Study On The Influence Of Collapsible Deformation Of Loess Strata On The Interaction Between Tunnel Surrounding Rock And Lining

Due to the large porosity,structural properties of loess and the water sensitivity of sudden drop in water intensity and sudden increase in deformation,loess has strong collapsibility,so a variety of engineering disasters caused by water often occur in loess areas.Approximately 1/4 of the second phase of the Hanjiang-to-Weihe River Water Diversion Project passes through collapsible loess strata.The collapsible grade of loess is grade Ⅱ-Ⅲ,which has adverse effects on the project construction.Many achievements have been made in the evaluation of loess collapsibility and the foundation treatment of water conveyance tunnel.However,due to the inhomogeneity of loess strata and the long-distance linearity of tunnel engineering,it is necessary to continuously improve the applicability of collapsibility evaluation method and foundation treatment method.Based on the Xianyangyuan Tunnel Project of the North Main Line of the Second Phase of Hanjiang-to-Weihe River Diversion Project and the immersion test of a field test pit in Xi’an,the seepage characteristics of the undisturbed loess site and the existing tunnel(different buried depths)loess site are analyzed by numerical simulation.According to the characteristics of Xianyangyuan tunnel project,three working conditions of different soil layers and different buried depths are established.Considering the surface immersion,the numerical analysis model of loess collapsible deformation under various working conditions is established,and the interaction between tunnel surrounding rock and lining structure during immersion is analyzed.The concrete replacement method is used to replace the soil layer below the tunnel base at different depths,and the deformation and internal force changes of the lining structure before and after treatment are analyzed.The main research results of this paper are as follows:(1)The collapsibility of the five sections of the tunnel along the Xianyang tableland is analyzed,and the collapsibility grade of each section is obtained.Among them,the foundation of the bottom of the three sections of the tunnel has obvious collapsibility:the buried depth of the B1+300～B9+948 section of the tunnel is generally 5m～10m,and the foundation collapsibility grade is mainly grade Ⅲ,local grade Ⅱ;the buried depth of tunnel in B9+948～B12+248 section is generally 12m～20m,and the collapsibility grade of foundation is grade Ⅱ～Ⅲ.The buried depth of the tunnel in B25+805～B33+837 is generally 6m～13m,and the collapsibility grade of the foundation at the bottom of the tunnel is grade Ⅲ.(2)Based on the in-situ immersion test results,the characteristics of water infiltration under the condition of with or without tunnel are analyzed by numerical simulation.The influence of tunnel depth and stratum combination on the water migration field around the tunnel is discussed in depth.The existence of the tunnel will increase the permeability rate of water in the soil on both sides of the tunnel.The larger the buried depth of the tunnel,the larger the diffusion range of water infiltration;the permeability coefficient of the soil layer is different,and the infiltration rate of the water in the vertical and horizontal directions is also different.Therefore,the infiltration areas obtained under different stratum combinations are significantly different.(3)According to the engineering information of Xianyangyuan tunnel and the saturated area of loess site in Xianyangyuan tunnel under the condition of surface seepage obtained above,the numerical analysis model of loess stratum collapse under the condition of existing tunnel is established,and the vertical pressure distribution function of tunnel basement with two different stratum distributions is given.The deformation and internal force variation of surface subsidence,tunnel basement and lining structure under different collapsible grades,different buried depths and different stratum combinations are studied,and the interaction characteristics of tunnel and surrounding soil under different collapsible grades,tunnel buried depths and stratum combinations are revealed.The structural failure mechanism of tunnel structure in the process of water collapsibility of loess is described by the characteristic indexes of deformation increment and uneven deformation of key parts.On the whole,the maximum settlement deformation position of the lining structure is at the arch foot.During the collapsible process,the uneven settlement caused by the loess saturation at the arch foot but the unsaturation at the center of the inverted arch is the most likely to cause the lining structure to fail.In the whole process of collapsibility,the position with the largest change of internal force before and after collapsibility is in the inverted arch and arch foot.The vault position is always pulled in the process of collapsibility,which should be paid more attention to avoid cracking damage.(4)Based on the analysis of the deformation and internal force of the tunnel lining structure under the collapsible condition of the loess stratum,the foundation treatment is carried out by using the method of concrete cushion replacement,and the numerical model is established to simulate the displacement of the characteristic points and the internal force of the tunnel before and after the replacement.After replacement,the collapsible depths of the vault tensile stress reaching the ultimate tensile strength of concrete in the three working conditions are about 15m,about 10m and about 12m respectively.After using the replacement method to treat the basement,the tensile stress at the vault position will increase during the collapse process,so the internal force change at the vault should be paid attention to.