The Influence Of Different Flooding Methods On The Deformation Of Loess Metro Tunnel

With the rapid development of urban subway,the construction of subway in loess regions faces a series of problems that still remain to be solved,such as homogeneous or inhomogeneous collapse resulted from soaking in the loess stratum,the deterioration of engineering properties of the surrounding rock after soaking resulted from the infiltration of surface water and the uplift of underground water level.Not only the successful completion of the subway construction is influenced by these problems,but also the safe operation is endangered.Therefore,it is very necessary to seek feasible treatments for engineering problems of the collapsible loess through engineering entities.The main research carried out is as follows:Based on the project of the large-thickness loess subway tunnel in Lanzhou Rail Transit Line 4.We established the finite element numerical simulations of two kinds of soaking models by using the MIDAS-GTS-NX and adopting the dense mode correction method,including the type of considering the soaking of surface water and the type of the uplifting of groundwater level,to study the deformations and structure stress characteristics of the Surrounding Rock.The results show: the surface subsidence of the two types of soaking all has sudden changes in different project conditions,and sometimes causes the second collapse.No matter the homogeneous collapse or the inhomogeneous collapse,the internal displacement of the surrounding rock is greatly affected by the uplifting of the underwater level.Because the bottom-up soaking has a great influence on the collapse of loess subway tunnel,the laboratory model tests are taken to simulate the collapse equivalently.The surface subsidence,the surrounding soil pressure and the tunnel stress-strain caused by the homogeneous and inhomogeneous collapse are studied.The results show that the cumulative settlement is basically the same in the stage of mild collapse,it is not consistent in the middle stage of collapse,and it changes sharply in the stage of strong collapse,making the differences increase gradually.By comparing homogeneous collapse and inhomogeneous collapse,we found that the left outflow rate of water gradually increased and were approaching the rate of right,and the cumulative settlement gradually transferred from "slant" to "parabola" with the increasing of it's value in inhomogeneous collapse.The final cumulative settlement of the inhomogeneous collapse of different monitoring points in different project conditions was greater than that of the homogeneous collapse,which means the inhomogeneous collapse has a greater harm to subway structure.The risk of soaking was evaluated,the damage possibility of soaking in subparts of subway engineering was explored,and the foundation treatment principles and treatment methods for subway projects crossing the collapsible loess area were put forward in combination with the commonly used foundation treatment methods in the loess area of subway projects.Numerical simulation and laboratory model test are used to analyze the effect of homogeneous and inhomogeneous collapse on the deformation of subway tunnels in loess areas.The research results can provide reference for the construction of subway tunnels in loess areas.