高填黄土明洞卸载结构土拱效应离散元分析

DEM Analysis on Soil Arching Effect of Unloading Structure of High-Filled Open Cut Tunnels

采用细观颗粒流软件PFC2D建立明洞卸载数值模型,从竖向土压力、颗粒竖向位移、颗粒间接触力链、孔隙率等细观层面研究了不同减载措施对土拱效应的影响,同时,以工况Y7(EPS板+土工格栅+混凝土柱)为基准,考虑不同密度、厚度EPS板进行参数化分析。研究结果表明:洞顶上方设置减载措施可有效减小洞顶土压力,不同减载措施的减载效果由强到弱依次为:EPS板+土工格栅+混凝土柱>EPS板+土工格栅>EPS板>低压实土+土工格栅+混凝土柱>低压实土+土工格栅>低压实土。其中,在原明洞边坡上设置混凝土柱可以同时加强土拱效应和土工格栅的拉膜效应,使大部分填土荷载转移至混凝土柱,进而减小洞顶与洞侧土压力。土拱效应的发展与洞顶土体孔隙率的变化相关,孔隙率随EPS板密度和厚度的增大先增大后减小,当EPS板密度ρ=15 kg/m^(3)、厚度T=15 cm时,孔隙率达到最大值,土拱效应最强,洞顶荷载分布最少。

The concentration of soil pressure on the tunnel roof caused by the stiffness difference between the highfilled open cut tunnel and the soil masses will often lead to the longitudinal cracking of the roof,which will lead to the occurrence of engineering accidents.This paper establishes a numerical model for the unloading structure of open cut tunnels through the Mesoscopic Particle Flow Code in 2D(PFC2D)software,and studies the effects of different load reduction measures on soil arching effect at the mesoscopic level of vertical soil pressure,vertical particle displacement,inter-particle contact force chain,and porosity,etc.At the same time,taking the working condition Y7(EPS plate+geogrid+concrete column)as the benchmark,this paper also takes into consideration the EPS plates of different densities and thicknesses for parametric analysis.The research results show that the load reduction measures above the tunnel roof can effectively reduce the soil pressure on the roof,and the load reduction effects of different load reduction measures are arranged from strong to weak as follows:EPS plate+geogrid+concrete column>EPS plate+geogrid>EPS plate>low compacted soil+geogrid+concrete column>low compacted soil+geogrid>low compacted soil.Among them,setting concrete columns on the side slopes of the original open cut tunnel can strengthen both the soil arching effect and the tensile membrane effect,so that most of the filling soil load can be transferred to the concrete columns and thus reduce the soil pressure on the top and side of the open cut tunnel.The development of soil arching effect is related to the change of porosity of the soil at the top of the tunnel.The porosity increases before decreasing with the increase of the density and thickness of the EPS plates,and it reaches the maximum value when the density of the EPS plateρ=15 kg/m^(3) and thickness T=15 cm,while the soil arching effect is the strongest and the load distribution at the top of the open cut tunnel is the least.