PFC3D Mesoscopic Simulation of Self-Boring In-Situ Shear Pressure-Meter Model Test
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摘要: 自钻式原位剪切旁压试验(self-boring in-situ shear pressure-meter)以其独特的多级加载方式, 能够直接测出土体的强度和变形参数, 然而, 目前对探头周围土体变形机理研究较少.为揭示自钻式原位剪切旁压仪试验过程中测定器周围土颗粒变形机理, 应用PFC3D(particle flow code in three dimensions)颗粒流程序对自钻式原位剪切旁压试验进行了仿真数值模拟, 对多级加载过程中探头周围土体的位移场和应力场发展变化以及数值试样各阶段变形模量和土颗粒运动轨迹进行了分析研究.试验结果表明: 随着剪应力的逐级施加, 中间区域颗粒的位移量不断增大, 且位移矢量方向性更加显著.径向应力在探头附近两侧形成近似呈对称分布的数个"应力核"; 竖向应力在探头两侧形成扁平状应力带, 且在肩部形成应力集中区.中部区域球颗粒的运动轨迹成台阶状, 且随距探头距离的增大由近到远可分为3个特征区域; 球颗粒的Z向和XY向位移量亦随之呈负指数形式衰减, Z向位移量衰减速率更快.Abstract: The self-boring in-situ shear pressure-meter test can obtain the strength and deformation parameters directly through its unique multi-level loading mode; however, the deformation mechanism of the soil surrounding the probe has seldom been studied. To reveal the deformation mechanism of the sand sample surrounding the probe of the self-boring in-situ shear pressure-meter (SBISP) during the loading process, the SBISP model test is simulated by PFC3D (particle flow code in three dimensions) program in this study. The development of the displacement and stress field of the soil surrounding the probe, as well as the deformation modulus of each grade of the numerical sample and the motion trails of the soil particles, are studied under multi-level loading process. The results of numerical experiments show that the displacement of particles in central area increases with the shear stress imposed stepwise, and the direction of displacement vector shows a clearly preferred direction. Several radial stress cores of approximately symmetrical distribution have formed near both sides of the probe. At the same time, the vertical stress has formed flat stress zones, and it has produced some stress concentration areas in the shoulder of the probe. The motion trails of the particles are step-like lines in the central area which can be divided into three characteristic zones as the distance to the probe increases. The vertical and horizontal displacements of particles descend in a negative exponential form, but the vertical displacement has a faster attenuation rate.
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图 6 球颗粒位移量与对应颜色关系
Fig. 6. The corresponding colors for different displacement of particles
图 7 各级径向应力作用下颗粒位移等值线
Fig. 7. Displacement contour sketch of numerical sample under different radial stresses
图 8 颗粒位移矢量分布与位移等值线对比
Fig. 8. Comparison of the distribution of displacement vectors of particles and the contour map under shear stress of each grade
图 10 各级径向应力与探头半径关系
Fig. 10. Relation of the radius of the probes and the radial stress of each grade
图 12 不同距离处球颗粒运动轨迹
id表示监测球的编号; r表示监测球距测定器中心的距离.各球运动轨迹在XZ面上的投影用黑色线表示; 各球运动轨迹在XY面上的投影用蓝色线表示
Fig. 12. Motion trails of particles located in different distances
图 13 球颗粒Z向和XY向位移量变化趋势
Fig. 13. Vertical and horizontal displacement of particles in different distances
表 1 冈垣砂物理性质指标
Table 1. Physical property indexes of okagaki sand
项目 颗粒密度ρs(g/cm3) 平均粒径D50(mm) 有效粒径D10(mm) 不均匀系数Uc 最大孔隙比emax 最小孔隙比emin 冈垣砂 2.63 0.26 0.16 2.2 0.94 0.56 
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表 2 模型墙细观参数
Table 2. Meso-parameters of model wall
墙编号 法向刚度kn(N·m-1) 切向刚度ks(N·m-1) 摩擦系数fc 1 1×1010 1×1010 0.6 2、3、5、6、7、8 1×1010 - -
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表 3 数值试样分区及细观参数
Table 3. Partitions and meso-parameters of numerical sample
区域 D的倍数范围 粒径(mm) 法向刚度kn(N·m-1) 颗粒密度ρs(kg·m-3) 摩擦系数fc 中间区域 1~10 2.0~3.2 2.0×105 2 630 1.0 外围区域 10~14 5.0~6.2 4.3×105 2 630 1.0 注：D为探杆直径.
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