Experimental And Theoretical Study On Mechanical Properties Of Coal Rock Under Water Soaking

As one of China's main energy sources,coal consumption in China accounted for 59%of total energy consumption in 2018.In coal mining,water injection softening process for hard coal seam,prevention and control of mine strata pressure and stability design of waterproof coal pillar all involve the physical and mechanical properties of coal rock under water soaking environment.Based on experimental research,this paper systematically studies the physical and mechanical properties of coal rock under water soaking,and considers the randomness of coal rock defects and damage to establish a damage constitutive model of soaking coal rock.(1)The fresh coal rock test block is retrieved from a coal mine in Shaanxi,and is processed into compressive and tensile test pieces according to the International Rock Mechanics Society(ISRM)recommended standard.Firstly,the coal rock soaking test is carried out to study the variation law of the longitudinal wave velocity and water saturation of coal rock with the soaking time.The results show that the longitudinal wave velocity and water saturation of coal rock increase with the increase of soaking time,the growth rate gradually slows down and finally stabilizes.Then,the coal rock specimens with different drying time and different soaking time were observed by scanning electron microscopy.The soaking process of coal rock experienced the process from the drying shrinkage of coal rock crystals to water swelling,then to the saturation of coal rock and damage,and analyzed the coal rock soaking damage mechanism.(2)Compressive and tensile testing of coal rock specimens with different soaking times(0,1,3,5,7,15,30,60,120 days),using acoustic emission system and VIC-3D non-contact full-field 3D strain measurement system measures the whole process of the failure in real time,compares the acoustic characteristics and strain changes of coal rock,and is analyzed the variation law of compressive strength,tensile strength and elastic modulus of coal rock with the soaking time.The research shows that the uniaxial compression process of coal rock is divided into four stages:pore fracture compaction closure stage,elastic stage,crack stability expansion stage and crack instability expansion stage.The compressive strength,tensile strength and elastic modulus of coal rock,as the soaking time increases,it increases first and then decreases,accompanied by the internal structural response of coal.(3)The crack closure stress point of coal rock is determined by the acoustic emission ringing count mutation point.Considering the randomness of coal rock defects and damage,the theory of imbalance probability is combined with the damage mechanics,and the soaking model of coal rock is established based on the Weibull distribution and the Drucker-Prager criterion.The damage constitutive model is obtained by using the extreme point geometry to solve the model parameters by multivariate function,and the rationality of the model is verified by experimental data,which provides a theoretical basis for mining engineering.