Study On Rock Bridge Fracture Characteristics And Mechanical Behavior Under Unloading Conditions

With the gradual transfer of China's hydropower construction to the Tibet region,an alpine and gorge region with complex geological environment,the stability of slope construction becomes particularly important.The valley at the right abutment side slope of Rumei Hydropower Station in Tibet is a deep “V” type,of which the lithology is dominated by dacite with hard rocks,steep slopes,as well as complex ground stress.The combined development of unloading cracks and slope joints(formation of rock bridges among fissures in the middle of the ravine)will result in cracks in the rock bridges among the fissures after the excavation of the slope and the formation of damage through the surface.The slope at the right abutment is a high rock slope,and its overall topography is gentle-and-steep,among which the cracks and faults are relatively developed.The whole slope is affected by tectonic action,rock structure,weathering and so on,leading to the fragmentation of the shallow surface rock mass.This paper takes the right abutment slope of Rumei Hydropower Station as the research object,carrying out field investigations and tests and grasping its engineering geological conditions,namely the nature of unloading fractures,the structure of rock mass,as well as the physical and mechanical characteristics of rock mass.Through indoor physical model tests,this paper makes a study of the mechanical properties and deformation failure characteristics of fractured rock mass under unloading conditions.The fracture characteristics of rock bridges were simulated by discrete element procedure,and a regression model of rock bridges' penetration strengths was established to provide theoretical basis for the fracture characteristics of rock bridges and their mechanical behaviors under unloading conditions and stability evaluations of slope rock mass under excavation conditions.The main findings are as follows:(1)According to the measured in-situ stress results,the stress field distribution characteristics in the lower cut process of the valley are inverted;And the stress variation characteristics are studied to provide a theoretical basis for the design of unloading experimental stress paths.In the early stage of lower cut in the valley,the stress in the river bed is relatively small,and the strong stress changes occur in the recent period of valley cutting process.The river bed stress concentration range is limited.When the river bed is high in historical evolution,it will not affect the ground stress state of the river bed elevation.When the river bed cuts to the designated site,stress concentration occurs at the bottom of the valley,forming a “high stress package” phenomenon.(2)The deformation and fracture phenomena of the dacite rocks and the development of acoustic emission under unloading conditions are proposed to provide theoretical bases for the judgment of rock bridges' rupture.Under low confining pressures,the fissures develop and the fractures penetrate both ends of the specimen.While under high confining pressures,shear failure is dominant.And there is a conjugated “X” or pressure-twisting goose-type fracture in the shear fracture surface,resulting in less fractures overall.During the test,the acoustic emission monitoring of the AE event was obvious,indicating that there was a strong brittle characteristic during unloading,and cracks generated by the release of crack propagation energy could be heard during the destruction.When the low confining pressures began to unload,the specimens produced tensile cracks in the axial direction.The AE events were active at the peak and there were fewer AE events before the peak.Under high confining pressures,specimens were destroyed with a single shear plane of a small width,and most of the monitored AE events were concentrated near the peak.(3)The cracking stress of rock bridges was broken and the crack development rule of rock bridges' fracture was ascertained.In the double-fissure specimens,the rock bridges among the fractures penetrated directly,and the fractures' combined modes controlled the failure of the specimens,followed by confining pressure influences.The moderately inclined and steeply sloping combined fissures exhibited stronger tensile properties in the experimental simulation,which was consistent with the development pattern of the moderately inclined structural planes in the actual turbulence.Based on two-dimensional discrete element program named PFC2 D 5.0,the deformation characteristics of single and double fracture specimens were simulated and compared with the actual tests.(4)Based on multiple linear regression analysis,a regression formula for rock bridge penetration strength was established.Using the multiple regression model in SPSS software,the author of this paper established the relationships among crack dip,confining pressure,rock bridge inclination,crack length,rock bridge length and rock bridge penetration strength.