| With the development of urban infrastructure,there are more and more construction projects of the double-line CRD(Center cross diagram)tunnel undercrossing the existing tunnel in close distance.In order to ensure the safety of the existing tunnel during the construction of the new tunnel and prevent disasters,it is necessary to further carry out the research on the key technology of the construction of the double-line CRD tunnel undercrossing the existing tunnel in close distance,and clarify the deformation law of the existing tunnel.Based on the project background of Chengdu Rail Transit Line 9undercrossing the existing Line 1,the key technologies for the construction of double track CRD tunnel undercrossing the existing tunnel in a close distance is studied by the method of combining theoretical analysis,numerical simulation and on-site monitoring in this paper.The main results are as follows:(1)In order to ensure the safety and stability of the Line 1 operation,the CRD method is proposed for undercrossing construction,and the advanced pipe shed and tremie grouting are used to control the deformation of the existing tunnel.In view of the complexity of the CRD method construction scheme of this project,the method of combining stress release,stiffness reduction and equivalent layers is proposed.The optimized three-dimensional dynamic construction numerical analysis model of CRD is established.The rock and soil layers of the actual site,the displacement stress field after the excavation of the existing tunnel,the excavation process of the new tunnel CRD method,the advance pipe shed(conduit)grouting reinforcement and the secondary lining construction are considered in this model.(2)Taking the deformation of the new tunnel as the control object,the discrete element numerical model is used to study the deformation of the new CRD tunnel in the normal operation of the existing tunnel.The results show that the horizontal displacement of the side wall of the new CRD tunnel is the largest,and the horizontal displacement of the vault and the bottom is small;the vertical displacement of the tunnel vault is the largest,followed by the bottom,and the vertical displacement of the side wall is small.In the process of tunnel excavation,the deformation characteristics of horizontal or vertical direction are basically the same with time,which is mainly composed of initial deformation stage,accelerated deformation stage and deformation completion stage.When the thickness of the grouting reinforcement is greater than 40 cm,the maximum deformation of both the existing tunnel and the new tunnel is less than 3mm,which meets the construction requirements.(3)Based on the CRD 3D numerical analysis model established in this paper,and the deformation of the existing tunnel was taken as the control object,and the reasonable delay step,initial support stress and closedloop time of the double-line CRD tunnel were studied considering various working conditions of the single-line and double-line CRD tunnel undercrossing construction,and an optimization scheme of the left and right line delay step spacing of 60 m,initial support stress of 0.3MPa and CRD closed-loop step of 200 is proposed.(4)Based on the technical specifications for tunnel monitoring and measurement,the monitoring scheme of the Chengdu Rail Transit Line 9undercrossing the existing Line 1 is designed,and the construction process of the CRD tunnel is monitored by means of the monitoring equipments.The monitoring results show that all the monitoring data of the existing tunnel projects are within the control standard value,which proves that the optimized scheme of the CRD method proposed in this paper has good applicability.The field monitoring data are in good agreement with the numerical calculation results,indicating that the optimized three-dimensional CRD dynamic construction numerical analysis model has a high accuracy,which can provide reference for the similar projects. |
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