Analysis On The Mechanism And Failure Probability For Longitudinal Deformation Of Existing Shield Tunnel Induced By Overcrossing Tunnelling

With the rapid development of urbanization in China,subway tunnels have increasingly become an important form of transportation to relieve urban traffic pressure in large and medium-sized cities.However,the limited underground space and the increasing number of tunnel lines will inevitably lead to the new tunnel overcrossing the existing shield tunnel.The stress relief of the new tunnel will cause the longitudinal deformation of existing shield tunnel below.It will lead to a series of adverse effects on the existing shield tunnel,including segmental lining cracking,bolt loosening,lining seepage and so on.Therefore,to ensure the safety of tunnel structure and subway operation during the construction of a new tunnel above is of great significance.Depending on the field measurement,theoretical analysis and probabilistic assessment,this thesis investigated the effects of longitudinal internal forces and deformation of existing shield tunnel due to overcrossing tunneling in soft soil areas.The work and research in this thesis are mainly as follows:(1)Based on the field measurement of rectangular pipe jacking overcrossing the existing metro shield tunnels,the vertical displacement variations of the existing shield tunnels are studied.The results showed that the vertical displacement of the existing tunnel presents obvious phased changes with the excavation of the pipe jacking,which can be divided into three stages: weak impact zone,strong impact zone and stable zone.Moreover,the horizontal distribution of the vertical displacement is similar to Gaussian distribution.(2)The analytical solution for longitudinal deformation of existing shield tunnel induced by overcrossing tunnelling considering circumferential joints is proposed.First,a simplified longitudinal beam-spring model(SLBSM)that can consider the opening and dislocation between rings simultaneously is established,in which the EulerBernoulli short beam is used to simulate the deformation of the segmental rings,and the rotation and shearing springs are introduced to simulate the rotation and dislocation of circumferential joints.The existing tunnel is then treated as the SLBSM resting on the elastic foundation,and the state equations including tunnel displacements and internal forces are constructed to solve the discontinuous deformation of circumferential jointsegmental ring.Finally,the feasibility of the proposed solution is verified through two well-documented cases.The predictions from the proposed method are also compared with other existing analytical methods,and some dominant parameters are further analyzed.It is found that the tunnel displacement curves obtained by the proposed method are neither smooth nor continuous,which consists of a series of short straight lines with inclined angles.The rigid displacement mainly occurs in the segmental rings,while the rotation and dislocation occur in the circumferential joints.The previous methods always give smooth and continuous tunnel displacement curves,which cannot truly reflect the real rotation and dislocation of the joints.(3)The analytical solution for longitudinal response of existing shield tunnel to overcrossing tunnelling considering the circumferential joints effects and continuity of soil is further proposed.The existing shield tunnel is first regarded as the SLBSM lying on the Pasternak foundation.Then,by virtue of the finite difference method,the complex tunnel-soil interaction and ring-to-ring interaction can be easily transformed into the numerical solutions.The longitudinal displacement,joint opening and dislocation between rings of the existing tunnel caused by the new tunnel are obtained.Finally,the feasibility of the proposed solution is tested by two well-documented cases and comparison with existing methods.Parametric analyses are further conducted to explore the effects on tunnel deformation.The results showed that the predicted longitudinal displacements from the present solution are consistent with those from the Timoshenko beam method and the field measurements,but the Euler-Bernoulli beam method slightly underestimates the tunnel longitudinal displacements.The dislocations obtained by the present solution are slightly lower than the Timoshenko beam method.Moreover,the obtained joint openings from the present solution are slightly higher than those from the Timoshenko beam method and lower than those from the Euler-Bernoulli beam method.(4)By the aid of the mathematical statistics,the failure probability assessment framework for the longitudinal deformation of existing tunnel due to overcrossing tunnelling considering the effects of the uncertainty of soil and tunnel structural parameters is constructed.First,based on the established deterministic model,a surrogate model is created by means of the sparse polynomial chaos expansion(SPCE)method.The surrogate model can significantly improve the computational efficiency of the deterministic model,and can be also used to perform the global sensitivity analysis(GSA).The Monte Carlo simulation(MCS)method is then combined with the surrogate model to calculate the failure probabilities of the existing tunnel deformations exceeding the allowable limits under different new tunnel diameters.Some dominant parameters are also analyzed to further investigate the effects on failure probabilities of existing tunnel deformations.The results indicated the uncertainties in the input random parameters associated with surrounding soils and circumferential joints have significant effects on the failure probability assessment of existing tunnel deformations.