Bursting liability indices of coal

Natural coal's ability to store and release rapidly elastic strain energy seems to be a fundamental condition for the occurrence of rock-bursts in deep underground coal mines. In Upper Silesia where a number of existing mines work below a critical depth, several indices have been proposed and their numerical values found by experiment in order to classify the potential liability of coal seams to create rock-burst hazards. Three of these indices are most popular, namely the Strain Energy Storage Index (WET), the Bursting Efficiency Ratio (η), and the Rheologic Ratio (ϑ). The first is defined as the proportion of strain energy retained to that dissipated during a single loading-unloading cycle of uniaxial compression. The second is the ratio of the kinetic energy of chips thrown out on failure to the maximum elastic strain energy. The Rheologic Ratio is calculated as a dynamic resistance stress rate (failure velocity) divided by an average stress relaxation rate. WET values may be obtained through laboratory tests or by direct (double-hole method), or indirect (rebound method) in-situ evaluations, while both η and ϑ values are obtainable in laboratory solely. Each of the indices quoted leads to classifications and/or criteria for the bursting liability of coal. When complete predictions of the rock-burst hazard for a specific location in a mine are to be made, data on in-situ stress concentrations must also be collected. Since direct stress measurements in coal are hard to recommend as a routine procedure in mines, a number of indirect methods are currently being practiced, drilling yield and seismic wave velocity methods giving the most reliable results.