Risk Analysis And Control Measures Of Underwater Shallow Shield Tunnel Construction

Shield method plays a more and more important role in urban tunnel construction because of its advantages,such as fast construction speed,little impact on the surrounding environment,high construction safety and excellent construction quality,In recent years,slurry balanced shield has been the first choice for the construction of underwater tunnels crossing rivers,lakes and seas.However,with the increase of the shield diameter,the uncertainty of the excavation layer and the complexity of the surface environment,the risk of underwater large-diameter shield construction is very high,which is prone to various risk events,especially the safety problems in the shallow stratum.Therefore,the construction safety of underwater large-diameter shallow shield tunnel under the condition of complex stratum is a basic subject that needs to be focused and studied in the process of tunnel construction in China.In this paper,a series of high-risk safety problems of slurry shield tunneling under the river bottom are studied systematically and deeply by means of data investigation,theoretical analysis,numerical simulation and engineering application.The theoretical and scientific basis is provided for the safe construction of the tunnel and the tunnel safety is ensured.The main research contents are as follows:(1)Based on the normal cloud model and entropy weight method,an evaluation model which can reflect the risk level in the early stage of underwater shield tunnel construction is proposed.From the aspects of river factors,tunnel design factors,hydrogeological factors,external environmental factors and construction management factors that affect the construction safety of the tunnel,an index system that can comprehensively reflect the risk factors of the underwater shield tunnel is constructed,and the range of each index under different risk levels is given.Combined with the normal cloud model method,the randomness and fuzziness of risk assessment process are effectively considered;Combined with entropy weight method,the regularity of evaluation data is considered to determine the weight coefficient of index,which avoids the subjectivity of expert weight method.(2)36 construction technology risk accidents in the process of shield tunneling in China in recent years are collected,and the occurrence time,accident situation,accident type and the direct cause of these accidents are statistically obtained.Combined with the engineering geological conditions,hydro geological conditions,unfavorable geology,tunnel construction methods and key construction difficulties of the underwater large-diameter shallow buried shield tunnel construction,the risk sources of the tunnel crossing the embankment section,shallow buried section and Karst Section during the shield tunneling construction are determined.Combined with the risk assessment model,the construction safety risk level of the shield tunnel is determined to be level II.(3)Combined with the numerical simulation method,taking the stability of the tunnel and the control of the surface deformation as the index,and considering the cost of the project,the influence of different water cement ratio and different reinforcement range on the construction safety of the shallow overburden underwater tunnel shield crossing the flood plain is simulated.Through the comparison and analysis of different control schemes,the best stratum reinforcement scheme considering construction safety and construction cost is determined.(4)The three-dimensional numerical model of parallel double line tunnel passing through karst area is constructed.The influence of karst cave on tunnel structure and stress is analyzed from the perspective of karst cave space location,karst cave space shape and multi karst cave.The risk of different positions of karst cave is obtained.The order of risk is: the risk of underlying karst cave > the risk of lateral karst cave > the risk of two axis karst cave > the risk of upper karst cave;The influence of the height and width of the karst cave on the stress of the tunnel structure is studied,which shows that the influence of the height of the cave on the deformation of the tunnel structure is greater than that of the width of the cave.In view of the karst disposal problem,the method of sand blasting and static pressure grouting is proposed for half filled and unfilled karst caves,and the method of static pressure grouting is adopted for fully filled karst caves with the density of filling material below the medium density.(5)Based on the limit analysis method,the model of "arc body +chamfered platform" is constructed for the passive instability of the limit thrust of the excavation face of shield tunnel in high water pressure and permeable stratum.According to the principle of statics and slice method,the solution formula of the limit thrust of the face is derived.Through the finite-difference software,the finite-difference calculation model of shield tunnel excavation in high water pressure and permeable stratum is constructed.The sliding instability of soil mass in front of the face of the face of the face under three levels of thrust is calculated,and the correctness of the calculation model of "arc body + inverted prism" proposed in this paper is verified.(6)The influence of depth,height of water level and permeability coefficient on the ultimate thrust of the face is analyzed by numerical method;Based on the stability coefficient method,the quantitative grade of the instability risk of the tunnel face is analyzed;In order to reduce the risk in the process of shield excavation and ensure the safety in the process of shield tunneling,the effective soil slurry ratio reinforcement,synchronous grouting anti floating and optimization of tunneling construction parameters are adopted.