| During the construction of a new tunnel overcrossing existing shield tunnels at close proximity,the existing tunnels should be protected by protective structures and ground improvement measures.However,the construction of these structures,ground improvement,and excavation may cause movement or deformation to the existing tunnels,potentially jeopardizing their operational safety,particularly under soft soil and sensitive ground conditions.Based on the Xingangjie tunnel overcrossing shield tunnels of subway Line 1 at close proximity in Suzhou,the field tests and field monitoring through the whole construction process were carried out.Combined with numerical simulation and theoretical analysis,this thesis systematically studies the deformation mechanism and deformation control methods of the underlying subway shield tunnel during the whole construction process of tunnel that constructed by the cut-and-cover method in soft soil ground.Specifically,the researches include the influence law of each construction stage on the existing shield tunnel,the influence of main construction parameters on the existing shield tunnel deformation,the theoretical prediction method and control methods of the existing shield tunnel displacement,etc.The main research results are as follows:(1)Based on the monitoring data,the response law of the underlying subway shield tunnel in the whole construction process of the new municipal cut-and-cover tunnel in the soft soil ground is revealed.The monitoring results show that each construction stage has different influences on the deformation of the shield tunnel.The construction of soil mixing wall(SMW)causes the largest settlement(3.6 mm),the construction of deep soil mixing(DSM)causes the largest horizontal displacement(4.7 mm),and the staged excavation causes the maximum uplift(3.5 mm)at the crown of the tunnel,the displacement of shield tunnel caused by the construction of bored pile is less than 1 mm.The maximum total displacement of the shield tunnel occurred in the DSM stage.The maximum horizontal displacement of the spring line is7.6 mm,which exceeds the warning value(7.0 mm).The maximum settlement of the crown and track are 6.4 mm and 2.8 mm,respectively.In the SMW stage,the deformation of the shield tunnel is mainly caused by the construction of SMW on both sides of the tunnel,the crown and the track settled,and the spring line moved outward the tunnel;In the DSM stage,the deformation of the tunnel is mainly caused by the installation of DSM on both sides of the tunnel and in the middle of the two tunnels,the tunnel settled and moved outside the DSM construction area.During the staged excavation,the tunnel uplift is mainly caused by the excavation of the area directly above it.(2)A refined numerical model considering the staggered joint assembly and bolt connection of shield tunnel segments is established,which realizes the refined analysis of the stress and deformation characteristics of the shield tunnel.The research shows that the simulation results of shield tunnel deformation caused by construction of SMW,DSM and staged excavation are in good agreement with the field monitoring data.The maximum compressive stress(at the invert)and tensile stress(at the crown)of the tunnel segment are 8.9MPa and 0.6 MPa,respectively.The maximum shear stress(at the invert)is 7.3 MPa.The maximum tensile stress(at the spring line)of longitudinal bolts is 36.3 MPa,and the maximum tensile stress(at the crown)of circumferential bolts is 193.4 MPa.The maximum openings of the longitudinal and circumferential joints(at the spring line)are 0.7 mm and 0.24 mm,respectively.(3)Through refined numerical analysis,the influence of the SMW depth and its distance from the shield tunnel,DSM depth and its construction sequence,the staged excavation sequence and divided pit number on the deformation of the underlying shield tunnel are discussed.The simulation results show that the displacement of the shield tunnel decreases with the decrease of the construction depth of SMW and DSM.When the depth of SMW or DSM is less than the depth of the shield tunnel axis,the displacement of shield tunnel is less than 0.5mm.When the distance between the SMW and shield tunnel increases from 1 m to 2.5 m,the settlement of the crown and the horizontal displacement of the spring line caused by SMW construction are reduced by 20.1% and 43%,respectively.When the DSM areas directly above the tunnel are installed first,the installation direction of DSM areas on both sides of the shield tunnel starts at the farthest row,and then approaches the shield tunnel,which can reduce the displacement of the shield tunnels.The tunnel deformation decreases with the increase of the divided pit number of staged excavation,and the excavation sequence of excavating the areas on both sides of the tunnel first and then excavating the areas above the tunnel has the best effect on controlling the tunnel deformation.(4)Based on the field test data and Timoshenko beam theory on elastic foundation,a method for predicting the displacement of shield tunnel caused by SMW construction is proposed,and the predictied results are close to the field monitoring data.In addition,considering the influence of shield tunnel joints,combined with the earth pressure theory and structural mechanics,the deformation of shield tunnel lining caused by SMW construction unloading is deduced.The maximum horizontal displacement of the spring line calculated by theoretical method is 0.8 mm larger than the monitoring data,with a relative error of 32%.(5)Based on Mindlin solution,Odemark equivalent ground layer method and equivalent elastic parameters,a theoretical analysis method for calculating the additional stress and displacement of the soil caused by staged excavation in reinforced foundations is proposed.Based on the theory of Timoshenko elastic beam in Pasternak foundation,the displacement differential equation of the shield tunnel under the action of additional soil displacement and stress derived.Engineering examples show that the additional stress of the soil caused by staged excavation is calculated based on equivalent ground layer method and Mindlin solution,and the displacement of the shield tunnel is calculated by the theory of Timoshenko elastic beam in Pasternak foundation,which can reflect the control effect of isolation structure,foundation improvement and staged excavation on the displacement of shield tunnel.(6)The control effects of different protection methods on the deformation of the existing underlying shield tunnel are discussed.For excavation in soft soil area,SMW isolation wall+DSM ground improvement+bored pile and pile-slab protection methods can control the deformation of shield tunnel well.When there are bored piles,the elastic modulus of the reinforced foundation should be 20 ～ 30 times that of the original soil,and the depth of reinforced foundation should be between 1 m and 1 times of tunnel diameter below the invert of the tunnels.The deformation control method for existing shield tunnel with layered foundation improvement and bored piles frame protection structure is proposed,which can be used to strict control the deformation of the existing shield tunnel in soft soil area. |
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