Study On The Interaction Between Surrounding Rock And Lining Of Collapsible Loess Tunnel Under Different Flooding Environment

Collapsible loess has mechanical and engineering properties such as macroporosity,structure,collapsibility and water sensitivity,Collapse deformation can not be ignored quantitatively and rapid development qualitatively,which are two outstanding problems affecting the stability of construction structures.Take the Hanjiang-to-Weihe River Water Transfers Project as an example,nearly 1/4 is located in the collapsible loess site,and the collapsible grade is generally Ⅱ～Ⅳ,which has an important impact on the project construction.At present,the core idea of "maximum collapsible amount",which is caused by shallow loess saturated with water,is considered in the evaluation and treatment of collapsible loess.The characteristics of excavation unloading,small underground space and no additional pressure on foundation are taken into account in underground engineering such as tunnel,At the same time,avoid unnecessary waste of resources caused by excessive foundation treatment,a more scientific and reasonable foundation treatment principle should be put forward based on the evaluation objective of‘actual possible amount of subsidence’.In this paper,a numerical simulation method is used to simulate the seepage in the possible flooding environment of the project,Determine the reasonable saturated area as the subsidence field,The interaction between surrounding rock and lining structure of tunnel is analyzed in detail.The main research results obtained in this paper are as follows:(1)According to the collected physical and mechanical parameters of the loess tableland,the possible flooding environment of the site was determined.On this basis,the seepage analysis was carried out to obtain the law of water migration and the influence range of water level under different seepage environments during the tunnel operation.(2)Three-dimensional seepage model of different seepage sources was established,Through the horizontal and vertical analysis and comparison of seepage range,volumetric water content,pore water pressure and other characteristics of seepage,the time-space infiltration law and water accumulation of water in loess field under different seepage conditions were reflected,and the corresponding emphases of foundation treatment under different seepage conditions were put forward.Taking surface water seepage as an example,a new idea of foundation treatment is put forward to guide water through sand Wells.(3)According to three lining sections and three seepage environments,nine finite element models were established to analyze the deformation development of surrounding rock at different flooding stages.The affected area radius of surface and basement settlement is determined by surrounding rock settlement curve,Based on the longitudinal settlement curves of the central feature points of the invert at different flooding stages,the displacement development process can be divided into four stages.(4)Based on the above model,the stress distribution law of surrounding rock at different flooding stages is analyzed,and the change process of the stress distribution law of surrounding rock around the tunnel is described in detail.Through the statistical analysis of the pressure values of the center and foot of the invert at different flooding stages,the pressure values of the characteristic points were compared with the initial dead weight stress.It is found that the pressure value of the arch foot exceeds the initial dead weight stress of the formation under the conditions of surface water seepage and pipeline leakage,which meets the condition of dead weight collapsibility,By analyzing the settlement deformation and internal force changes of the lining structure with different sections and comparing the stress values of different parts in the process of water immersion,the position of the lining structure that may produce tensile crack is determined.