Water-induced Disaster Mechanism And Safety Risk Control For Shield Tunnel Construction

Aiming at the safety risk control of EPB(earth pressure balance)shield tunneling in urban water-rich stratum,this paper highly generalizes the water inrush mode of typical urban stratum and systematically studies the mechanism,control measures and risk management system of water inrush by means of data investigation,numerical simulation,theoretical analysis and laboratory soil test.The main research contents and conclusions are as follows:(1)Based on the statistical analysis,the water inrush during shield tunneling is divided into two modes: erosion mode in sand layer and tension-shear failure mode in clay layer.(2)In view of the seepage-erosion characteristics of sandy soil,a triaxial test device considering complex stress state and seepage-erosion effect was developed,and the starting conditions and development rules of internal erosion of sandy soil were studied.Discrete element method(DEM)was used to carry out cubic triaxial shear tests,and the stress dependence of critical hydraulic gradient for sand erosion was explained from a mesoscopic point of view.The results show that the development of erosion has obvious phase characteristics and stress correlation characteristics.When the specimen approaches the failure state,the critical hydraulic gradient of erosion decreases with the shear ratio increasing.Under the same shear ratio,the critical hydraulic gradient of erosion increases with the confining pressure increasing.The stress transfer coefficient of fine particles shows obvious anisotropy with the shear ratio increasing,which is the fundamental reason for this phenomenon.(3)For the water inrush of erosion mode in sand layer,the stress criterion for erosion starting is established from the microscopic scale,and applied to simulate the water inrush evolution process at the macroscopic scale,presenting the process of initiation and expansion of hydraulic channels.The results show that the internal erosion of sandy soil generally occurs at the outlet of seepage and develops continuously upstream.The area where internal erosion occurs becomes the channel for concentrated flow of confined groundwater.(4)For the water inrush of tension-shear failure mode in clay layer,the built-in Mohr T model in FLAC3 D 6.0 is used to simulate the water inrush evolution process.The results show that the shear failure zone develops upward from the bottom of the cutting face,the tension failure zone develops downward from the groundsurface.When the two failure zones connect,there is a channel for concentrated flow of surface river.(5)Based on the three-dimensional rotating failure mechanism proposed by Mollon(2011)and the equivalent calculation method for the power of seepage force proposed by Viratjandr(2006),the upper limit solution of effective support pressure for cutting face considering groundwater seepage is derived.Through the secondary development of TFSS program(MATLAB open source code),the optimal solution of effective support pressure on cutting face is realized.The results show that the effective support pressure on the cutting face increases with the seepage flow increasing and decreases with the soil cohesion increasing.(6)Based on the above research results,the risk management system of water inrush during shield tunneling is put forward and established.In the construction stage,the dynamic control of water inrush safety risk can be realized by the fusion and feedback of multi-monitoring measurement information.The risk management system has been successfully applied in the construction of an underwater shield tunnel of Tianjin metro line 6.