Study On The Influence Mechanism Of Loess Collapsing On Metro Structure Stability And The Control Standard Of Remaining Collapsing Amount

With the rapid development of Xi’an,the subway construction has become an urgent mission in Xi’an.There are a large number of tunnels passing through collapsible loess stratum in subway construction.And the potential collapsible loess deformation will seriously threaten the safety and stability of the tunnel structure during operation.But most of Xi’an is covered with loess.Therefore,it is very important to evaluate the damage of loess collapsibility to the tunnel lining structure correctly.However,the current standard evaluation method of the collapsible deformation of loess foundation is more suitable for shallow-buried loess building foundations.It is difficult to make a reasonable evaluation on the foundation treatment of deep loess tunnel,and it can not provide effective guidance for the treatment of collapsible loess tunnel foundation.Based on the background of Xi’an Metro Line 10(Yangjiazhuang-Shuijing Park section)This paper studies the influence of different collapse conditions on the stress and deformation of the tunnel structure through the finite element numerical simulation;then analyzes the damage degree of tunnel at a certain collapsible depth under different treatment depth of tunnel foundation,to find out the regular pattern of the foundation treatment depth of the loess tunnel and formulate the control standards for the remaining collapsibility of the loess tunnel.The main research contents and conclusions of this paper are as follows:1.Based on the existing research on the influence of loess collapsibility on tunnel structure,this paper analyzes the influence law of the loess collapsibility on the deformation and stress of the shield tunnel structure,and proposes the deformation and stress standards of shield tunnel structures,as well as the standard and calculation method of tunnel segment crack width in combination with "Metro Design Code" and "Concrete Structure Design" "Code".2.IN combination with the difference between the ground structure foundation and the collapsible collapse of the loess tunnel,the control standard of the remaining collapsible amount in "Construction Code for Collapsible Loess Area" is analyzed,and the control standard of the remaining collapsible amount control in the code is obtained which is mainly aimed at the requirements of surface buildings for uneven settlement of loess.The controlstandard for the remaining collapsible amount of the loess layer at the bottom of the tunnel needs to be further studied.3.With the aid of ABAQUS system,the stress and deformation analysis of the tunnel structure under the condition of submerged subsidence was carried out,and it was concluded that each subsidence condition would cause different degrees of settlement of the tunnel.The settlement caused by the full width collapse is larger than that caused by the half width collapse;the settlement caused by the tunnel bottom collapse is larger than that caused by the surrounding and overlying collapse;the influence of the tunnel bottom and surrounding collapse on the stress of the tunnel structure is greater than that caused by the overlying collapse,and the influence of the half-column collapse on the stress of the tunnel structure is greater than that caused by the full-width collapse.No matter the surrounding tunnel collapse or the tunnel bottom collapse,it may have an adverse effect on the tunnel structure.Therefore,effective foundation treatment and seepage control measures must be taken to avoid the adverse effects of the collapsible loess caused by the submergence of the water.4.Through the analysis of the influence of loess collapse on the stress and deformation of tunnel structure under different treatment depth,combined with the tunnel structure control standards,the reasonable treatment depth of the tunnel foundation is obtained.And according to the analysis of four groups of different working conditions,with the increase of the treatment depth,the bearing capacity of the foundation increases,the remaining collapsibility decreases,and the lateral and vertical deformation of the segment decreases.the effect of increasing the treatment depth is not obvious for improving the bearing capacity of the foundation and reducing the collapsibility.So it is of great significance to select the appropriate treatment depth for shield tunnel construction in the loess area.According to the analysis of numerical simulation and theoretical calculation results,the treatment thickness of collapsible loess at the bottom of tunnel is 65% ~ 80% of collapsible loess at the bottom of tunnel.