Study On Deformation And Failure Mechanism Of Surrounding Rock Under Full-section Excavation In High Geo-stress Section Of Xiangjiawan Tunnel Of Zhengzhou-Wanzhou High Speed Railway

The Xiangjiawan tunnel is one of the foremost controlling tunnels of the Zhengzhou-Wanzhou high-speed railway,and is characterized by high geo-stress and complex geological conditions in the tunnel site.In areas experiencing high geo-stress,revealing deformation and failure mechanisms of the surrounding rock mass using the full-section construction method is important for preventing engineering problems in the rapid construction process,guaranteeing the successful conducting of the railway and the safe use in the later stage.The Xiangjiawan Tunnel was described and analyzed as a typical case in the Zhengzhou-Wanzhou high-speed railway.This study aimed to propose the functional relationship between the point load strength and uniaxial compressive strength of the rock mass in the limestone section in the tunnel site,clarify the distribution characteristics of geo-stress in tunnel site,and explore the causative factors and failure mechanism of surrounding rock mass in the typical high geo-stress region of Xiangjiawan tunnel under full face excavation method.This paper conducted site investigation of engineering geological condition involving on-site tunnel face geological sketch records,surrounding rock mass deformation monitoring,laboratory rock mechanical experiments and multiple numerical simulations.The results suggested that:（1）The lithology of the tunnel entrance and exit area of Xiangjiawan Tunnel is complex,and joints and fissures are well developed.Most of which are GradeⅣorⅤsurrounding rock masses,while the lithology of the middle area is relatively single,which is dominated by limestone with few development of structural planes,mainly Grade III surrounding rock masses.（2）The empirical formula for the conversion of point load strength and uniaxial compressive strength of limestone in Xiangjiawan tunnel site is R_c=21.88I_s(₅₀)^0.74.（3）Along the axis direction of Xiangjiawan tunnel,the stress distribution is"large in the middle,small at both ends".The maximum stress mainly concentrates in the mileage of D2K583+560～D2K583+710.（4）The entire tunnel area of Xiangjiawan tunnel is mainly distributed by high geo-stress,following by the usual degree geo-stress,and the local area is distributed by extra-high geo-stress.（5）The deformation of surrounding rock mass is more intense in the mileage of D2K583+758～D2K583+788 in the high geo-stress area of Xiangjiawan tunnel,and the convergence degree of tunnel mass near section D2K583+788 is the largest.（6）For the surrounding rock mass at the entrance and exit of the tunnel with relatively developed structural plane and low grade of surrounding rock mass,the deformation and failure mainly occurred in the spatial combination conditions that the interval of the structural planes is less than 1.5m and the inclination of J1 is in the range of 45°to 75°.（7）After the full-section excavation in the high geo-stress section of Xiangjiawan Tunnel,the geo-stress is redistributed to varying degrees in the surrounding rock mass at different parts of the tunnel.Due to the stress concentration,the surrounding rock mass on both sides of the tunnel and the face of the tunnel presents the plastic failure phenomenon with shear failure as the main pattern.In addition,because of the unloading effect at the vertical position of the arch crown and inverted arch,a large number of tension failure elements developed in the unloading zone accompanied by the slip of structural plane,showing the brittle failure with tensile failure as the main pattern.