Research on experimental and nonlinear creep constitutive model of coal at depth

The study of creep law in deep coal is important to control the deformation of surrounding rock,prevent coal and gas outburst and improve the effect of gas extraction. Based on the rheological model of fractional derivative,the nonlinear creep constitutive equation of coal under three-dimensional stress is deduced. Considering the occurrence of high stress environment and stress variation path of coal at depth,the creep test of coal under constant stress and multi- stage unloading confining pressure in three direction stress state is carried out. The experimental results show that in the three dimensional high stress state,the axial and circumferential creep strains of specimens are characterized as in- itial and steady state,and finally occur to accelerated creep stage;under the condition of same differential stress,the creep deformation of the coal decreases with the confining pressure increasing. The parameters of the creep constitutive equation are fitted by the experimental data of unloading confining pressure of coal. The results show that the creep constitutive equation can well describe the three stage of creep of coal,especially the accelerated stage. Furthermore, the influence of stress level,fractional derivative order and viscosity coefficient on creep characteristics of coal is ana- lyzed under the premise of determining the parameters of the creep constitutive equation.