Analysis And Control Of Surrounding Rock Stability For Safe And Efficient Tunneling Through Thick Coal Seam

With the further promotion of the strategy of strengthening transportation,China’s infrastructure construction such as highways,high-speed railways,and urban rail transit has entered a period of rapid development.As a key area for transportation construction,the western region is dominated by mountains and hills,and the number of tunnel projects is increasing.The coal measures strata are widely distributed in the western mountainous areas,which are one of the unfavorable geological conditions often encountered during tunnel construction,and are prone to disasters such as large deformation of surrounding rock,gas outburst,and explosion.Therefore,how to ensure the safe and efficient passage of tunnels through coal measures is worth in-depth and systematic research.Based on the Tianchengba Tunnel of Zhengxi Expressway in Guizhou Province,combined with theoretical analysis,numerical simulation,model test,and on-site monitoring methods,this paper conducts a systematic study on the safe and efficient crossing of tunnels in extremely thick coal seams.The main research contents and achievements are as follows:(1)The physical and mechanical properties and failure characteristics of coal and rock are summarized.Based on the theories of coal mechanics,elastic mechanics,rock mechanics,and material mechanics,the mechanical model of coal seam interface is simplified,and a prediction formula for large deformation of surrounding rock considering gas strength is proposed.The three stages of surrounding rock deformation are clarified: compression deformation stage,compression shear deformation stage,and shear slip stage.(2)A three-dimensional numerical calculation model for coal seam rock mass tunnels is established using FLAC 3D software to study the effects of coal seam thickness,inclination angle,and construction methods on the displacement,stress,support internal force,and plastic zone of the tunnel surrounding rock.The results show that the vertical and horizontal displacement of surrounding rock,the stress of surrounding rock,the internal force of support,and the maximum value of plastic zone area are positively correlated with the thickness of the coal seam,and negatively correlated with the dip angle of the coal seam.After a comparative study of tunnel construction methods,it was found that the three-step reserved core soil has less disturbance to surrounding rock,is more suitable for safe and efficient excavation of tunnels in extremely thick coal seams,and is conducive to the stability of the tunnel structure.(3)Carry out indoor excavation model tests for tunnels in extremely thick coal seams,determine the similarity ratio of surrounding rock and coal seams through similarity theory,and study the influence rules of surrounding rock stress,displacement,and initial support internal force variation characteristics under different excavation methods.The results show that when the tunnel is excavated to the coal seam section,the surrounding rock stress,displacement,and initial support internal force reach the maximum value,and the maximum value of the surrounding rock stress,displacement,and initial support internal force of the coal seam section using the three-step reserved core soil method is less than the three-step method,reducing the disturbance to the surrounding rock.(4)Combining numerical simulation and model test excavation results,it can be seen that the three-step reserved core soil method is more suitable for safe and efficient excavation of tunnels in extremely thick coal seams.In order to further ensure the safety of the tunnel structure,the combined support form of pipe shed method and small conduit radial grouting is used to reinforce the surrounding rock in the extremely thick coal seam section.From the on-site monitoring data,it can be seen that the deformation of surrounding rock is within the safety regulations,ensuring the safety of the tunnel support structure,effectively controlling the disturbance of tunnel construction to surrounding rock,and achieving efficient and safe passage of tunnels in extremely thick coal seams.