基坑土体卸荷平面应变试验离散元数值分析

对基坑开挖影响范围内土体的应力路径进行平面应变试验离散元数值模拟，以研究结构性与卸荷形式对坑周土体宏微观力学特性的影响。首先，将一个描述土颗粒间胶结效应的简单三维胶结接触模型植入三维离散元软件PFC3D；其次，对初始K0固结状态的重塑土、结构性土试样分别进行常规三轴以及平面应变条件下4种不同卸荷应力路径的离散元模拟；最后，对经历不同卸荷形式的坑底土体单元进行再加荷模拟。模拟结果表明，在卸荷过程中，被动区土体峰值强度以及破坏时竖向应变随卸荷比增大而增大，且其强度小于主动区土体强度；在卸荷、再加荷过程中，被动区土体峰值强度随卸荷比增大而增大，但均小于不卸荷而直接加荷条件下的峰值强度；由于结构性的存在，土体由应变硬化向应变软化过渡，且强度增长；结构性与卸荷形式显著影响土体体积改变。在微观尺度，增大卸荷比或结构性均会增大垂直大主应力方向的平面上的法向接触力，进而提高其强度。

Distinct element analysis of plane strain test on soil unloading around a foundation pit

To investigate the influence of structure properties of soil and unloading form on macro- and micro- mechanical behaviors of soil around a foundation pit, a series of plane strain simulation tests were conducted on the soil within the region affected by pit excavation using three-dimensional (3-D) distinct element method. First, a simple 3-D bond contact model, representing inter-particle bonding effect, was implemented into 3-D distinct element analysis code PFC3D. After that, conventional triaxial and plane strain tests on remolded and structured soil were simulated under four stress paths. Finally, the reloading process was simulated on the bottom soil element after unloading. The results show that the peak strength of the soil in passive region and the vertical strain corresponding to failure during unloading increase with unloading ratio, and the strength is smaller than that of the soil in active region. During reloading, the peak strength of the soil in passive region increases with unloading ratio, and is smaller than that without unloading or immediate loading. In addition, volume changes are significantly influenced by structure effect and unloading form. On the microscopic scale, increase in unloading ratio and enhancement in structure degree will increase the normal contact forces in a plane perpendicular to the direction of the maximum principal stress and thus enhance the strength of the soil.