Analysis Of Mechanical Properties Of Deepburied And Large-span Tunnel Excavation And Study On Rapid Construction Method

In the process of tunnel construction,tunnels often have the characteristics of large excavation span,deep burial,difficult construction,complex construction mechanical properties and long construction period.Based on the qiaopingshan tunnel of Chongqing East Ring Road,this paper analyzes the applicability and supporting effect of three common excavation methods of large-span tunnel,namely,three bench method,CRD method and double side heading method.Combined with the actual situation of the site,this paper studies the conversion of the original design method of double side heading method,and puts forward three methods for the section with better geological surrounding rock condition Step rapid construction method,and through the numerical simulation analysis results,predict the deformation sensitive parts and the occurrence period in the construction process,and design a targeted monitoring scheme.The main research contents and achievements of the three-step rapid construction method are as follows:(1)In mechanical mechanism,the distribution of surrounding rock stress corresponding to different construction methods is qualitatively analyzed by axial strain theory.The analysis results show that the stress distribution of surrounding rock is more uniform and the stability of surrounding rock is better by double side heading method and CRD method than by three-step method.Through numerical simulation method,the distribution characteristics and potential of surrounding rock stress of double side heading method,CRD method and traditional three-step method are compared and analyzed The results show that the surrounding rock deformation and stress distribution caused by the three-step method is slightly worse than the CRD method and the double side heading method after the retaining structure is closed into a ring,while the adverse effect caused by the three-step method is much greater than the CRD method and the double side heading method before the retaining structure is closed into a ring,especially when the upper step is excavated.It is inferred that the optimization of the supporting measures of the three-step method can improve the stability of the surrounding rock The three-step method is used in the field construction of this kind of large-span tunnel.(2)The support parameters of the traditional three-step method are optimized to adapt to the large-span tunnel.In the aspect of bolt,from three aspects: whether the bolt can be anchored into the stable surrounding rock,providing enough anchoring force and forming the overall stability between the bolt and surrounding rock,through the method of numerical simulation,the influence of bolt length,bolt diameter and bolt circumferential spacing on the tunnel is studied.The results show that the bolt length should be controlled at about 8m,if the bolt is too short,it can not be anchored to the deep layer The stability of surrounding rock leads to large overall deformation and Instability Risk;the diameter of bolt is at least 22 mm,if the bolt is too small,it can not provide enough anchorage tension;the circumferential spacing of bolt should be controlled at about 1m,if it is too sparse,the integrity of bolt and surrounding rock is poor.In the aspect of initial support,the influence of steel arch type is studied from the aspect of whether it can provide enough bending and compressive stiffness.The results show that the steel arch type should be i22 b.If the steel arch type is too small,the initial support structure is unfavorable in compression and bending,its internal force is too large,and the safety factor is low.(3)The results show that: after the excavation of the upper step,two locking anchor pipes are laid at the two arch feet of the upper step,which can enhance the bending rigidity of the tunnel arch waist,reduce the bending movement and indirectly control the horizontal displacement;after the excavation of the middle step and the lower step,two locking anchor pipes are laid at their arch feet It is beneficial to improve the bending moment distribution of the side wall and reinforce the surrounding rock outside the two walls,so as to reduce the risk of instability.The influence of excavation footage and bench length is studied.The results show that the excavation footage should be controlled at about 1m;if the footage is too long,the initial support can not be carried out in time,which leads to the long exposure time of the burrow and the risk of instability;if the footage is too short,although the support can be carried out in time,it increases the number of construction blasting,which will disturb the surrounding rock many times and is not conducive to the stability of the surrounding rock The length should be controlled at about 10m;if the bench is too short,the working face of each bench is relatively close,and their mutual disturbance during construction is relatively large;if the bench is too long,the initial support can not be closed as soon as possible.(4)This paper summarizes the construction steps of the three-step rapid construction method,and combined with the numerical simulation results,designs the monitoring scheme for the sensitive position and excavation period of the tunnel deformation,guides the dynamic construction of the project by monitoring the real-time data,and summarizes the key technologies of the three-step rapid construction method,which is used to guide the construction of the project.