Time-dependent deformation and shear strength along weakness planes in the roof of a coal mine

Time-dependent deformation of ground is widely observed in mining but poorly understood. Often this deformation is concentrated at joints and along planes of weakness. The importance of time is recognized by empirical concepts such as “stand up” time and in practice by manipulation of excavation rates. However, time-dependent deformation is poorly handled by analytical and support design methods.; Time-dependent deformation in the form of roof sag at two sites in a coal mine was studied in an effort to improve understanding of this phenomenon. In this case, the deformation led to eventual roof failures before mining approached the failure site.; Cores taken from the mudstone roof near one of the sites contained several planes of weakness. Direct-shear tests in the form of constant-velocity tests, creep tests, and creep-recovery tests were performed on the cores. Results suggested the need for a strength model with hardening friction, softening friction, and softening cohesion. Creep tests showed both transient and constant-rate creep at one site, and predominantly constant-rate creep at the other site, suggesting that different material conditions were present between sites.; A joint model was modified to simulate the important features of the direct-shear test results. Of note were a continuous function that represented hardening and softening of friction according to inelastic shear displacement and a cohesion function that softened according to inelastic shear displacement to a residual amount that may be other than zero. A Burger's model was used to represent deformation over time and a plastic unit was used to represent sliding along a joint.; A method of determining parameters of the modified model is presented. This method was tested with a synthetic problem and used to determine average parameters of the mudstone weakness planes. Differences between model simulations of direct-shear tests on mudstone weakness planes were within the range of sample scatter.