52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: The City of Red Lodge, Montana, is underlain by up to five levels of room and pillar coal mines, ranging in depth between 55 meters to over 200 meters, that have been closed for nearly 75 years. Potential trough-type subsidence estimates ranged between 75 to 175 millimeters and rates were suspected to be near 3.5 millimeters/year. A leveling network was designed to measure subsidence rates of this order. Pre-survey precision was simulated to gauge a suitable network geometry and level of redundancy. Measured survey elevations were computed with a routine that used weighted least squares (L2-norm) to find the best linear unbiased estimates for subsidence rates. Gross measurement errors were identified with two methods: 1) comparing residuals from the L2-norm with those from the least absolute error method (L1-norm); and 2) examining L2-norm standardized residuals. Several elevation estimates showed small downward movements, many of which had a 95% confidence interval (2-sided) where the entire range was downward. Estimated movements ranged between no movement to 10 mm (downward) with an average of 4 mm (downward). This paper presents a case history of the methods used to evaluate subsidence above historic, abandoned coal mines where surficial evidence is not definitive.
The City of Red Lodge is in south-central Montana, approximately 97 kilometers southwest of Billings. As many as 7 coal seams underlying part of Red Lodge were mined in the early 20th century. The seams vary in depth and condition, with depths below ground surface ranging from 15 meters to over 210 meters. The coal seams are located in sandstone and claystone bedrock, which is overlain by alluvial gravel and boulders.
Underground mining operations started in the 1880s and effectively ceased in the early 1930s. The resulting underground workings lie beneath an area of nearly 8.5 square kilometers. Of that, approximately 0.8 square kilometers lie beneath the city limits, much of which is residential. Figure 1 shows a plan view of the known mine workings in the area immediate to the city. According to historic documents, until around 1908, the method of working the seams was room and pillar, where pillars were left in place. The distance of rooms from center to center was 21 meters and the general distance between levels was 168 meters long (Rowe, 1908). This geometry yields a 41% extraction ratio. The segment of mine map shown in Figure 2 shows typical room and pillar geometries used for extraction in Red Lodge.
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