Study On The Influence Of Deep Excavation Adjacent To Existing Shield Tunnel In Loess Area

Rail transit has maintained rapid growth in recent years,and it is foreseeable that the subway lines will be denser in the future.Meanwhile,with the rapid development of new urban buildings,the available space of the city will gradually shrink.At the same time with urban development,in the process of engineering construction,there will be a lot of deep excavation engineering.Deep excavation will not only lead to huge excavation unloading,resulting in soil stress redistribution,but also in the construction process,groundwater will be extracted,which will cause soil deformation behind the excavation and at the bottom of the excavation,posing a serious threat to the adjacent subway shield tunnel segments,and even cause segment cracking and bolt damage,which will have a huge impact on the safe operation of the subway.At present,in the study of the effect of excavations on the existing shield tunnel behind the excavations,the surrounding soil is often equivalent to the load in the theoretical calculation,while the equivalent stiffness method is often used to simulate the tunnel in the numerical calculation,which cannot reflect the weakening effect of the stagger-jointed.Finite element software is used in this paper,the beam spring method is applied to simulate shield tunnel,and the stagger-jointed installation pattern tunnel is considered.The deformation of the existing shield tunnel behind the excavations caused by excavations and the internal force change law of segments and joints are studied in detail.The main discrimination basis of segment joints is summarized,and the specific results are as follows:(1)Read a lot of literature and related books,calculate the axial stiffness of shield tunnel segment joints using Koizumi’s analytical formula and relevant empirical formulas,respectively,rotational stiffness and shear stiffness of shield tunnel segment joint.The axial stiffness adopts the tensile stiffness of the joint,and the rotational stiffness is the result before joint separation.Midas GTS NX output spring internal force results are converted into joint axial force,shear force,bending moment.(2)Based on the foundation pit of a station in Xi ’an Metro High-tech Zone,the parameters of the small strain hardened soil constitutive model were obtained by relevant empirical formulas,and the calculation model of the excavations with two-dimensional plane strain was established.The comparison between the numerical calculation results and the measured results shows that the deformation law of foundation excavation in Loess Regions can be well reflected by the small strain hardening soil,and the soil parameters calculated by the above method are reasonable and reliable,which provides certain support for the next stage of research.(3)The two-dimensional model of a station in the high-tech zone is extended to analyze the change rules of joint shear axle ratio and bending moment under different stitching angles,and the analysis of adverse stitching is selected.Under the support of double-row piles,the deformation of the tunnel,the internal force of the segment,the internal force and the deformation law of the joint are studied under the different horizontal net distance of the tunnel,the buried depth of the tunnel,and the excavation distance of the inner and outer excavations.There are obvious differences in shear force and bending moment between the two splicing modes.Compared with the joint failure basis,the shear shaft ratio of segment joint has more than 50% safety margin.The safety margin of joint opening is only 20% under the maximum tunnel buried depth condition,and more than 50% under other conditions.The minimum bolt pressure is 400 k N,and all of them are under pressure.(4)Based on the case of multiple subway lines behind a pit in pit in Xi ’an Gaoxin,two typical sections are selected for numerical analysis.On the east side: the outer excavation has a great influence on the deformation and the internal force of the segment of the Metro Line 6tunnel.The effect of inner excavation is small.West side: The excavation has a great influence on the deformation of subway Line 8 tunnel and the internal force of segment.When the excavation exceeds the buried depth of the tunnel,the vertical deformation trend of the two tunnels changes.The bolts of all joints are under compressive stress;When the excavation reaches the bottom,the compressive stress of the joint bolts decreases compared with the initial state,which is greater than 400 k N.The shear shaft ratio of all joints is less than 0.25,with more than 50% safety margin,and the shear shaft ratio of the tunnel joint near the excavation side is the largest.The joint opening is less than 1mm,with more than 50%safety margin.If the tunnel deformation is further increased,there is a risk of relative dislocation,so it should be strengthened in time to ensure the safe operation of the subway tunnel.(5)Combined with the analysis results,the joints of the top block are easy to misplace each other;The position of the sealing block of the staggered joint shield tunnel should be set in the arch area.The compressive stress reserve of segment joint bolts in the vault area is lower.Therefore,the tunnel arch joint should be focused on and protected.