An experimental investigation of the creep behavior of an underground coalmine roof with shale formation

Stability of the roof in an underground coalmine is imperative for safe and efficient mining. Mining of the coal seam disturbs the natural equilibrium of the in-situ stresses in the surrounding rock. Because of the opening, the surrounding stratum deforms to fill the opening and return to the pre-mining stress state. Such deformation, if allowed to continue, will lead to failure of the strata, which creates an unsafe work environment. Therefore, supports are used to reinforce the deforming strata and help prevent any further deformation in the rock.;However, various mines have reported failures in the supported entries (MSHA, 2013). These failures have been mostly called as cutter-failure (Peng, 2008) and are attributed to high horizontal stresses; however, surprisingly, another factor which is suspected to have an effect on such failure is the time-dependent deformation. Its effect on the roof fall activity is unknown and to the authors knowledge limited research has been reported on this subject. Additionally, organizations such as the ISRM (International Society of Rock Mechanics) and the ASTM (American Society for Testing and Materials) have not recommended any creep testing procedure. Therefore, the objective of this research was to investigate the time-dependent deformational behavior of immediate coal measures rock (shale) and then use phenomenological equations to fit the experimental data and produce input properties for numerical modeling.;Creep, or time-dependent, experiments were performed on shale and sandstone specimens under constant load and stress conditions. Both rock types showed development of creep strain; however, the shale specimens were more sensitive to the change in the stress conditions. Under uniaxial stress conditions, the specimens showed an increase in the creep strain as the stress increased. In triaxial conditions, the creep strain increased with an increase in the deviatoric stress conditions. It is believed that this research will provide some preliminary information on the time-dependent behavior of shale