _S1硬粘土（）和α _S2）。还研究了在轴向压缩和拉伸作用下沿桩-土界面的径向应力的变化，欠扩孔桩中轴向力的分布以及土壤中塑性剪切破坏的状态。预期这项研究的结果将有助于估算均匀黏土和硬质黏土所覆盖的软黏土中扩底桩的承载力和抗拔能力。