Study On Mechanisms And Chain Effect Of Disasters Due To Subway Tunnel Excavation

At present,the development and utilization of urban underground space dominated by the subway construction in China has entered a climax.Due to the complexity and uncertainty of urban underground engineering,when the subway tunnel is excavated in urban area under the condition of the high-reliability operational requirements and the extremely fragile environment,improper control can easily induce various types of disasters,resulting in heavy casualties and economic losses.The safety problem has become the key problem that must be solved first during the subway construction in China.To improve the safety of the subway tunnel excavation,it is necessary to conduct an in-depth study on the mechanisms and the chain effect of disasters due to subway tunnel excavation.This paper takes the structural system composed of the subway tunnel,the ground and the existing building as the research object,and focuses on the engineering disaster,the environmental disaster,and the chain effect of environmental disaster caused by engineering disaster transmission due to subway tunnel excavation.Different methods including theoretical analysis,numerical simulation,model test and field monitoring are used in combination.The characteristics of the three-dimensional ground deformation due to subway tunnel excavation,the mechanism of the tunnel face collapse due to subway tunnel excavation,the characteristics of the mechanical responses of the building induced by the ground deformation,and the disaster chain of the subway tunnel excavation and the chain-cutting disaster mitigation measures are systematically investigated.The main research work and the results are as follows:(1)The prediction method of the three-dimensional ground deformation due to subway tunnel excavation is proposed.Using the subway tunnel deformation pattern considering the nonuniform convergence and the ovalization,the prediction method of the three-dimensional ground deformation due to tunnelling are proposed based on the virtual image technique.The proposed method is suitable for both clayey soil and sandy soil.The characteristics of the three-dimensional ground settlements and ground transverse displacements due to tunnelling are illustrated.The variation laws of three-dimensional ground deformation with typical factors are revealed.The results indicate that the transverse surface settlements mainly occur within H/tan(45°+φ/2)+R from the tunnel axis on both sides of the tunnel,and the variations of the longitudinal surface settlements are mainly produced within ±2H from the tunnel face.The positions of the maximum surface transverse displacements move towards the tunnel axis and are fixed as the internal friction angle and Poisson’s ratio increase,respectively,and move away from the tunnel axis as the cover depth and diameter of the tunnel increase.The maximum subsurface transverse displacement always occurs near the tunnel spring line,regardless of the variations of the four typical factors mentioned above.(2)The three-dimensional analysis model of the face stability of the subway tunnel is established.A three-dimensional log-spiral prism model considering unsupported length is established based on the limit equilibrium method.The shape of the proposed model is more consistent with that of the actual collapse zone in sandy soil.The proposed model is applicable to the tunnels excavated by shield method and shallow tunnelling method.The calculation formulas of the limit support pressure for the homogeneous ground and the multilayered ground are presented.The variation laws of the limit support pressure and the collapse zone with different factors,and the sensitivities of the limit support pressure to different influencing factors are revealed.The results indicate that the limit support pressure appears linear relationships to the soil unit weight,cohesion and surface surcharge,and increases significantly with the increases of the tunnel diameter and aspect ratio.The cover depth ratio has little effect on the limit support pressure when the cover depth ratio exceeds 2.0.The extent of the collapse zone decreases noticeably with the increase of the internal friction angle.The log-spiral slip surface turns steeper and the collapse zone above the tunnel extends behind the tunnel face as the unsupported length increases.(3)The calculation method of the mechanical responses of the building induced by the ground deformation is proposed.The strip foundation building is equivalent to a Euler-Bernoulli beam on the Pasternak model.The calculation method of the mechanical responses of the masonry building and the framed building induced by the ground deformation are proposed based on the two-stage method.The proposed method takes into account the tunnelling process,and can be applied to the condition that the angle between the building axis and the tunnel axis is arbitrary.When the relative positions between the building and the tunnel are different,the characteristics of the settlements,rotation angles,bending moments and shear forces of the building are illustrated.The variation laws of the deformation and internal force of the building with typical factors are revealed.The results indicate that the differential settlement of the building reaches the maximum and the rotation angles are symmetric with respect to the building centerline when the tunnel face arrives at the middle of the building.The maximum bending moments occur at the middle of the building,while the maximum shear forces occur at about 1/5 and 4/5 of the length of the building when the tunnel face is located at the two ends of the building.The angle between the building axis and the tunnel axis,elastic modulus and Poisson’s ratio of the soil,bending stiffness and gap parameter have great effects on the mechanical responses of the building.(4)The mathematical model of the disaster chain of the subway tunnel excavation and the mechanical model of the chain-cutting disaster mitigation measure are established.The chain-styled theory of the disaster is introduced herein.The structural relationship and the evolution principle of the disaster chain are clarified.Three ways of the chain-cutting disaster mitigation are presented.The mathematical model of the disaster chain of "tunnel face collapse→ground collapse→building damage" is established based on the catastrophe theory.The evolution process of the disaster system is analysed.The mechanical model of the horizontal grouting reinforcement measure is established based on the equivalent layer method.The evaluation index of the blocking effect of the reinforcement layer on the ground deformation transmission,which is named the blocking efficiency,is presented.The variation laws of the mechanical responses of the building and the blocking efficiency with the parameters of the reinforcement layer are revealed.The results indicate that the chain-cutting disaster mitigation ways consist of three types: improving the state of the external environment,blocking the disaster transmission path,and improving the bearing capacity of the disaster-bearing body.The mathematical model of the disaster chain of "tunnel face collapse→ground collapse→building damage" is a cusp catastrophe model.As the elastic modulus and thickness of the reinforcement layer increase,the deformation and internal force of the building gradually decrease,whereas the blocking efficiency gradually increases.The distance between the bottom of the reinforcement layer and the tunnel crown has no effect on the mechanical responses of the building and the blocking efficiency.