Creep law and model of coal under triaxial loading at different gas pressures

There are too many creep models at present,but none has summarised these models’ applicability,advanta- ges and disadvantages yet. Also,the relation between coal rock creep and low gas pressure is unclear. Therefore,the creep test of coal rock under different gas pressures at triaxial stress was carried out to study the influence of gas on creep,and the applicability of various common creep models. The results show that the larger the gas pressure is,the greater the axial strain of coal and rock. The coal creep is positively correlated with the gas pressure. Then,the experi- mental data were used to test these common creep models. It was found that the empirical equations,such as the power function,exponential function and polynomial function,can only fit one of the decay or non-decaying creep. The Nishi- hara model cannot fit the non-decaying creep,and the Burgers model and its improved model has a few defects in fit- ting the decay and non-decaying creep curves. The Nishihara acceleration model has hysteresis,when it is used to fit the non-decaying creep of coal rock. And the Nishihara acceleration model is suitable for sandstone creep but not for coal rock creep. In this paper,the Nishihara acceleration model was improved. It is concluded that the improved Nishhara acceleration model can better describe the creep of coal rock. In addition,the theory of creep inflection point was presented. The creep curve at least has one creep inflection point,which is the necessary and sufficient condition for the non-decaying creep of rock. It not only enhances the theory of acceleration creep,but also has some guiding signifi- cance to prevent rock steady failure and other engineering practices.