Experimental study on the acoustic emission of prefabricated fracture sandstone under hydraulic coupling
[Objective]The hydraulic coupling behavior of rocks is closely related to the safety and stability of various rock engineering projects,including tunnel excavation,dam construction,oil and gas extraction,and rock slope stability analysis.The failure of rock masses and the evolution of permeability are part of a gradual dynamic process that is interrelated.On the one hand,the external load applied to the rock mass alters its microphysical,macrophysical,and mechanical properties,thereby affecting its permeability.On the other hand,changes in permeability can alter the distribution of permeability pressure,affect the effective stress within the rock mass,and consequently influence its stability.Additionally,the rock mass contains numerous fractures and fissures resulting from diagenetic processes,tectonic movements,and excavation disturbances,which complicates the assessment of the stability and reliability of the rock mass.Therefore,fully quantifying the mechanical properties and permeability evolution of prefabricated fractured rocks under hydraulic coupling is essential for minimizing damage in rock engineering projects.Despite recent advances in rock mechanics testing technology,measuring the microstructural changes in rocks under hydraulic mechanical loads during laboratory tests remains a challenge.[Methods]The rock failure process is closely associated with acoustic emission(AE),which is defined as transient elastic waves generated by the rapid release of energy within a material.Owing to its high sensitivity to crack initiation,propagation,and coalescence in loaded rocks,AE monitoring is a powerful tool for studying brittle rock failure.This technology has been widely applied in rock mechanics research and engineering applications.This article employs AE technology to conduct triaxial hydraulic coupling tests on intact sandstone and prefabricated fractured sandstone samples.The primary objective is to explore the micro-mechanisms of permeability evolution under hydraulic coupling loads through a series of laboratory tests.The strength and deformation characteristics,macroscopic failure modes,and permeability evolution patterns of prefabricated fractured sandstone under hydraulic coupling were examined.Additionally,the spatiotemporal evolution of AE events and the statistical distribution of fracture types during the failure process of prefabricated fractured sandstone were analyzed using AE technology.[Results]The experimental results indicate the following:1)Antiwing cracks occur when the inclination angles of prefabricated cracks are 45°,75°,and 90°,whereas specimens with other prefabricated crack inclination angles predominantly exhibit wing crack failures.2)During the spatiotemporal evolution of AE events,the compression events resulting from the closure of cracks and pores during the initial compression stage represent the primary microscopic mechanism for the decrease in permeability.3)The increase in permeability is mainly attributed to tensile and shear microcracks associated with volume expansion,whereas the decrease in permeability is caused by shear microcracks associated with volume compression and compression microcracks.[Conclusions]Under triaxial hydraulic coupling,the strength and deformation characteristics,macroscopic failure modes,and permeability of prefabricated fractured sandstone specimens are influenced by the inclination angle of the prefabricated fractures.The decrease in permeability mainly results from compression events that lead to fracture closure and pore collapse,whereas the increase in permeability is attributed to the continuous accumulation of tensile and shear events,which is also influenced by the inclination angle of pre-existing fractures.
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