各向异性砂土宏微观特性三维离散元分析

各向异性对砂土强度和变形特性有显著的影响,为了研究各向异性砂土的宏微观特性,基于三维离散元法,对7个不同沉积角的试样进行了一系列的三轴模拟试验。利用"Clump"命令生成近似椭球形状颗粒,并且采用三维抗转动模型来模拟颗粒间的抗转动能力。离散元模拟结果与已知室内试验结果吻合很好。结果表明:随着沉积角的增大,偏应力和轴向应变的关系逐渐由应变软化向应变硬化发展。沉积角较小的试样剪胀性更强并且容易到达临界状态,颗粒组构–应力联合不变量(表征颗粒长轴组构张量和应力张量的相对角度)的值接近于-1,且颗粒长轴组构各向异性先增大后减小;然而对于沉积角较大的试样,在轴向应变50%处,仍不能达到临界状态,并且联合不变量的值大于-1,颗粒长轴组构各向异性先减小后不断增大。对于法向接触组构,组构主轴方向迅速向应力主轴方向偏转,组构各向异性的演化规律与偏应力随轴向应变的演化规律相似。

Macro and micro-behaviors of anisotropy granular soils using 3D DEM simulation

Anisotropy has a significant effect on the strength and deformation characteristics of granular sand. In order to study the macro- and micro-behaviors of anisotropic sand, a series of numerical triaxial tests are performed on seven specimens with different bedding angles using the three-dimensional discrete element method (DEM). The approximate ellipse-shaped particles are generated using the command of clump, and a 3D rolling resistance model is implemented to simulate the local rolling resistance between particles. Good agreement is achieved between the present DEM simulation results and previously published experimental results. The relationship between deviator stress and axial strain changes from strain softening to strain hardening with the increase in bedding angel. The specimens with smaller inclination angels are more dilative and easier to reach the critical state. The combined invariants representing the relative orientation of particle orientation fabric tensor and stress tensor can approach nearly to -1. Their fabric anisotropies of particle orientation increase first and then decrease. However, for the specimens with higher bedding angels, the critical state cannot be reached even when the axial strain reaches 50% and the combined invariants are much larger than -1. Their fabric anisotropies first drop and then continuously increase. For the contact normal based fabric tensor, the principal axe rotates instantly toward the principal axe of the stress tensor, and the evolution of fabric anisotropy is similar to that of deviatoric stress against the axial strain.