Study On Crack Propagation And Resistivity Variability Of Uniaxially Loaded Sandstone

The natural rocks contain certain micro-defects and fissures.The instability of rocks is usually caused by the expansion and penetration of internal micro-fissures,which in turn form macro-fissures.Therefore,studying the law of the initiation,expansion and penetration of micro-cracks in the rock during compression and understanding the meso-mechanism of the rock’s macro failure can provide a certain reference for the stability evaluation of rock engineering.Under high stress conditions,the macroscopic damage of rocks is closely related to the mesoscopic microfracture expansion.Therefore,the study on the law of initiation,expansion and penetration of micro-fractures in rock during loading can further understand the mesomechanism of rock macroscopic damage,which can provide a reference for the stability evaluation of deep rock excavation engineering.For water-bearing rocks,electrical and acoustic properties can change sensitively due to changes in internal pore structure.Based on this,this paper first makes a theoretical analysis,taking the porosity change of a rock under pressure as a bridge,and deducing the theoretical relationship between resistivity and damage factor of sandstone during compression;The traditional quadrupole method for measuring the resistance of the rock sample is improved for multi-channel measurement of the resistivity change of the rock under pressure,and the anisotropy of the resistivity can be used to judge the expansion direction of the macroscopic crack of the rock;On this basis,the uniaxial compression tests of sandstone rock samples with different initial damage,different loading rates,constant amplitude repeated loading,dead load and grading loading were carried out,and the law of resistivity change under different compression stages was studied.Combining information,research and analysis of micro-fracture expansion law;The PFC2D model is established according to the relevant parameters of the rock sample and the simulation test was carried out to further understand the micro-fracture initiation,expansion and penetration process of the rock sample under different loading rates.The paper mainly achieved the following results:The theoretical relationship between resistivity and damage factor in rock compression process is deduced,and the resistivity of rock measured by traditional quadrupole method is improved,real-time measurement and recording of multi-channel resistivity during test is realized.The resistivity measured in different channels will exhibit anisotropy,and the resistivity of the split surface will be significantly weaker than that of the adjacent surface.According to the different resistivity measured by different channels,the crack propagation inside the rock can be judged.The resistivity and ultrasonic information of the rock failure process are closely related to the initiation,expansion and penetration of the micro-cracks inside the rock sample.In the initial stage and the elastic deformation stage,the internal crack of the rock sample is compacted,the resistivity decreases slowly,and the ultrasonic wave velocity and the amplitude of the main frequency increase gradually.During the plastic deformation stage,as the crack expands,the resistivity decreases significantly,and the ultrasonic wave velocity and the main frequency amplitude begin to decrease.When the crack gradually penetrates to form a macroscopic crack,the resistivity suddenly drops.As the rock sample breaks,the resistivity suddenly increases.Ultrasonic information is sensitive to the degree of rock damage.When the stress reaches 40%～60%of the peak stress,the wave velocity begins to decrease.The microcracks in the rock sample begin to expand,and the resistivity change will suddenly decrease when the stress reaches 80%of the peak stress.At this point,the rock is on the verge of destruction.For sandstone samples with different loading rates,the elastic modulus and peak strength of sandstone increase gradually with the increase of loading rate,and the Poisson’s ratio decreases gradually.The lower the loading rate,the larger the number of micro-fractures within the rock sample.When the crack is initiated but not penetrated,the resistivity remains basically unchanged,and the fracture degree is high after the rock sample reaches the peak strength;The higher the loading rate,the more obvious the localization of crack propagation inside the rock sample,and the more obvious the decrease of resistivity,the less the fracture surface after the rock sample reaches the peak strength.When the ultrasonic wave velocity becomes negative,it indicates that the stress level of the rock sample has exceeded 0.8σc,and the rock sample is seriously damaged and close to the damage.Due to the influence of time-effect characteristics,the strain will gradually increase with time as the sandstone is in the dead load stage,and the resistivity and ultrasonic wave velocity and main frequency amplitude will gradually decrease.The micro-fracture inside the sandstone gradually increases with time.Expansion;the higher the stress level,the more obvious the strain,resistivity,ultrasonic wave velocity and main frequency amplitude change during constant load,and the faster the micro-crack expansion inside the rock.