Study On Mechanical Behavior Of Tunnel Excavation In Shield Tunnel With Complicated Conditions

Shield Tunneling Method,with its advantages of fast tunneling speed,safety and reliability,and good construction quality,has been mostly used in the construction of subway tunnels.However,due to the existence of special cross-section tunnels such as subway stations,re-entry lines,and bifurcation tunnels,the shield construction section was severely divided,causing excessive excavation time,reduced average excavation speed,delayed construction schedule,and increased cost.At the junction of the left line and tie line of Zhengzhou Metro Line 2 are affected by the line and surrounding construction environment,and face difficulties such as tight construction period and difficult construction.There are no precedents for similar projects in China.In order to effectively solve the contradiction between shield tunnel construction and special tunnel section construction,and shorten the construction period,this paper tries to apply the concept of "tunnel first and station later" to the expansion and excavation of the interval tunnel according to the characteristics of the project.That is to say,the shield machine is first used to pass through the special section directly,and then the mining method is used to demolish the shield tunnel at the side of expanding the tunnel in the opposite direction.This method not only gives play to the efficiency of the long-distance tunneling of the shield machine,but also makes full use of the mature experience of the mining method construction.Taking the construction scheme as the research object,this paper analyzes the construction mechanics’ characteristics of the large cross-section tunnel excavated by the shield machine interval.The stress transformation law of lining and segment,construction risk,and the influence of the construction process on the surrounding environment are studied in depth.The main results achieved are as follows:(1)Based on numerical simulation,by analyzing the characteristics of surface displacement,dome settlement and structural stress,the influence law of the excavation section excavation sequence on the tunnel is revealed.The results show that the sequence of step 1(the upper left Part-the upper right part-the lower left part-the lower right part)is more reasonable.(2)On the basis of the shield tunnel,the adaptability analysis of a large cross-section tunnel constructed by mining method under different surrounding rock levels is carried out.The conclusion is as follows: the upper half of the excavation tunnel and the dismantling support are particularly prominent for the stratum disturbance.Under the parameters of surrounding rock of Ⅵ,the environmental impact caused by shield expansion is more intense.Therefore,in order to carry out shield excavation construction in Ⅵ grade surrounding rock,the necessary auxiliary construction measures should be taken and the data and information of monitoring should be strengthened before the expansion construction can be carried out.(3)The reinforcement effect analysis of large-section tunnels under different pre-reinforcement measures was carried out by shield expansion and excavation.The results show that the construction method 4(simultaneous application of pipe shed and advanced small pipe grouting reinforcement)is most beneficial to the expansion of shield tunnels.The reinforcement effect is the most ideal,and it is a safer one-stage pre-reinforcement measure.(4)According to the design scheme of shield tunneling construction,a three-dimensional finite element numerical model is established to analyze the force and deformation of the tunnel structure,and the stability of the surrounding rock and the rationality of the support design is judged.The results show that the safety factors of the segment,initial support and temporary support meet the safety requirements of structure strength in 《Code for Design of Railway Tunnel》(TB-10003-2016).the deformation of the segment and the initial support also meet the requirements of《Code formonitoring measurement of urban railtransit engineering》GB50911-2013.It shows that the structure is in a safe state,and the design scheme of expansion and excavation is reasonable.(5)According to the design scheme of shield excavation construction,a three-dimensional finite element numerical model is established,and the influence of shield excavation on surface settlement,pipeline settlement,building settlement and inclination,and displacement of pile foundation is analyzed.The results show that the tunnel expansion has the obvious impact on the surrounding environment,but the impact of the existing construction scheme on the surrounding environment is within the scope of design control standards.(6)According to the design of the shield excavation construction,based on the on-site monitoring results,the ground settlement,arch settlement,pipeline settlement,building settlement,surrounding rock pressure,concrete stress and steel arch stress caused by shield expansion are analyzed.The variation characteristics show that the settlement of each monitoring point has stabilized,the deformation of surrounding rock tends to be stable,and the design scheme of tunnel expansion is reasonable.The measured results and numerical analysis results are compared and analyzed.The conclusions show that the field measurement and numerical analysis results are in good agreement,which verifies the feasibility of the research method and the reliability of the conclusion.(7)Based on the measured data,regression analysis was carried out,and the general law of settlement deformation was obtained.The results show that the lateral ground settlement of each section satisfies the Gaussian curve law of Peck formula,and the range of ground subsidence obtained by fitting analysis is 12.24～18.13 mm.The change of ground subsidence with time satisfies the variation of the BiDoseResp function curve.The settlement of the vault satisfies the logistic curve law,and the fitting settlement shows that the range of the vault settlement is between 16～26mm.