Abstract
Appropriately designed support systems are required to control potentially unstable ground around entries in coal mines. Alternative support systems can be evaluated using various approaches, including empirically based methods and advanced numerical models. However, conducting numerical model analyses can be time consuming, requires special software, and requires specialist expertise for setting up models and evaluating the results. In this study, the need for a more rapid method of assessing support alternatives is addressed. A regression-based equation has been developed based on the output of more than 600 numerical model analyses that investigated entry stability in various ground conditions, depths of cover, stress conditions, and support systems that might be encountered in US coal mines. The model outputs were verified against actual performance of coal mine excavations in a variety of geotechnical settings. Least squares curve fitting procedures were used to find the unknown parameters of a nonlinear equation that includes parameters for the rock mass strength, support characteristics, and geometric layout of the excavation and support system. The developed equation can satisfactorily predict the numerical model calculated stability factors. This allows practitioners to rapidly assess support alternatives for a range of conditions using spreadsheet software.
1. INTRODUCTION
Ground falls remain a significant factor in underground coal mine injuries and fatalities. In 2013 ground falls accounted for 4 of the 14 fatalities and 166 of the 1577 reported lost-time injuries in underground coal mines [1]. The stability of underground coal mine excavations is improved by the provision of support systems that control potentially unstable ground. Support systems may consist of rock reinforcement such as rock bolts and cable bolts and may be supplemented by standing supports such as engineered wooden cribs or cement-based columns. The design of appropriate support systems is complex because of the variable nature of the rock mass and the difficulty of estimating the interaction between the rock mass and the installed support system. Over the past 25 years multiple design approaches have been used in coal mine ground control. The approaches include empirical mechanistic methods, empirical statistical analysis, and rules of thumb and numerical methods, after Hebblewhite [2]. Of these approaches, numerical methods are increasingly used to supplement empirically based designs.
