| The construction of underground reservoirs in coal mines plays an important role in maintaining the ecological environment of arid and water scarce mining areas in the western region.Ensuring the long-term stability of safe coal pillars is a challenge in the construction of underground reservoirs.When the axial direction of the safety coal pillar is distributed at a certain angle with the direction of the maximum principal stress,the coal pillar will be subjected to a combination of compressive and shear loads,and its mechanical behavior is significantly different from that of pure compressed coal pillars.In addition,the safety coal pillar is severely eroded by groundwater,and under the action of compressive and shear composite loads,it is easy to cause the water bearing coal pillar to rupture and lose stability,posing a significant threat to the safety production of coal mines.Although domestic and foreign scholars have conducted a series of studies on the mechanical properties of water-bearing coal and rock,there are few reports on the mechanism of deformation and strength evolution of water-bearing coal and rock under compressive shear composite loading from the perspective of small laboratory specimens due to the lack of experimental systems.This article takes the safety coal pillar of the underground reservoir of Daliuta Coal Mine in Shendong Mining Area as the research object.Firstly,an inclined uniaxial compression test was conducted on the water-bearing coal rock to obtain the variation law of the mechanical properties parameters of the water-bearing coal rock.With the help of scanning electron microscopy experiments,the strength attenuation mechanism of the water-bearing coal rock was revealed;Then,combined with the acoustic emission characteristic parameters of water-bearing coal and rock under compressive shear composite loads,the micro fracture evolution characteristics of water-bearing coal and rock were studied;Finally,based on the creep mechanical properties test of waterbearing coal rock under compressive shear composite load,the characteristics of axial deformation,failure stress,deformation rate,and long-term strength of water-bearing coal rock were explored,and the strength formula of the coal pillar was modified using theoretical analysis.The research results obtained are as follows:(1)By using the improved compression shear coupling loading test system(CCAST),the response laws of mechanical characteristic parameters such as peak stress,elastic modulus,and peak strain to loading inclination angle and water content were analyzed.It was found that peak stress and elastic modulus were negatively correlated with loading inclination angle and water content,while peak strain was negatively correlated with loading inclination angle and positively correlated with water content;Summarized the macroscopic fracture modes of coal and rock,with shear failure being the main mode under high water content and high loading inclination,and tensile failure being the main mode under low water content and low loading inclination;Analyzed the micro mechanical behavior of water-bearing coal and rock,and revealed the strength attenuation mechanism of water-bearing coal and rock under compressive shear composite loads through scanning electron microscopy experiments.(2)We studied the variation of acoustic emission characteristic parameters(ringing number,cumulative ringing number,energy,and cumulative energy)during coal rock fracture process with loading inclination angle and water content.Based on the number of acoustic emission ringing numbers and energy characteristics at different loading stages,the acoustic emission signal is divided into acoustic emission quiet period,low amplitude growth period,and high amplitude growth period;The stress thresholds for crack closure,initiation,and damage of coal and rock under different loading angles and moisture content were determined,and it was found that none of the above three thresholds varied with loading angles and moisture content;The distribution characteristics of tensile and shear cracks in coal and rock were studied,revealing the influence of loading inclination angle and water content on the micro fracture behavior of coal and rock.(3)We conducted graded loading creep tests on water-bearing coal and rock under compressive shear composite loads,systematically studying the axial deformation characteristics,deformation rate characteristics,failure stress characteristics,and longterm strength characteristics of coal and rock under different loading angles and water content;Based on the acoustic emission signals during the graded loading process,the characteristic parameters of coal rock acoustic emission and the evolution law of microcracks were explored;Based on the complex loading environment,the formula for coal pillar strength has been revised.The research results can provide theoretical guidance for the construction of coal mine underground reservoirs.There are 71 figures,23 tables and 108 references in this dissertation. |
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