Study on damage characteristics of parallel double fracture sandstone under cyclic loading and unloading

Periodic disturbance will lead to the decrease of stability of fractured rock mass due to cyclic loading. In order to explore the influence of cyclic loading with different amplitudes on the service performance of parallel fractured rock mass, the damage characteristics of parallel double fractured sandstone under cyclic loading and unloading with different upper limits of stress were studied. Uniaxial variable amplitude cyclic loading and unloading tests and acoustic emission tests were carried out on intact and 45 ° parallel double-fractured sandstone. The failure mode, strength and deformation parameters and acoustic emission characteristics under different upper limit stress ratios were analyzed, and the failure damage characteristics were clarified. The results show that : ( 1 ) The upper limit stress ratio fatigue damage threshold of parallel double-crack sandstone is lower than 0.75, which is significantly lower than that of intact sandstone. ( 2 ) The strength growth ratio, elastic modulus and deformation modulus of intact sandstone show a trend of increasing first and then decreasing, while the above parameters of parallel double fracture sandstone show a decreasing trend. ( 3 ) There is no anti-wing crack in the parallel double-crack sandstone during the loading period, and the tensile failure is the main failure mechanism. The expansion of the outer wing crack and the connection of the overlapping cracks between the parallel double cracks are the main failure mechanisms. ( 4 ) The acoustic emission characteristics of sandstone during loading can be divided into three stages : rising period, stable period and severe period. Under cyclic loading, it mainly corresponds to the period of severe fluctuation. The acoustic emission count of parallel fractured sandstone is about 2 times that of intact sample. The dissipated energy and acoustic emission count increase synchronously with the increase of upper limit stress ratio, indicating that the internal damage increases with the increase of stress level. The research results can provide a theoretical basis for the safety early warning of deep geotechnical engineering.