在实际工程中，常见天然的或通过人工换填形成的上部砂土、下部黏土的层状地基，目前关于这种双层地基极限承载力和破坏机制研究还不够深入。通过有限差分法建立双层地基数值模型，分析基底粗糙程度、砂土剪胀角和超载对地基破坏模式及极限承载力的影响，并根据有限差分法计算结果对强度加权平均法、应力扩散法和冲剪破坏法等现有实用计算方法的估算正确性进行评价。研究结果表明，基底粗糙程度对极限承载力的影响随着砂土内摩擦角的增大而减小；当剪胀角较小时，剪胀角变化对承载力的影响更为明显；当下层黏土强度较小时，超载的作用更明显。强度加权平均法由于低估破坏面影响深度导致砂土层权重较大，计算结果偏大；应力扩散法忽略了砂土剪切强度，在砂土层较厚时出现低估；冲剪破坏法由于可较为精确地计算砂土破坏面上抗剪强度和被动土压力，是3种实用方法中计算最准确的，当砂土厚度和黏土强度较大时，建议按太沙基经验公式对下卧黏土进行局部剪切破坏修正后确定极限承载力。
In engineering practice, a layered soil with sand overlying clay is often witnessed in naturally deposited or artificially backfilled grounds. However, limit insight has been gained into the failure mechanism and bearing capacity of two-layer soil. A numerical model on the basis of a finite difference method is developed for studying the influences of the footing roughness, dilation angle and surcharge on the failure mode and ultimate bearing capacity of the soil foundation. Based on the numerical results, the existing practical methods are evaluated, including the weighted average method, the load spreading method and the punching shear method. Our results indicate that the influence of the footing roughness on the bearing capacity decreases with the increase of the friction angle of soil. The dilation angle has remarkable effect on ultimate load when the dilation angle is small. Besides, the influence of surcharge becomes significant for low-strength clay. The weighted average method overestimates the bearing capacity, since it overestimates the weight of the strength of sand due to a conservative prediction of the depth of the failure slip. The load spreading method is considered to underestimate the bearing capacity when the depth of sand is large due to neglecting the shearing resistance of sand. The punching shear method, in which the vertical side sand block is utilized and shearing resistance of sand is taken into account, gives a reasonable solution for small sand depth and low-strength clay. In addition, the accuracy of the calculation can be improved if the local shear failure in the clay subgrade is taken into account.
