Energy West Mining Company (EWMC) dedicated significant resources to collecting rock mechanics data for planning longwall layouts. To aid EWMC within the last twenty-five years, the authors implemented a variety of rock mechanic programs for site-specific evaluation of longwall stress conditions, and implementation of alternative mine designs and operating procedures for controlling stress, seismicity and stability. This program has been successful for safe extraction of two-seam reserves under stiff stratigraphic conditions at depths approaching 2600-ft locally. The rock mechanics program consisted of stress and deformation monitoring at five locations in two neighboring mining districts located in the Hiawatha and Blind Canyon seams used for verification of numerical models. The work improved our understanding of load transfer, seismicity and yield pillar performance critical to two-seam mine designs. Analyses examples include load transfer analyses, solid-gob crossings under high abutment loading, and stability evaluation of two-seam partially columnized mains.
To evaluate rock mass response to longwall stress in the first two mining districts, Energy West mining staff implemented a geotechnical monitoring program in cooperation with Maleki Technologies, Inc. (MTI) staff during early 2000's. This program consists of underground measurements at five locations in two neighboring mining districts located in the Hiawatha and Blind Canyon seams, separated typically with 24 m (80 ft) of interburden (Figure 1). Monitoring results have been used for the study of load transfer, seismicity, coal pillar performance and the calibration of numerical models including boundary-element code MULSIMTI (Maleki et al 2003) and FLAC (Itasca). The regional seismic system developed by the University of Utah is used for monitoring mining-induced seismicity.
Pseudo three-dimensional boundary element code MULSIMTI has been used over the last 25 years for routine analyses of mine layouts at the EWMC operations. The code assumes homogenous, elastic, and massive overburden rock properties, compatible with the geologic setting of the Wasatch Plateau Coal Mines. The model has been calibrated using data collected from geotechnical instrumentation over the last three decades at the EWMC operations providing important guidelines for analyses of cave conditions, coal pillar strength and yielding pillars with full stress-strain characteristics. When combined with FLAC analyses, the modeling procedures incorporate key elements of the geologic site model for the Wasatch Plateau mines including the presence of stiff, competent stratigraphic units in the roof and floor. Modeling and calibration procedures are described elsewhere (Maleki and others 2009, Maleki and Lewis 2010). The focus of this paper is on the application of a practical rock mechanics program in development of two-seam mine designs.