Linkage mechanism of mechanical response of deep granite under high stress and high water pressure

Under high stress and high water pressure,the rocks are usually in the post-peak stage,some cracks are developed and some internal structures are broken. The variability of mechanical behavior of rock mass is seriously presented. Therefore,this paper performed a series of triaxial cyclic loading and unloading experiments under hydraulic-mechanics coupling to investigate the post-peak characteristics of granite drilled from more than 1 000 m in metal mine. Through the complete stress-strain curves,the mechanical and hydraulic parameters are correspondingly calcu-lated. By obtaining the threshold values of crack initiation stress,dilatancy stress,peak stress and residual stress,the stress-strain stages of rock are divided. Given the high residual strength of granite after failure,the ratio of residual strength to peak strength under different confining pressures is analyzed and compared,and the influence curve of different confining pressures on the strength loss of granite is built. By comparing the difference between the initial permeability at the residual stage and the maximum permeability before the peak stage,the permeability change affected by confining pressure factors is revealed,and the relationship between different high confining pressures and permeability changes is obtained. Finally,the response mechanism among the rock expansion,damage evolution on residual stages and permeability are explicated. The results show that under the coupling of high confining pressure and high water pressure,the cataclastic granite shows the mechanical characteristics of no compaction section,low initiation point,long yield stage and expansion stage,and high bearing capacity after failure. The elastic modulus shows a downward trend in the pre-peak stage,and an upward and then downward trend in the residual stage,and the evolution law of Poisson's ratio is opposite. With the increase of the number of cycles,the damage variable of rock shows a trend of decrease first and then increase,and increases steadily in the residual stage after peak,the permeability shows similar law. The relationship between the permeability and damage variable of rock can be described by logarithmical function in the expansion stage and an exponent function in the residual stage. There is a negative correlation between the ratio of residual strength to peak strength and the confining pressure,while the permeability loss rate has a positive correlation with the confining pressure.