Study on mechanical properties and finite deformation behavior of the coal under temperature-confining pressure and gas pressure

Deep mining is imperative due to the exhaustion of the shallow coal resources. Temperature, in situ stress and gas pressure are the factors that have great effects on the mechanical properties of the coal. Meanwhile, the nonlinear behavior of the coal under such environment is the key and basic task of deep mining. Therefore, based on the orthogonal experiment principles, the temperature-confining pressure-gas pressure coupled mechanical test of the coal is designed and carried out. The effects of the mentioned three factors are investigated.The results show that temperature, confining pressure and the gas pressure perform the significant effects on the elastic modulus and the peak stress, the significant level of each factor is greater than 95%. The significance level of the effects on the Poisson’s ratio induced by the temperature and the confining pressure is above 95%. The significance level of the effect on the Poisson’s ration induced by the gas pressure is about 90%. The evolution of peak stress, elastic modulus and Poisson's ratio with the three factors can be described by linear models. Furthermore, the mean rotation angle, which can be employed to describe the nonlinear deformation of the coal, is calculated and studied based on the finite deformation theory and the experimental data. The evolution of the mean rotation angle with stress and stain can be divided into three partitions, i.e. the rapid increase partition, the moderate partition and the accelerating growth partition. The three partitions can be taken for the understanding of the mechanism of the compression zone, the linear deformation zone and the yield zone of the coal. The tangent modulus varies greatly with the mean rotation angle and with an increase zone and a decrease zone, which may reflect the process of compression stage, the hardening stage and the brittle-plastic transition stage. Afterwards, the mean rotation angle is taken as an internal variable to establish the constitutive model of the coal under the effects of temperature, the confining pressure and the gas pressure. The comparison of the model and the experiment data shows that the proposed model can be used to describe the entire process of the coal deformation and failure.