Test study of creep properties of gas-bearing coal specimens under triaxial compression

A self-developed triaxial creep system of gas-bearing coal is introduced firstly. A series of triaxial creep tests on gas-bearing coal specimens are carried out by using the creep system, and results of the creep tests under different creep stresses, confining pressures and gas pressures are obtained. The results show that creep stress, confining pressure and gas pressure are the important factors affecting the creep properties of the coal specimens; the creep behaviors of the specimens can be split into two parts mainly: one is the attenuating creep, the other is the non-attenuating creep. Decelerating creep stage is caused by elastic aftereffect; by the actions of creep stress, confining pressure and gas pressure during creep tests, steady-state creep rate increases with the increasing of confining stress and gas pressure, and decreases with the increasing of confining pressure. Accelerating creep stage marks the starting of the failure of coal specimens. Based on the creep results, the creep rates at decelerating creep stage and steady-state creep stage are analyzed in detail; and in addition, a power function and an exponent function are proposed which can effectively describe the creep rates at transient creep stage and steady-state creep stage respectively. By a normalized method, a mathematical equation is constructed which can model the relationship among the steady-state creep rate, creep stress, confining pressure and gas pressure. And using this equation, the value of steady-state creep rate under various stress conditions can be estimated easily.