Analysis On Earth Pressure Of High-filled Cut-and-Cover Tunnel For The Coupling Of Load Reduction And Structure Optimization Based On Discrete Element Method

In recent years,the transportation network in the Loess Plateau has rapidly increased because of the rapid economic development and the needs of the China’s western development.In addition to the complex terrain of the ravines in the area,there have been more and more Cut-and-Cover Tunnels(CCTs)with high-filled,heavily loaded and weakly grounded have appeared.Moreover,the high earth pressure generated by the high-filled loess will result in insufficient High-Filled Cut-and-Cover Tunnel(HFCCT)strength and foundation bearing capacity,resulting in structural cracks and uneven foundation settlement,which will seriously affect the normal use and durability of the open tunnel,and it causes structural cracks and uneven settlement of the foundation,which seriously affects the normal use and durability of the CCTs.But when the height of the backfill is too large,only a single load reduction method cannot fully meet the requirements of the project,it is necessary to further increase the height of the backfill and improve the structural force through structural optimization and load reduction coupling methods.Therefore,considering the discreteness and inhomogeneity of the soil in this paper,the particle flow software-2D(PFC2D)was used to establish a numerical simulation models,load distribution with different load reduction forms,different CCT section types,and coupled effect load reduction forms and CCT section types were analyzed.The mechanism of earth pressure change was explained in meso-level view,and the main research contents are as follows:(1)The numerical biaxial test and uniaxial lateral compression test were established for the backfill materials involved in this paper(loess and EPS board).Through comparison with the macro-mechanical properties(stress-strain curve,elastic modulus,poisson’s ratio,cohesion and internal friction angle)obtained from the indoor model test done in the previous period,the requirements of loess and EPS board materials in the particle flow program were determined,which provided a basis for the numerical simulation parameters of the HFCCT.(2)Numerical analysis models of particle flow in HFCCTs with different cross-sections were established.By analyzing the changes of related macro-indicators and micro-indicators,the force of different cross-section types of HFCCTs were systematically analyzed.The simulation results show that the change of the cross-sections of HFCCT will have a certain impact on the force of itself,which the vertical earth pressure on the top of the CCT and the lateral earth pressure on both sides of the CCT will change.In addition,there is an optimal section type of CCT,it is height-span ratioλ=1.0.Under this optimal section type,the vertical earth pressure,relative displacement and moving speed of soil particles are the smallest,and height of the settlement surface for soil particles is the lowest,and the distribution of contact force chain on the top of the CCT is sparsest,and the force on the HFCCT is the most reasonable.(3)A numerical analysis model for particle flow of load reduction HFCCT was established,and the characteristics of the load reduction material EPS board(EPS board density,thickness,width and laying position)and the size of the concrete wedge(height and width)were analyzed for the effect of earth pressure and micro-contact between soil particles.Influence of displacement and micro-contact between soil particles.The simulation results show that when the EPS board is laid on the top of the CCT,due to its own large deformation,part of the earth pressure on the top of the CCT is relieved,and the load on the top of the CCT is transferred to both sides of the itself,which significantly reduces the vertical earth pressure on the top of the CCT,increases the settlement of the internal soil column.The side wall of the HFCCT bears a relatively large load,which increases the lateral earth pressure.While arranging EPS board on the top of the CCT,concrete wedges are set on both sides of the CCT,which will transfer the load on the top of the CCT to the top of the concrete wedges,effectively reducing the settlement of the soil on both sides of the CCT,and indirectly increasing the settlement of the internal soil column,forming a larger settlement difference,and further reducing the vertical earth pressure and the lateral earth pressure of the CCT.In addition,EPS board characteristics(EPS board density,thickness,width,and laying position)and concrete wedge dimensions(height and width)both have optimal values with the smallest earth pressure and the largest soil particle displacement.Among them,the optimal thickness,width and density of the EPS board are T=0.1m,W=0.3m(1.5S),ρ=15kg/m~3,respectively.And the optimal height and width of the concrete wedge are H=0.32m and B=0.17m,respectively.The closer the EPS board is to the top of the CCT,the better the load reduction effect.(4)A numerical analysis model of particle flow in a HFCCT under the coupling effect of load reduction and cross-section was established,and the soil pressure,soil particle displacement and microscopic contact changes under different conditions were analyzed.The simulation results show that the optimal value of the characteristics for the EPS board and the size for the concrete wedge have not changed under the coupling effect.However,the optimal value of the section type of CCT has changed.When the EPS board is coupled with the section type of CCT,the optimal value of the section type of CCT is the height-span ratioλ=0.8.When the EPS board and concrete wedge is coupled with the section type of CCT,the optimal value of the section type of CCT is the height-span ratioλ=0.6.