Experimental Study On Mechanical Properties Of Sandstone Under Coupling Of Shear Stress And Water Injection Pressure

In recent years,with the complexity,diversification and enlargement of geotechnical engineering,such as ground construction,underground space transformation,energy mining,CO₂ capture and storage,and dam repair,most of these projects involve underground injection activity,which will lead to significant changes in rock mass environment near the engineering area.Furthermore,the most destructive form of rock mass is shear failure.Therefore,the shear test of sandstone under the different water injection pressure level and the hydraulic fracturing test under different shear stress levels are carried out on the basis of the shear stress and water injection pressure coupling effect.The stress,deformation,flow,acoustic emission of the sandstone under different stress levels are analyzed.And the spatial and temporal evolution of fracture and the cracking form of specimens after failure were observed,combined with acoustic emission location technology and CT scanning technology.Simultaneously,the coupling mechanism of the shear stress and water injection pressure under the two stress paths is discussed.The results obtained are as follows:?1?The peak shear stress decreases with the increase of water injection pressure,for the shear test under water injection condition.However,the peak water injection pressure increases first and then decreases with the increase of shear stress level,for the hydraulic fracturing test under shear load.?2?During the shear process,the transition from the elastic phase to the unstable rupture stage occurs roughly between the 69%⁸5%of the peak shear stress.And a large number of acoustic emission events gather in the stage of unstable rupture development,forming nuclear or banded point clouds.?3?The water pressure in the shear test is mainly manifested in weakening the connection of sandstone particles,increasing the pore water pressure,reducing the effective normal stress,and promoting the development and expansion of the crack.The concrete performance is to weaken the shear properties of the sandstone,the greater the water injection pressure level,the greater the weakening effect.The shear stress in the hydraulic fracturing test shows the compression and compaction of the specimen at the low shear stress level,while the high shear stress level shows the large scale damage to the specimen,and there is a middle shear stress level as the turning point of the two effects,which is 60%in the test.?4?For the shear test with or without water injection,the failure modes of the specimen are transverse shear fracture and transverse shear fracture with longitudinal secondary fissure,and the connectivity of the longitudinal secondary fissure increases with the increase of the water injection pressure level.For hydraulic fracturing test,with the increase of shear stress level,the failure mode of the specimen is converted from longitudinal splitting fracture to splitting-shearing mixed mode fracture,and then transformed into transverse shear fracture with longitudinal secondary cracks.The failure modes of the two types of experiments are different.