Study On Stability Of Tunnel Surrounding Rock Crossing Coal Gob Strata

In the past ten years,the number of tunnels built has increased rapidly,especially in highway tunnels,with an annual increase of more than 1000 km.Frequent tunnels are inevitably crossing the coal gob strata.The softness of the surrounding rock in the coal strata and the spatial variability of the coal gob will has an extremely unfavorable effect on rock stability.Based on the application and development of Chongqing City,the key project "the key technology research of gas control and gob treatment through the coal seam formation",based on Huayan tunnel crossing the coal gob strata for the engineering background,based on the theory of tunnel mechanics,through the establishment of tunnel crossing the coal gob strata elastic-plastic model by FEM,combined with on-site monitoring and analysis,stability of tunnel surrounding Rock crossing coal gob strata is systematically studied.The specific research contents and achievements are as follows:(1)According to the theory of elasto-plastic mechanics and the theory of excavation mechanics,the stress distribution characteristics of tunnel surrounding rock with distance are discussed under different size conditions of coal gob in the coal strata.(2)Based on the numerical analysis model of the coal gob strata,the variation of displacement,stress and plastic zone with different distance between gob and tunnel is analyzed,in the different position of the coal gob(tunnel top,arch waist,side wall,arching corner,bottom)and different size conditions.(3)From the stability of the tunnel surrounding rock,based on the Drucke-Prager yield criterion,taking whether the surrounding rock yield and yield approach index as standard,the threshold of the strong impact zone,weak influence zone and no influence zone of the coal gob in different locations around the tunnel are gotten.The yield approach index of YAI = 0.70% YAI_?(yield approach index of no gob)is used as the cutoff value of strong influence zone,weak influence zone and no influence zone.(4)Based on the results of finite element model,According to the division criteria of the existing structure to the new tunnels affected zone,in the different position of the coal gob(tunnel top,arch waist,side wall,arching corner,bottom)and different size conditions,the strong and weak influence scope of the coal gob on the the new tunnel was systematically obtained.The conclusions of the study can be used as reference for similar engineering design and construction in the future.(5)The influence zoning results applied to the Huayan tunnel crossing the coal gob strata,the measures of tunnel reinforcement in the area of strong influence zone are analyzed and the tunnel for reinforcement measures taken were carried out to evaluate the effect by the three-dimensional construction numerical model and field measurement.The results show: the pre-reinforcement treatment of the advancement of the pipe roof is effective in controlling the safety of the construction.The research results give guidance on the design and construction of Huayan tunnel.