碎屑流冲击柔性网的离散元仿真研究

Numerical investigation on flexible barriers impacted by dry granular flows using DEM modeling

柔性网和碎屑流相互作用包括碎屑流散体运动冲击和柔性网连续大变形两个复杂的力学过程。由于目前柔性网和碎屑流相互作用的力学理论计算方法尚不成熟,为此提出一种利用Hertz–Mindlin黏结接触模型模拟柔性结构,利用无滑移的Hertz–Mindlin模型模拟碎屑流的离散元仿真方法。选择有横向支撑锚索的沟道碎屑流防护结构进行模拟计算,并定义碎屑流动能变化率Wk和碎屑流死区质量与碎屑流总质量之比Fm来对比碎屑流冲击柔性网和刚性挡墙的动态响应过程。结果表明:与冲击刚性挡墙不同的是,碎屑流冲击柔性网时冲击荷载首先沿坡面方向冲击,使承力锚索在水平方向和竖直方向均产生较大的变形。随后冲击荷载作用方向逐渐转变为以水平冲击为主,使堆积区上部锚索的水平偏移值和碎屑流在水平向的堆积范围增大。利用经验公式求得的作用于刚性挡墙的最大法向冲击合力与数值计算结果较为一致,而利用经验公式求得的作用于柔性网的最大法向冲击合力比数值计算结果大45%以上,因此利用经验公式计算碎屑流作用于柔性网的最大法向冲击力时,需要重新确定动土压力系数CD和弗洛德数Fr之间的关系。

The interaction of flexible barriers and granular flows has two complex mechanical processes: the continuous large deformation of flexible barriers and the discrete motion of particles. Owing to the fact that the theoretical method for the interaction of flexible barriers and granular flows is immature, a DEM method is proposed. In this method, the Hertz-Mindlin bonding particle model is employed to simulate the flexible barriers. The no-slip Hertz-Mindlin model is used to simulate the granular flows. The flexible barrier with lateral anchorage cable is selected. The change rate of kinetic energy Wk and the ratio of the dead zone mass friction Fm to the total mass of the granular flows are defined to compare the dynamic impact response of flexible barriers to retaining wall. The results show that the impact of granular flows causes large horizontal deformation and vertical deformation of cables firstly. Then, the direction of impact load converts to the horizontal one, so that the horizontal deflection in the upper dead zone of cables and the horizontal accumulation range of granulars increase. The total normal force impacting on the retaining wall calculated by the empirical formula agrees with that of the numerical method. Based on the results of numerical simulation and theoretical calculation, the maximal total normal force impacting the flexible barrier calculated by the empirical formula is over 45% greater than the maximum total normal force calculated by numerical simulation. Therefore, it is needed to reappraise the relationship between dynamic pressure coefficient CD and Froude number Fr before calculating the maximum normal force using the empirical formula.