Study On Applicability Of Early Ring Closure For Tunneling In Squeezing Rocks

When tunnels are built in squeezing surrounding rocks,the initial deformation rate of the tunnel is rapid,the cumulative deformation is large,and it is difficult to control because of the small strength-stress ratio of the surrounding rock and the remarkable time-space effect of the deformation.How to control the squeezing deformation and ensure the construction safety of tunnel is an important research topic.In recent years,with the improvement of mechanized construction level,tunneling in squeezing surrounding rock actively adopted the large cross-section excavation and early ring closure,together with secondary support(multiple support)to control deformation and avoid support damage.However,the mechanical mechanism and applicability of the early ring closure in squeezing surrounding rock tunnels still need to be further studied.Based on the Yuelongmen Tunnel of Chenglan Railway,this paper makes a comparative analysis of tunnel deformation,plastic zone and support force under different closure distance conditions by using three-dimensional numerical simulation technology.Studies the main influencing factors of adopting early ring closure.At the same time,the applicability of early ring closure is analyzed and compared according to different mechanical characteristics of surrounding rock.Taking the left line tunneling of Yuelongmen Tunnel as the object,through the comparison of numerical simulation calculation and in-situ monitoring data,the reinforcement effect of secondary support under the condition of yielding of primary support is analyzed,and the application effect of early ring closure in squeezing deformation tunneling is discussed.The main results are as follows:(1)The closure distance is the main factor affecting the early ring closure.Through three-dimensional numerical simulation,the effects of different closure distances on tunnel deformation,plastic zone and support force are obtained.When the closure distance is less than one time of the tunnel diameter,the control effect of tunnel deformation is remarkable,and the shorter the closure distance,the better the control effect When the closure distance exceeds 1 or 1.5 times the tunnel diameter,the control effect of tunnel deformation will be significantly weakened.The control effect of plastic zone of early ring closure is obvious:the shorter the closure distance is,the smaller the plastic zone range is.In addition,the closure distance also has a great influence on the support force:when the closure distance is less than 1 times the diameter of the tunnel,the support force is very large,and the support may reach into yielding.(2)The applicability of early ring closure is closely related to the mechanical properties of surrounding rock.For the surrounding rock with the same strength-stress ratio,the three-dimensional numerical simulation analysis shows that the surrounding rock with the dominant cohesion strength(cohesion)is more obviously affected by the early ring closure than the surrounding rock with the dominant friction strength(internal friction angle).The early ring closure is more effective in controlling the deformation of squeezing surrounding rock which is dominated by cohesion strength.(3)With the actual construction situation of the large deformation section where the primary support of Yuelongmen Tunnel has been destroyed,the validity of using secondary support(multiple support)in the early ring closure is studied.It is concluded that the additional radial stress of secondary support to primary support can effectively improve the strength of primary support.At the same time,after the construction of secondary support,the deformation rate of the tunnel decreases greatly and the deformation value converges quickly.Secondary support and primary support form a multiple support system together,which is soft first and then rigid.It can be seen that multiple support is an effective means to control severe squeezing deformation.