Analysis On Meso-Mechanism Of Soil Arching For Unloading Structure Of High-filled Cut-and-Cover Tunnel Based On DEM

Recently,High-Filled Cut-and-Cover Tunnel(HFCCT)is widely used in highway and railway engineering.However,the stiffness difference between Cut-and-Cover Tunnels(CCT)and backfill will cause the earth pressure concentrating on the top of the CCT,which will lead to the longitudinal cracking of the top of the CCT,and then lead to the occurrence of engineering accidents.For this reason,scholars have studied the unloading measures mechanism of a HFCCT,and concluded that the soil arching is the main load transfer mechanism in the top of the CCT.However,the previous studies were based on the continuity hypothesis and have ignored the discrete characteristics of soil.Therefore,on the basis of the previous studies,I established a numerical simulation model by using the PFC.The soil arching is analyzed and studied in depth from the perspective of granular mixtures and mesoscopic view,so as to grasp the load reduction mechanism of load reduction measures on earth pressure in essence.The main research contents are as follows:(1)Numerical biaxial test,uniaxial lateral compression test and tensile test were established respectively for three test materials of loess,EPS board and geogrid.By comparing the macroscopic mechanical properties(stress-strain curve,elastic modulus,poisson’s ratio,cohesion and internal friction Angle)of the materials obtained from the laboratory tests and the numerical tests,the mesoscopic parameters required for each material in the PFC were determined,which provided a basis for the numerical simulation parameters of the HFCCT.(2)Based on the laboratory model test conducted by the research group in the early stage,the numerical analysis model of HFCCT is established by using PFC.By analyzing the variation law of related macro-indexes(soil pressure,fill displacement)and mesoscopic indexes(contact force chain,porosity and coordination number),the general variation law of soil arching and its influence factors(slope angle and B/D ratio)are systematically analyzed.The simulation results show that the initial equal-settling plane gradually decreases and becomes stable with the increase of the height of backfill and the soil arching gradually appears.When the height is constant,with the increase of the slope Angle,the position of the plane first decreases and then rises,the soil arching first increases and then decreases,and the contact force chain distribution at the top of the CCT is dense and then sparse.With the increase of B/D ratio,the position of the plane gradually rises,the soil arching gradually weakens,and the distribution of contact force chain on the roof of the cave gradually becomes dense.(3)Based on the laboratory model test conducted by the research group in the early stage,the numerical analysis model for Unloading Structure of HFCCT is established by using PFC.the general variation of the soil arching is comprehensively analyzed from the macromesoscopic perspective.At the same time,the influence of EPS board characteristics(density,thickness,width and laying position)and geogrid characteristics(stiffness and number of laying layers)on the soil arching and earth pressure is further explored.The simulation results show that when relatively low-compacted(RLC)soil or EPS board is laid in a certain height and width above the CCT,the relative settlement direction of the inside and outside soil columns will be changed,and the soil arching effect will appear.At the same time,the earth pressure on the CCT will be further reduced by layering geogrids.The optimal thickness and width of EPS board were T = 0.1m and W = 0.3m(1.5D),respectively.The presence of geogrid weakens the soil arching to some extent,but this effect is weaker than the carrying capacity provided by geogrid.There is an upper limit for the influence of grid stiffness on the earth pressure.For the number of grid layers,under the same geogrid layout conditions,the more the number of grid laying layers,the better the load reduction effect.(4)In view of the phenomenon of load reduction on the top of CCT and load loading on it’s side when EPS board is laid on the top of CCT.A new type of unloading structure of HFCCT is proposed.The concrete columns are set on both sides of the slope,so that the concrete columns and the slope become a whole structure.Under the unloading system,the sphere of influence of soil arching is further expanded.most of the load above the CCT is transferred to the concrete column,and the stress on the CCT and it’s side wall will be significantly reduced.