ABSTRACT:

In this study, 3DEC modeling in conjunction with the Discrete Fracture Networks (DFNs) technique was performed to better understand the true anisotropic behavior of the specimens acquired from a bump-prone underground coal mine. The spatial characteristics of the discontinuities (i.e., cleats and bedding planes) as input data for the 3DEC model are estimated based on the results of the laboratory tests and field observations. The DFNs explicitly generated the coal seam that was poorly or well cleated, indicating the different spacing between cleat apertures using the probability distribution functions on fracture density (or frequency) and size. The heterogeneity of the engineering properties (i.e., cohesion and tensile strength) are also considered by Monte Carlo simulations. As a result, the 3DEC model and DFNs technique demonstrated that the results of the simulations agreed well with the results of the laboratory test. These calibrated results can be used as we seek to evaluate bump risk by modeling at field scale.

1. Introduction

U.S. underground coal mines reported 25, 171 accidents related to ground control between 2000 and 2019. Of these accidents, 158 involved fatalities or resulted in permanent or total disability [1]. Although techniques and practices of mining are highly advanced, coal pillar bursts or bumps continue to occur. Many uncertainties remain because of the highly anisotropic characteristics of coal seams that are associated with geologic structure and the mining-induced spatial redistribution of stress in coal pillars [2]. Thus, to prevent fatalities in underground coal mining, continuous efforts are required to better understand the catastrophic failure mechanisms in coal mines.

This paper is developed as part of an effort by the National Institute for Occupational Safety and Health (NIOSH) to identify risk factors associated with bumps in the prevention of fatalities and accidents in highly stressed, bump-prone ground conditions. Kim et al. [3] and Kim et al. [4] characterized the behavior and brittleness of a coal based on the laboratory testing results. They presented that the behavior of coal varied with respect to the angle between the geologic structure (such as cleats) and the loading direction (major principal stress). Kim et al. [4] modified the approach proposed by Kim et al. [5] and Kim and Kaiser [6] to understand how joint persistence affects the overall rock mass strength. They developed a 3DEC model that randomly generates cleats in the coal seam with various lengths that follow a given degree of persistence. Although the results of the modeling agreed with the observations of the laboratory test, the bedding planes in the coal seam were ignored but were implicitly considered by assigning the equivalent material properties for the bedding planes in the model.

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