Experimental study of the creep properties of coal considering initial damage

In underground tunnels, caverns or pillar goaf, the time-dependent deformation or failure of the rock or coal mass is a major safety concern, which affects the mechanical properties and stability of the coal and rock in the underground excavations. However, the damage surrounding rock or coal usually occurs from the time mining starts. This initial damage is rarely considered during the current creep tests used for coal. In the present study, an experimental specimen processing method to identify the initial damage to a coal deposit is suggested. The effect of the strain history on the deformation of coal samples with initial damage during multi-level loading triaxial compression creep tests was studied. The laboratory tests showed that the strain versus creep curve of the coal samples experiences three typical creep stages of deceleration creep in the early stage, constant velocity creep in the medium term and catastrophic accelerated creep in the later stage. With the increase in initial damage, it was observed the gradual decrease in total creep time, deviating stress of creep failure, stress threshold of accelerated creep and starting time of the volume expansion. When the initial damage value increased from 0.0 to 0.50, the deviating stress of creep failure, stress threshold of accelerated creep and starting time of the volume expansion of the coal decreased by 42.3%, 48.5% and 51.4%, respectively. The study is of great significance to conduct researches on the deformation, mechanical characteristics and long-term strength of coal pillar with initial damages under long-term pressure, and also provides insight into the underground excavations and the design of supports to the opneings under such disturbance.