ABSTRACT:

In this paper, the authors present a review of data-gathering programs and subsidence analyses in support of a mine permit. Using analyses of corehole and subsidence data, overburden deformation characteristics are addressed. It is shown that surface subsidence reaches a maximum of 2.3 m (7.5 ft) over the longwall panels. Subsidence is less than 0.06 m (0.2 ft) over the gate pillars at shallow cover areas, but reaches 1.1 m (3.6 ft) under deeper cover. The amount of subsidence is influenced by variability in overburden thicknesses and pillar designs adopted for the Upper D Seam. Ground movements are influenced significantly by topographic features.

INTRODUCTION

Over the last decade, Bowie Resources, LLC (BRL), has been extracting coal reserves from the Upper D and B seams, located within the Somerset Coal Field near Paonia, Colorado. After extraction of the Upper D Seam reserves in the No. 2 Mine, BRL has recently initiated mining in the B Seam approximately 90 m (300 ft) below the D Seam workings. The mine has employed the longwall mining method for the extraction of the Upper D Seam since 1999 using yield abutment gate pillar designs while monitoring both surface vertical and horizontal deformation with precision surveying techniques (GPS or Global Positioning Satellite Survey System) as shown in figure 1. In the gateroads of these longwall panels, BRL used yield abutment pillars while gradually increasing abutment pillar width from 30 m (98 ft) in the D3 headgate to 35 m (114 ft) in the D5 headgate and beyond. Yield pillars were 11 m (37 ft) wide, and the longwall face was 250 m (824 ft) wide. Historic workings in the U.S. Steel Mine and the King Mine are also shown in figure 1.

Results have been analyzed on a routine basis to evaluate surface response over the panels and above the gate roads under the variable topographic conditions overlying the mine. Over the Upper D Seam reserves, the overburden ranges from few hundred to 426 m (1400 ft). The surface includes numerous deeply incised drainages at various orientations to the mine workings, and thus the influence of topographic conditions on ground movements has been of particular interest to this study.

In addition to surface monitoring, BRL, in cooperation with MTI staff, implemented a geotechnical program consisting of drilling three continuous coreholes from the surface in one area, logging the depositional and structural characteristics of the core, geologic mapping, rock mechanics testing, and hydrologic monitoring. Overburden characteristics were evaluated using the Rock Mass Rating (RMR) system. Massive overburden units were identified capable of transferring loads and reducing differential movements for future extraction of the B Seam.

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