Study On The Stability Of The Face And Pre-Reinforcement Method In Soft Surrounding Rocks

With the rapid development of tunnel construction and technology,tunnel engineering under soft surrounding rock geological conditions will face mechanized large-section/full-section excavation construction.How to ensure the stability of the tunnel face is the key issue to realize large-section/full-section excavation.This paper adopting the method of combining literature investigation,numerical simulation and filed monitoring to analyze the stability of the face of large-section tunnel and the pre-reinforcement method and its construction parameter's influence on the deformation and stress of surrounding rock.The main contents and results are as follows:(1)Briefly describe the research status of the stability of tunnel face and pre-reinforcement method domestic and abroad.(2)The development law of the failure characteristics of tunnel face and the variation law of surrounding rock stress in front of the face under different buried depths and different surrounding rock grades are analyzed.The results show that both the surrounding rock grade and the buried depth have a significant influence on the failure characteristics of the face.The final failure depth above the crown of class IV surrounding rock is 7.5m,and the failure depth in front of the face is 8m.The final failure depth above the crown of the class V surrounding rock is 14m,and the failure depth in front of the face is 13m;After excavation,the vertical compressive stress of the surrounding rock in front of the face is obviously increased,which is about 1.5 to 2.0 times of the original rock stress,the horizontal stress is significantly reduced,and the horizontal stress at the face is reduced to zero.(3)Comprehensively analyzed the mechanical characteristics of deformation,stress and supporting structure stress in soft surrounding rocks under different excavation methods.The results show that the pre-reinforcement method through enhancing the rockmass strength of the core zone in front of face by glass fiber bolts,owning certain advantages in controlling the deformation and stress of the surrounding rocks;At the same time,compared with the three-step method,the pre-reinforcement method has a large construction space and can introduce large-scale construction machinery,which can effectively control the project schedule,project quality and construction safety,and has a larger comprehensive advantage.(4)Based on the pre-reinforcement method,the influences of reinforcement parameters of the bolts and the excavation footage on the deformation and stress of the surrounding rocks under different surrounding rock conditions are systematically analyzed.The safe excavation footage of class IV and class V surrounding rock tunnel is confirmed according to the stress of surrounding rocks under different excavation footages,and reasonable reinforcement parameters of the bolts of class IV and V surrounding rock tunnel is confirmed according to the deformation of the surrounding rocks under different parameters of the bolts.(5)Based on the monitoring and measurement,the field monitoring datas were analyzed,and the surrounding rock deformation and the stress state of initial steel arch were mastered and compared with the numerical simulation results to verify the rationality of the numerical simulation.