Geology Oriented Loading Approach for Underground Coal Mines

Abstract

Mining-induced stresses in underground coal mines play a significant role in pillar and support design, hence in the safety of mining operations. Adequate design of pillars and roof control plans rely on the accurate assessment of the mining-induced loads, as well as the load-bearing capacities of the supports. In pillar design tools frequently used by the underground coal mining industry and Mine Safety and Health Administration (MSHA), overburden loading is estimated by geometric concepts such as the tributary area and the abutment angle method. These methods do not explicitly consider important mechanical responses such as specific overall ground behavior. The mine-specific overburden geology is one of the major influencing factors for these complex mechanical responses. In this research, to include the effect of geology in the overburden load estimations, a new parameter was defined as the total strong layer thickness (tstr) that represents the strength and stiffness of the overburden with a single value. This parameter is a function of the strength of overburden layers, thicknesses of strong beds, relative locations of the strong beds in the overburden, and the panel width and overburden depth. In this study, to develop a site-specific tstr, 13 field measurement case studies from 12 different U.S. longwall mines with different overburden geologies were used. 2D numerical models of the case studies were verified against the field measurements such as surface subsidence, and stress. After verifying the numerical models, parametric studies were performed to be able to assess the influence of panel dimensions on mining-induced stresses and to simulate different panel conditions such as critical, subcritical, and supercritical. Overburden stress redistribution on the pillar system, gob, and adjacent solid coal is estimated from the modeling results. Using these results, a regression analysis is conducted for a loading model with the tstr as a variable, and a method to estimate the percentages of loads carried by the gob was constructed. The proposed method is used to calculate the percentage of load carried by the gob and was found to have a coefficient of determination (R2) of 85% when compared to the field measurements and the parametric runs. The new methodology was compared against the empirical estimates for the field study cases and was found to give better results. The method was then tested with field measurement case studies that were not included in the initial analysis. One case was used to test the estimation accuracy and