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Study on microscopic deformation mechanism of sand soil shear zone in direct shear test
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摘要: 剪切带的形成与发展是影响土体变形破坏的关键因素。利用数值模拟试验方法，对砂土直剪试验中剪切带的形成和演化机制进行研究。试验中采用条块法对试样进行标记，以观测其变形及发展过程；并记录剪应力及剪切位移的变化，分析试样的应力应变特征；最后从细观角度对颗粒试样的平均转动量、配位数及孔隙变化率等参数进行统计分析，以揭示试样的细观变形规律。研究表明：砂土剪切带的形成是一个渐进的过程，呈现出累进破坏的特征；且剪切过程中颗粒试样会发生局部变形并形成狭长的剪切带，剪切带的厚度受平均粒径及颗粒摩擦系数的直接影响，当颗粒粒径为1.25倍平均粒径且颗粒摩擦系数为0.5时，剪切带厚度达到最大值；此外，剪切带区域的强力链较为密集，通过对强力链进行光密度及面积占比分析发现峰值时刻最为显著（平均光密度约为1.4，强力链面积占比约为52%），这表明力链网络的结构变化与剪切带的形成及演化具有同步性；同时发现剪切带内、外颗粒体系的平均转动量、平均配位数及孔隙变化率等细观参数存在显著差异，说明剪切带对外荷载的响应较为强烈。Abstract: The formation and development of shear zone is the key factor affecting the deformation and damage of soil. We use numerical simulation test method to study the formation and evolutionary mechanism of shear zone in direct shear test for sand. In the test, the sample was marked by the strip method to observe the process of deformation and development, to record the shear stress and shear displacement, and to analyze the stress-strain characteristics of the sample. Finally, the statistical analysis is made on the average rotation, coordination number, porosity change rate and other parameters of the sample from a mesoscopic angle to reveal the mesoscopic deformation law of the sample. The research shows that the formation of sand shear zone is a gradual process, showing the characteristics of progressive failure. During the process of shearing, the particle sample will be deformed locally and form a long and narrow shear zone. The thickness of the shear zone is directly affected by the average particle size and the frictional coefficient of the particles. When the particle size is 1.25 times the average particle size and the frictional coefficient of particle is 0.5, the thickness of the shear band reaches a maximum value. In addition, the strong force chain in the shear zone region is denser, and the time of peak is most significant by analyzing the optical density and area ratio of the strong chain (the average optical density is about 1.4, and the area ratio of the strong chain is about 52%), which indicates the structural changes of the force chain network are synchronous with the formation and evolution of the shear zone. At the same time, it is found that the mesoscopic parameters such as the average rotation amount, the average coordination number and the porosity change rate of the internal and external particle systems in the shear zone are significantly different, which reflects that the shear zone is more strongly affected by external loads.
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Key words:
- sand /
- discrete element method /
- direct shear test /
- shear zone /
- mesoscopic mechanism
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图 6 剪切带厚度与平均粒径及摩擦系数的关系
Figure 6. Relationship between shear band thickness and average particle size and friction coefficient
表 1 模型参数
Table 1. Model parameters
初始
孔隙率平均粒径/
mm颗粒密度/
（kg·m−3）颗粒法向
刚度/（N·m−1）颗粒切向
刚度/（N·m−1）粒间摩擦
系数颗粒
数量/个墙体摩擦
系数墙体法向
刚度/（N·m−1）墙体切向
刚度/（N·m−1）0.16 0.7 2 643 1.4×108 1.0×108 0.5 9 683 0 1.4×108 1.0×108 
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表 2 剪切状态特征
Table 2. Shear state feature
时刻 剪位移/mm 体积变化/mm3 剪应力/kPa 所处状态 A 0 0 0 初始固结阶段 B 0.10 −0.007 32 体变率为0 C 0.76 0.101 50 峰值点 D 1.78 2.120 28 软化 E 4.83 3.623 20 软化 F 6.00 4.216 25 残余阶段
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