Research On The Environmental Effects Of Soft Ground Shield-driven Tunnel And Its Responses To Adjacent Excavation

Shield tunnellling in soft ground condition will inevitably disturb surrounding ground and cause surface ground movement and deep soil movement, which will threat the safety and serviceability of existing nearby structures and underground facilities. When the construction is completed and the tunnel is on operation, adjacent construction such as shield tunnel crossing over existing shield tunnel, shield tunnel drown crossing existing shield tunnel, nearby excavation and so on will induce additive internal forces and movements of the existing shield tunnel. It will lead to a series of adverse effect on existing shield tunnel, including segmental lining cracking, bolt loosening, lining seepage and longitudinal uneven settlement. Therefore, it essential to propose reasonable and simple methods to predict shield-driven induced environmental effects and assess the adverse effects of shield tunnel associated with adjacent construction after the completion of shield tunnelling. Based on the previous studies, the shield diving induced environmental disturbance effects and performances of shield tunnels subjected to adjacent construction (including new shield tunnel acrossing over existing shield tunnel, nearby excavation, shield tunnel crossing beneath existing shield tunnel) are investigated and analyzed by field measurements and analytical methods. Based on the analyses, some main and useful conclusions are drawn as follow:(1) Combination of the vi method and linear-curve fitting method is suggested to identify the boundary between immediate and consolidation settlements. It is found that in the Hangzhou soft ground the ground surface settlement can mainly be divided into 3 stages and 8.5-9 day after the passing of shield tail can be recognized as the ending of immediate settlements and the start of consolidation when the shield advances in normal condition and no significant heave is induced. However, when the heave of ground surface is caused by back-fill grouting, the development of ground settlement can be divided into 5 stage and due to the grouting disturbance 11～12 day after shield tail passage can be considered as the boundary between immediate and consolidation settlement which is 2.0～3.5 day delay when compared to the normal shield tunnelling condition.(2) Analytical method is proposed to calculate the tunnelling induced excess pore water pressure considering the ground volume loss. Based on the analyses, it is found that the calculated excess pore water pressures based on the Skempton’s theory are in close agreement with measured data and Finite Element Analyses results. However, Henkel’s theory cannot give reasonable results and overestimates the values of induced excess pore water pressure.(3) Analytical method for calculating the ground surface movements and deep soil movement is proposed by considering the shield tunnelling parameters. The proposed solution accounts for the additive pressures induced by the compressing effect of cutter-head, the non-uniform distribution of softening shield skin frictions along the shield in soft soils, the tail grouting pressures and the soil movements caused by soil loss. It is found that the proposed method is able to reflect the heave of caused by shield-driven induced and back-fill grouting; Moreover, the horizontal deep soil movement caused by shield tunnelling can also be reflected by the proposed method.(4) The analytical method for predicting the shield tunnel longitudinal movement associated with the over-crossing tunnelling is proposed. In the proposed analytical method the existing shield tunnel is simulated by Euler-Bernoulli beam and the tunnel-ground interaction is considered by Pasternak foundation. The over-crossing tunnelling induced unloading loads on the existing tunnel is computed using the Mindlin’s elastic solution. The equilibrium differential equation of shield tunnel longitudinal movement is established and it is numerically solved by using the Finite Difference method. The feasibility of the proposed method is verified by two published case histories. The parametric analysis is also given to study the influences of different factors on the tunnel longitudinal deformation.(5) Due to the existence of segmental joints, the general shearing stiffness of shield will be weakened. In order to further take the segmental joint into account, the analytical method for the responses of shield tunnel subjected to adjacent excavation is proposed by considering the shearing effect of shield tunnel. In the proposed method, the shield tunnel is simulated by Timoshenko beam with two generalized displacement. The unloading loads due to excavation are computed using Mindlin’s solution and the corresponding loads are imposed on the existing tunnel. The equilibrium differential equations of shield tunnel longitudinal movement are established due to adjacent excavation. The equilibrium differential equations are solved numerically by using Finite Difference method. The measured results from three well-documented published case histories are selected to compare with the predicting results calculated by the proposed method and it is found the computed results are in good agreement with the measurements. It shows that the shield tunnel’s bending moment and shear force are remarkably overestimated by using the Euler-Bernoulli beam theory to simulate the tunnel deformation. Moreover, the tunnel longitudinal segmental lining dislocation distribution due to excavation can be obtained by using the proposed method.(6) The analytical solution for the shield longitudinal responses due to drown-crossing tunnelling is proposed. The existing shield tunnel is simplified as Timoshenko beam that is able to take account of the tunnel shearing effect. Two-stage analysis method is applied to analyze the deformation and internal forces of shield tunnel caused by the tunnel excavation. First, the tunnelling-induced ground settlement is predicted using the analytical method proposed by Loganathan and Polous (1998). Second, the induced ground settlement is imposed on the existing tunnel and the equilibrium differential equations that consider tunnel shearing effect are established based on the Timoshenko beam theory. Eventually, the tunnel longitudinal internal forces and displacement due to the corresponding induced settlement are solved numerically using the Finite Element method. The field measured results from two well-documented published case histories are especially selected to verify the feasibility of the proposed method. It is found that the calculation results based on the Euler-Bernoulli beam theory will overestimate the tunnel’s bending moment and shear force.