Abstract: 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.

Key words: pit excavation, plane strain test, unloading, distinct element method