| In recent years,the rapid development of urban rail transit construction in China has led to a large number of lines crossing each other,and the number of cases of new subways crossing existing tunnels is increasing.In view of the construction of a new double-line shield tunnel under the main road of a city in Beijing underneath the existing large section subway mining method tunnel section project,the numerical simulation method is used to analyze the deformation law of the existing structure during the construction process before and after reinforcement.Based on this,the grouting reinforcement of the small guide hole outside the existing subway tunnel and the reinforcement control measures of the steel plate inside the tunnel are put forward.The monitoring data show that the deformation control measures are effective,which can provide reference for similar projects.The main research contents and results are as follows :(1)Based on the actual project,a refined simulation model is established to analyze the influence of shield tunneling on the deformation of existing tunnels and track structures.Numerical analysis shows that without any measures,the maximum settlement of existing tunnel and track structure caused by shield tunneling is 7.35 mm and 6.98 mm,respectively,which exceeds the specified deformation limit of subway.The maximum calculated tensile stress of the existing tunnel structure is 2.56 MPa,which is greater than the limit value of the tensile strength of the concrete used for the second lining of the subway tunnel.Due to the track deformation and the tensile stress of the existing tunnel structure exceeding the limit,the measures to strengthen the soil between the shield and the existing tunnel and the existing tunnel structure are formulated.(2)With the construction of double-line shield tunnel,the settlement trough curve of existing subway tunnel structure gradually changes from "V" type to "W" type.The deformation of shield excavation face is most obvious in the range of-1.5D～1.5D on both sides of the existing subway tunnel center line,accounting for about 70%～80% of the total deformation.(3)The project is located below the main road of the city,does not have the conventional reinforcement conditions,and the small pilot tunnel undercrossing technology has been relatively mature.The small pilot tunnel is excavated between the new shield and the existing tunnel as a grouting face for soil reinforcement,and the existing tunnel is reinforced by steel plate.The simulation results show that the cumulative maximum settlements of the existing tunnel and track structure are 2.87 mm and 2.78 mm,respectively,under the grouting reinforcement of the small pilot tunnel outside the tunnel and the steel plate reinforcement in the tunnel,which are reduced by61.1% and 60.3% compared with those without reinforcement measures.The maximum tensile stress of the secondary lining concrete in the existing tunnel structure is 0.97 MPa,which is 62.1% less than that without reinforcement.(4)The settlement deformation of the existing tunnel structure decreases with the increase of the earth chamber pressure and synchronous grouting pressure of the shield machine.However,when the pressure increases to a certain range,the settlement control effect is no longer obvious when the pressure continues to increase.The optimal range of the earth chamber pressure and synchronous grouting pressure used by the shield machine in this project is 0.08MPa～0.10 MPa and 0.20MPa～0.24 MPa,respectively.(5)The engineering monitoring data show that the overall settlement trend and the maximum settlement value of the existing tunnel and track structure are consistent with the simulation results.The settlement monitoring data are less than the simulation results,and the deviation rate is less than 15.2%,which indicates that the reinforcement control measures are feasible. |
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