Direct shear and acoustic emission characteristics of sandy mudstone under the effect of moisture content

Shear failure is one of the main types of rock failure in coal mining.The structure and the integrity of roof strata will be destroyed by mining disturbance,which leads to the change of flow mode of water in aquifer from interlayer runoff to vertical runoff.The waterrock interaction in this process will change the shear failure characteristics of rocks significantly.In order to study the evolution law of shear failure characteristics of rocks under water effect,the sandy mudstone of 3-1 coal roof of Wulanmulun coal mine was taken as the study object.The composition and microstructure characteristics of sandy mudstone were obtained by the microscopic identification test of thin section.The variation of moisture content of sandy mudstone with time was tested by nondestructive immersion test equipment.The moisture content of sandy mudstone was determined,which provided a possibility for studying the evolution law of the direct shear characteristic of sandy mudstone under the effect of moisture content.The direct shear deformation and failure law,strength damage and acoustic emission characteristics of sandy mudstone under the influence of moisture content were discussed with the selfdeveloped rock direct shear test system and acoustic emission monitoring results.The result show that sandy mudstone has horizontal bedding structure,and the content of silty debris is 40% and the content of argillaceous is 60%.Clay minerals in sandy mudstone mainly include kaolinite and illite,accounting for 75% of total mineral constituent.According to the variation law of moisture content of rock specimens with immersion time,the moisture content immersion time curve can be divided into three stages:rapid growth (0-18 h),slow growth (18-42 h) and stable (42 h-).With the increase of moisture content,the shear strength,the cohesion and the internal friction angles of rock specimens decrease exponentially,negative exponentially and linearly,respectively.From dry to saturated,the cohesion and the internal friction angle of sandy mudstone decrease by 77.9% and 33.3% respectively.With the increase of moisture content,the cumulative count of acoustic emission of the rock specimen decreases.With the increase of moisture content,the flatness of macro main crack decreases after the rock specimen is destroyed.In the peak failure stage,the rock specimens with low moisture content produce largescale tension cracks,while the rock specimens with high moisture content produce smallscale tension cracks.The research findings provide a useful references to waterrock interaction problem in the engineering such as the waterenriched roadway of coal mine,water preserved mining and dam design of underground reservoir.