Time-dependent Analysis Of Earth Pressure For High-filled Cut-and-cover Tunnel Based On DEM

Recently,High-Filled Cut-and-Cover Tunnel is widely used in highway and railway engineering,the researchers reference the load reduction measures of the culvert structure and adopted many similar load reduction methods to study the unloading measures and unloading mechanism of the Cut-and-Cover Tunnel,which effectively reduced the earth pressure on the top of the Cut-and-Cover Tunnel.However,the backfill soil of HFCCTs is placed over a short time compared to natural soil that has been sedimenting for millions of years,and thus,such backfill soil experiences significant consolidation and creep.The induced relative settlement between the soil column above the High-Filled Cut-and-Cover Tunnel and both sides of the High-Filled Cut-and-Cover Tunnel in turn generates additional frictional force or shear stress that can affect the safety and stability of the high-filled structure.In addition,with the increase in the amount of backfilling above the Cut-and-Cover Tunnel,the changes in earth pressure and displacement in the high backfilling range caused by the creep of the backfill cannot be ignored,and the larger the backfilling,the longer the period of unsteady creep.Therefore,this paper uses the particle flow software PFC2D based on the discrete element theory to conduct an in-depth study on the transmission mechanism and change law of the earth pressure around the high-filled Cut-and-Cover Tunnel before and after the load reduction under the effect of loess creep in the macro and meso multi-scale.In order to improve the long-term safety of the Cut-and-Cover Tunnel structure.The main research contents are as follows:（1）First,the software PFC2D used in the discrete element numerical experiment,related theories and the determination of material macro and mesoscopic parameters are introduced.Then,a numerical biaxial test was established for the backfilled loess with different degrees of compaction involved in this article,and the macro-mechanical properties were compared with those obtained from the Zhang Yuchuan^[29]creep test.The meso-parameters of the Burger’s model required for loess materials in the particle flow program under different working conditions are determined,which provides a relevant basis for the establishment of the post-civilization Cut-and-Cover Tunnel model.（2）Establish a numerical analysis model of particle flow in high-filled loess Cut-and-Cover Tunnel.By analyzing the changes of macro-indicators such as earth pressure and soil particle displacement and the change laws of meso-indicators such as contact force chain,soil particle velocity vector,porosity and coordination number.Obtain the earth pressure transmission mechanism around the Cut-and-Cover Tunnel in high-filled loess and the influence law of soil creep effect on the earth pressure around the Cut-and-Cover Tunnel.At the same time,the influence of the change of section type and filling properties on the earth pressure around the Cut-and-Cover Tunnel under the action of creep is analyzed,and the influence characteristics of various factors on the earth pressure around the Cut-and-Cover Tunnel are obtained.The simulation results show that the consolidation creep of the backfilled soil redistributes the stress around the high-filled Cut-and-Cover Tunnel.In turn,the redistribution of the stress induces further cycle of initial consolidation and creep of the soil.With the progress of the creep process,the stress concentration on the top of the Cut-and-Cover Tunnel and the slope effect are weakened,finally the vertical earth pressure is balanced with the ground stress.After the creep is stable,the vertical displacement of the soil particles is in an inverted"V"shape,and the largest post-construction settlement appears at the center of the H-H section at 0.98m,which accounts for 93%of the total settlement,and the inner and outer soil columns the settlement difference gradually increases with the passage of time.In addition,the degree of compaction and section type have a certain effect on the earth pressure of the Cut-and-Cover Tunnel.（3）Based on the research team and Zhang Yuchuan’s previous research,a numerical analysis model of particle flow in Cut-and-Cover Tunnel with unloaded loess was established.Study the mechanical characteristics of the soil after load reduction,focusing on the analysis of the influence of different load reduction measures（relatively low-compacted soil,relatively low-compacted soil+concrete column）on the earth pressure around the Cut-and-Cover Tunnel.From the macroscopic and mesoscopic perspectives,the earth pressure transmission mechanism and change law around the Cut-and-Cover Tunnel of high-backfill loess unloading are analyzed.The simulation results show that when the low-pressure solid soil is set on the top of the high-filled Cut-and-Cover Tunnel,due to its own deformation,part of the earth pressure on the Cut-and-Cover Tunnel top is relieved,and the load on the top of the Cut-and-Cover Tunnel is transferred to both sides of the Cut-and-Cover Tunnel.The earth pressure is significantly reduced.However,with the development of soil consolidation and creep,the load reduction effect of low-pressure solid soil will be severely weakened.After the creep is stabilized,the earth pressure on the top of the Cut-and-Cover Tunnel will increase to nearγH,and the existence of low-pressure solid soil will also increase.Increased the lateral earth pressure of Cut-and-Cover Tunnel.Therefore,while low-pressure solid soil exists in this article,concrete columns are set on both sides of the Cut-and-Cover Tunnel,so that the load on the top of the Cut-and-Cover Tunnel is transferred to the top of the concrete column,which effectively reduces the settlement of the outer soil column above the Cut-and-Cover Tunnel.The lateral earth pressure of the Cut-and-Cover Tunnel and the vertical earth pressure on the top of the Cut-and-Cover Tunnel are used to optimize the Cut-and-Cover Tunnel system.