| With the rapid development of urban subway and the continuous improvement of the subway network,the construction of new tunnel crossing over existing tunnel will increase gradually.The excavation of the new tunnel will inevitably disturb the equilibrium stress field of surrounding strata,which will result in additional internal force and deformation of existing metro tunnel structure;thereby how to effectively predict and control the longitudinal heave deformation of existing tunnel due to constructin of upcrossing new tunnel is very utgent in the area of crossing construction.In order to ensure the safe operation of the existing tunnel,in this paper,the longitudinal deformation mechanism is studied systematically,and the factors analysis is also preformed;and then some the corresponding control measures and suggestions are put forward.The main contents in this paper are as follows:The unloading model of the existing subway tunnel is established,and the unloading stress of the existing subway tunnel is solved based on the Mindlin’s elastic stress solution.The parametric analysis is also preformed to study the effect rules of the different upcrossing unloading factors by solving the unloading additional stress coefficient.The study indicates that the average Poisson’s ratio of the soil and the longitudinal slope of the new tunnel have little effect on the unloading effect of the existing tunnel.For the construction of new tunnel crossing over existing metro tunnel,the existing metro tunnel is simplified as an Euler-Bernoulli beam on a Lifking foundation model.The simple two-stage method is adopted to study the longitudinal heave displacement of existing metro tunnel.The dimensionless parameters of Lifking foundation model are adjusted by the comparison of the calculating-example results and numerical solution,and the discontinuity defect of Winkler model foundation which was used in traditional simple two-stage method is improved.The applicability of the presented method is verified based on three well-documented practical projects,and the presented method can predict the tunnel heave displacement quickly and effectively.Based on the proposed project of Shenzhen Metro Line-6 crossing over Metro Line-4 tunnel,the mechanical models of the construction of new tunnel up-crossing existing tunnel are established by MIDAS/GTS NX software.The parametric analysis of the different up-crossing unloading factors is preformed to study the longitudinal deformation effect rules of existing tunnel.The study indicates the peak of the tunnel heave is approximately proportional to the excavation cross-sectional area,and the excavation areas and distance should be lessened in practices.The tunnel peck heaves increase obviously when the excavation section is 1.5 times outer diameter of new tunnel ahead or away from the centerline,and it suggests that the measures such as applying ballast in this area of new tunnel should be implemented.Increasing skew angle between the new tunnel and existing tunnel can reduce the longitudinal heave displacement and the maximum displacements decrease smoothly when the skew angle is greater than 45°.With the increase of the thickness of the interlayer soil,the maximum heave displacement of the tunnels is gradually reduced and the rate of change becomes slow.In practices,the thickness of the interlayer should be greater than 1.5 times outer diameter of new tunnel.With the increasing spacing between double-tube parallel tunnels,the maximum displacements decrease gradually.The spacing between double-tube parallel tunnels should be increased to eliminate the superposition effect of tunnels. |
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