Safety Analysis Of Shallow Shield Metro Tunnel Under River In Sand-Sebble Layer

According to the implementation of national policies,the city’s urbanization process is accelerating,which also promotes the improvement of underground rail transit network.But the number of subway construction is increasing and the topography of China is complex.Because of these reasons,it is inevitable to cross water rich strata such as rivers,lakes and oceans in route planning.During the whole construction process,high underground water pressure and continuous supply of water source may cause great risks.Especially in sandy cobble stratum,because of its high permeability and low cohesion,water gushing and tunnel leakage are easy to occur.Therefore,it is necessary to study the safety of tunnel construction in water rich sandy cobble stratum.Taking the Yellow River section of yingmentan Matan section of Lanzhou Metro Line 1 as the research background,the numerical model is established by finite difference software.Through theoretical analysis and orthogonal test,this paper analyzes and studies the response law of stratum and structure under different soil pressure without considering the seepage effect,compares and analyzes the influence on stratum and structure in the process of shield excavation with or without seepage effect,and focuses on the deformation and stress characteristics of stratum and structure in the process of shield construction with the change of different factors.The main research work is as follows:(1)The basic concepts and principles of seepage are summarized,and according to the different forms of seepage,the causes and calculation formulas are summarized.Finally,the calculation theory of seepage in sandy cobble stratum is discussed.(2)The influence of the change of soil bin pressure on the formation and structure deformation without considering the seepage effect is analyzed.It is found that the increase of soil pressure in a certain range will reduce the surface settlement and vault deformation,but has little effect on the arch bottom uplift.The variation of displacement field and stress field in the process of shield excavation with or without seepage is compared.It is found that after the completion of shield excavation,the surface settlement,vault settlement and peripheral convergence increase by 49%,61% and 51% respectively when considering the seepage effect compared with those without considering the seepage effect and according to Mohr’s circle principle,it is concluded that the arch bottom is easy to reach the yield strength and fail under the action of seepage.(3)Through the numerical simulation,a variety of working conditions are established,and the variation laws of seepage field,displacement field,stress field and plastic zone are compared under different soil thickness and water head height.The results show that: 1)When the water head is fixed,the distribution of pore water pressure outside the lining is as follows: arch waist > arch bottom > arch crown.With the increase of tunnel depth,the pore water pressure outside the tunnel,the lateral displacement of stratum and the segment deformation also increase.The surface settlement is distributed in the form of "V",the horizontal displacement is in the form of "double bow",and the maximum convergence value of the surrounding appears at the arch waist.The axial force values of segments from large to small are: arch waist > arch shoulder >arch foot > arch crown > arch bottom and the tunnel and surrounding rock are damaged by shear.2)When the tunnel depth is fixed,the dissipation of pore water pressure increases with the increase of water head height.The pore water pressure dissipates about 31 k Pa ～ 35 k Pa with the increase of 2m head height.And the range of plastic zone is increasing,and the development of plastic zone at the arch waist and arch foot is obvious.(4)Based on the orthogonal test,the influence of various factors on tunnel construction is studied by numerical simulation.It is found that the construction interval,equivalent layer thickness and soil pressure have great effects on the surface settlement,the most important factor affecting the lateral displacement of stratum is the soil pressure,and the most important factor affecting segment bending moment is construction interval.In the actual construction,the pressure value of soil bin should be reasonably selected according to the actual situation of the site,so as to ensure the safety of tunnel construction.