International Journal of Rock Mechanics and Mining Sciences
Pillar strength in underground stone mines in the United States
Introduction
There are more than 120 operating underground stone mines in the United States that use the room-and-pillar method of mining. Mining is typically conducted in flat laying or gently undulating deposits ranging from highly siliceous limestone to chemical-grade dolomite and limestone. Large mechanized equipment, such as 50-ton dump trucks and 5-m3 wheel loaders, are used to achieve the required economy of scale. The mining equipment requires adequately sized openings for effective operation. Where the thickness of the deposit allows it, second pass bench-mining of the floor may be carried out.
The function of pillars in room-and-pillar mines is to provide both local and global stability. Local stability is defined as the provision of stable ribs (pillar walls) and stable roof conditions, allowing safe access to working areas. Falls of ground from the roof and pillar ribs account for about 15% of all injuries in underground stone mines [1] and are related to local instabilities. Regional stability is defined as the need to support the weight of the overburden rocks up to the ground surface. Inadequate regional stability can result in the collapse of multiple pillars over large sections of a mine, which can result in significant safety hazards [2], [3]. A review of the literature revealed that two cases of multiple pillar failures have occurred in stone mines in the United States [3], both without injury or fatalities. One of the cases was assessed to be related to punching of the pillars into a weak floor stratum, while the other case appears to have been the collapse of irregular sized pillars [4]. Insufficient data are available to estimate the pillar stress at the time of failure of the latter case.
The design of slender pillars in hard brittle rocks has received considerable attention in recent years [5], [6], [7], [8]; however, a widely accepted design method does not exist for pillars in stone mines. The research described in this paper had the objective to reduce the rock fall hazard and the potential for multiple pillar collapse in stone mines by providing a method for designing pillars. The research was carried out by first conducting a detailed survey of mining practices and pillar performance in operating stone mines in the eastern and midwestern United States. Data were collected on the mining dimensions, pillar and roof stability conditions, spacing and orientation of discontinuities and the rock mass conditions using the rock mass rating (RMR) system of Bieniawski [9]. Rock samples were collected for laboratory strength testing. The field data together with numerical modeling results were used to develop a method for estimating the pillar strength and selecting an appropriate safety factor for design.
Section snippets
Geological setting
The stone mines included in this study are concentrated in the Interior Plains and the Appalachian Highlands physiographic regions [10]. Stone deposits located in the Interior Plains region are generally flat laying or only gently dipping and include rocks ranging across most of the Paleozoic Era, from the Ordovician Age to the Pennsylvanian. Overall, the rocks encountered in the Appalachian Highlands region are similar in age to those found in the Interior Plains region. They differ in that
Rock mass characteristics
The characteristics of the rock mass were evaluated at ninety-two investigation sites in the thirty-four different operating mines. Rock samples were collected for strength testing at the National Institute for Occupational Safety and Health (NIOSH) laboratory in Pittsburgh, Pennsylvania. Uniaxial compressive strength (UCS) tests were conducted on nominally 50-mm-diameter cores drilled from the rock samples. Cores were prepared and tested in accordance with ASTM standards [11]. The results are
Observed pillar performance
A survey of stable and unstable pillars in underground stone mines within the Eastern and Midwestern United States identified the causes of pillar instability to provide data for estimating pillar strength [15]. Mines that were likely to have unstable pillars owing to their depth of working or size of pillars were identified as targets for the survey. Data were collected that included both the intended design dimensions and the actual pillar and room dimensions in the underground workings. In
Pillar strength estimation
The stability of a pillar can be evaluated by calculating a factor of safety (FOS), which is the ratio of the pillar strength to the average stress in the pillar. The average stresses in pillars that are of similar size and are located in a regular pattern can be estimated with relative ease using the tributary area approach, which assumes that the overburden weight is equally distributed among the pillars. This provides an upper limit of the pillar stress and does not consider the presence of
Adjustment for large angular discontinuities
An adjustment for the presence of large discontinuities in pillars should account for both their inclination and spacing. Large discontinuities can be widely spaced and do not necessarily intersect each pillar in a layout. The two-dimensional UDEC [28] software for geotechnical analysis was used to assist in investigating the potential effect of a single large discontinuity on the strength of pillars with width-to-height ratios of 0.5–1.5. In these models, the discontinuities were assumed to be
Adjustment for rectangular pillars
Rectangular pillars are used in stone mines to provide ventilation control and to assist with roof control. Rectangular pillars can be expected to be stronger than square pillars of the same width. Strength adjustments to account for the increased strength of rectangular pillars have been suggested by several researchers [29], [30], [31]. These methods all assume that the pillar strength will increase if the length is increased, regardless of the width-to-height ratio. However, the stone mines
Factor of safety for design
An appropriate factor of safety for designing pillars can be selected from failure statistics if a sufficient number of failed and stable case histories have been observed [25], [34]. In the case of stone mines, the few individual pillars that have failed are not sufficient to conduct this kind of study. A more pragmatic approach was followed in which the FOS of all the layouts and failed pillars was recalculated using Eq. (4) and the results were assessed by comparing the FOS values of the
Pillar width-to-height ratio considerations
Fig. 10 shows that there has been a natural tendency for mines to avoid slender pillars. Nine of the layouts that had width-to-height ratios of less than 0.8 are no longer in use for various reasons, while only four mines included in the study are currently operating with these slender pillars. In addition, this study has shown that slender pillars are more severely affected by the presence of discontinuities than wider pillars. Studies have also shown that as the width-to-height ratio
Conclusions
The slender pillars used in stone mines in the United States present a unique challenge to mine designers. This study has shown that isolated pillar failures have occurred, which have resulted in safety and production concerns. An equation has been developed which can be used to estimate the pillar strength. The equation takes into account the intact rock strength and the potential impact of large angular discontinuities on pillar strength. An adjustment procedure is proposed to account for the
Disclaimer
The findings and conclusions in this report are those of the authors, and do not necessarily represent the views of the National Institute for Occupational Safety and Health.
Acknowledgements
The authors wish to thank the stone mine personnel who were willing to share their experiences and failures with the research team and who accompanied us to the various underground mine sites. Without their cooperation this research would not have been possible.
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