不同胶结砂土力学特性及胶结破坏的离散元模拟

DEM simulation of mechanical behaviour and bond breakage of different cemented sands

采用离散元法研究不同胶结砂土的宏观力学特性以及微观胶结破坏,其中一种胶结砂土胶结特性由离散元商业软件PFC2D中的胶结接触模型(Contact bond model)进行控制,另一种则采用蒋明镜等提出的无厚度改进胶结接触模型(改进的蒋氏模型)控制。首先,将改进的蒋氏模型引入PFC2D;其次,使用PFC2D对不同胶结强度和不同围压下的上述两种胶结砂土进行双轴压缩试验模拟,对比分析了两种不同胶结砂土的模拟结果。结果表明:蒋氏试样(胶结特性由改进的蒋氏模型控制)应变软化和体积剪胀显著,峰值内摩擦角基本随胶结强度增大而增大;而PFC试样(胶结特性由Contact bond model控制)在高胶结强度时应变软化和体积剪胀性比较显著,低胶结强度时表现为应变硬化和体积剪缩,峰值内摩擦角随胶结强度增加而减小。离散元模拟结果在一定程度上与已有室内实测结果相符,蒋氏试样更能反映胶结砂土的主要力学特性。由胶结破坏微观信息统计可知,蒋氏试样中胶结点受拉破坏率远大于受剪破坏率,而PFC试样中两者相当,且蒋氏试样中的总胶结点破坏速率峰值要大于PFC试样。通过对改进的蒋氏模型参数分析可知,低围压条件下,试样宏观力学特性与胶结破坏形式对切向胶结强度与法向胶结强度的比值较为敏感。

A numerical investigation of the macromechanical behavior and bond breakage of different cemented sands using distinct element method（DEM） is presented.These sand specimens are cemented by means of the contact bond model introduced in a DEM commercial software,PFC2D and the modified bond model without thickness proposed by Jiang et al（modified Jiang model）,respectively.First,the modified Jiang model is incorporated into PFC2D;Then,a series of biaxial compression tests are performed on specimens with different bond strengths under various confining pressures,and the DEM results are compared and analyzed.The results show that the Jiang sample,whose bond behavior is controlled by the modified Jiang model,exhibits pronounced strain-softening,and shear-dilatancy,and its peak frictional angle increases with bond strength;the PFC sample,whose bond behavior is controlled by the contact bond model,demonstrates strain-softening,shear-dilatancy with high bond strength,but strain-hardening,and shear-shrink under with low bond strength,and its peak frictional angle decreases with bond strength.The simulated results are in accordance with the experimental ones to some extent,especially the Jiang sample can well capture the major mechanical behavior of bonded sands.In addition,the number of broken bond due to tension is lager than that due to shear in the Jiang sample,while they are the same in the PFC sample.The studies on microscopic parameter of the modified Jiang model show that under low confining pressures,the macromechanical behavior and bond breakage form are sensitive to the ratio of tangential bond strength to normal bond strength.