| The collapse of soft rock tunnel is one of the core problems that plagued the safe construction of the tunnel in China.In view of the high level of collapse accidents in China in the past and in the current period,the research on the collapse control of the soft rock tunnel is particularly urgent.This paper takes the collapse of the excavation face of the soft rock tunnel as the research object and based on more than 180 typical cases of tunnel collapse disasters,the occurrence time,symptoms,location,scale,influence scope and geological description of the area are analyzed.The characteristics and influencing factors of the collapse disaster at the excavation surface of the soft rock tunnel are determined.Combined with the basic theory of convergence-constraint method,the simplified assumption model is solved to obtain the analytical solution of the maximum extrusion deformation of the excavation face of the soft rock tunnel and a detailed analysis is carried out.The numerical analysis and field monitoring data are used to supplement and demonstrate the theoretical analysis.At the same time,the control effect of different construction conditions on the deformation of tunnel face is studied,and the influence and mechanism of pre-reinforcement bolts on the maximum displacement of excavation face are also discussed.The following important conclusions have been drawn:(1)Based on the characteristics and influencing factors of collapse in soft rock tunnel which obtained from the statistics of the collapse accident cases.The characteristics of soft rock tunnel collapse are summarized as 3 types,the collapse of tunnel portal section,the excavation of excavating chamber behind the excavation face,and the collapse of excavation face.The caverns collapse accounted for 32%,tunnel portal accounted for 7%,and the excavation surface and the rear area of the surrounding rock accounted for 61%.The collapse of the excavation surface is the main form of the construction accident of the soft rock tunnel.On this basis,the factors that affect the collapse of tunnel excavation surface are mainly including engineering geological conditions,supporting conditions and construction procedures.(2)Taking the surrounding rock to be excavated in front of the tunnel and the pre-reinforcement bolts of the excavation surface as the research object.According to the convergence-constraint theory,six analytical models for different stress states of surrounding rock and pre-reinforcement bolts in the front of a circular tunnel are established.The expression of radial stress and tangential stress under different analytical models were derived and also the expression of dimensionless U*for describing the extrusion motion of the excavation face were derived.The effects of the tunnel depth,cohesion,and the length,stiffness,strength of the pre-reinforced bolts on the stress and displacement of the working face were analyzed.The results show that the pre-reinforced bolt can effectively control the maximum extrusion displacement of the excavation surface.(3)Using F1ac3D to analyze the effects of different excavation conditions,different depth conditions,and different pre-reinforced bolt arrangement conditions on the extrusion deformation of the tunnel excavation surface,the maximum extrusion displacement of the tunnel excavation surface obtained from the theoretical analysis agree well with the numerical analysis results.The step method can effectively control the extrusion deformation of the tunnel excavation surface and maintain the stability of the surrounding rock;the maximum extrusion deformation of the excavation surface occurs in the center of the excavation surface;the largest extrusion deformation will reducedwith the increase of the length and density about the bolt,but when the length of the anchor bolt is greater than the tunnel radius,The maximum extrusion deformation value maintains a stable value even through the length of the anchor rod is continued to increase.the maximum extrusion displacement value of the excavation surface which under different depths and bolt density iscompared with the theoretical calculations in Chapter 3,the maximum relative error between the two analysis modes is 22%.(4)The numerical simulation results are basically consistent with the on-site monitoring data of the Youzhushan tunnel,indicating that the model can better reflect the actual situation of the Yuzhushan tunnel.At the same time,with the advancing of the excavation surface,the extrusion displacement of the study section gradually increased. |
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