Study On Construction Stability Of Three-lane Small Clear Distance Highway Tunnel

In order to comply with the growing economic and transportation development requirements,the frequency of three-lane small clear distance highway tunnels has become higher and higher in China's highway construction.Because such type of tunnel structure has special characteristics compared with ordinary tunnels,the tunnel with twohole construction has great influence,and it is necessary to solve the key problems in its design and construction.It is necessary to carry out systematic and in-depth analysis and discussion.Through theoretical analysis,numerical simulation and on-site monitoring and measurement,this paper relies on Menggudao tunnel of reconstruction and expansion project of Zhamalong to daotanghe in Qinghai Province and analyzes and studies several problems in the design and construction of super-large section and small clear tunnel,and completes the following main tasks:(1)For the three-lane small clear tunnel with large cross section and small clearance,the numerical simulation method is used to explore the super large section and small clear tunnel.The advantages and disadvantages of the combined method of benching tunnelling method,CD method and double drift method for the surrounding rock sections of tunnels of Grade IV and V are analyzed.The characteristics of settlement and convergence of surrounding rock of tunnels,stress distribution of concrete structure and bolt structure and distribution of plastic zone are analyzed.By comparing the construction mechanical properties of various tunnel construction methods,the three benching tunnelling method + three benching tunnelling method is selected as the construction scheme of the V-class surrounding rock section,and the step method + step method is the construction scheme of the IV-level surrounding rock section.(2)The distance between the two faces of the super-large section and small clearance tunnel affects the construction safety of the tunnel.In this paper,numerical simulation is used to analyze the characteristics of surrounding rock settlement and convergence,concrete structure stress and plastic zone distribution under four different working conditions(0D,0.5D,1D,2D).It is recommended to use a right hole first construction method with a 2D distance of the face and strengthen the vault and arch support at the junction of the V-class surrounding rock section and the IV-class surrounding rock section.The influence of blasting construction of V-class surrounding rock section and IV-class surrounding rock section on the tunnel concrete structure was analyzed by numerical simulation.The vibration velocity and force characteristics of the concrete structure are analyzed.It is concluded that the position of maximum vibration velocity and the force are located in the arch waist and arch foot,which is the key monitoring object in the tunnel blasting construction.(3)The displacement data of the V-class surrounding rock section and the IV-class surrounding rock section are obtained from the monitoring of the tunnel and the rationality is verified by comparison with the simulation data.The vibration speed of the first hole and the existing slope were monitored during the construction of the second hole.Time-frequency characteristics and instantaneous frequency were analyzed using HHT method.The date was contrasted with the requirements of the specification to judge the stability of the surrounding rock and the existing slope.The ultrasonic detector was used to monitor the loose zone of the tunnel,determine the size of the loose zone and verify the rationality of the bolt length setting.