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Unconfined compressive strength of roof rock in the Midwestern and Southern Appalachian coal basins (after Rusnak and Mark, 2000).
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Mines with exceptionally low-strength roof (UCS
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... variation in roof fall rates is clearly related to geologic conditions. One recent study compared the uniaxial compressive strength of roof rocks in the Illinois basin with those in southern West Virginia (Rusnak and Mark, 2000). The study involved more than 800 rock units and 10,000 laboratory strength tests. It concluded that for each of the three major roof rock types (shale, siltstone, and sandstone), the strength of the Illinois basin variety was less than half that of the same rock in West Virginia (see figure ...
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Citations
... Ground failures can be prevented by conducting regular roof inspections; examining the roof, face, and ribs before work is begun; training workers how to identify hazardous roof and rib conditions (MSHA 2022); and prohibiting anyone from being under unsupported roof conditions (MSHA 2020) or in areas where the ribs and equipment are in close proximity (MSHA 2019). Some of the constructive measures that can be taken are optimizing the roof support and installing additional support at intersections that provide a way to exit the mine (Mark et al., 2004), removing underground mine overhangs, scaling roof and ribs from a safer location (MSHA, 2022), and installing rib bolts with adequate surface area and following a consistent plan while installation (MSHA 2019). Optimizing the roof support Installing additional support at the intersections of exits/escapes Removing overhangs in underground mines ▪ Scaling loose roof and ribs ▪ Installing rib bolts ...
... The average injury rate in the five-year period between 2010 and 2014 was about 0.92, while it was about 0.72 between 2015 and 2019. Improvements in roof control technology, including the widespread use of cable bolts particularly in the intersections, a persistent educational campaign organized by MSHA (Mark and Gauna, 2017;Mark et al., 2004;Zhang et al., 2019 ), and the reduction in mine worker exposure to hazardous areas are generally considered among the main reasons for reduced accidents and injuries in room-and-pillar mines. employee-hours is significantly lower than the rest of the ten-year period, making the injury rates significantly higher in these three years. ...
Mining has been recognized among the most hazardous industries in the United States, despite the significant reduction in injury and fatality rates over the past century. This study focuses on the ground-fall incidents classified in the Mine Safety and Health Administration (MSHA) database as “fall of roof, back, or brow from in-place” and “fall of face, rib, pillar, side, or highwall.” The main objective of this study is to conduct a comprehensive statistical analysis of the reported ground-fall incidents from 2010 through 2019 including both coal and metal/non-metal mines. Since most of the ground-fall injuries and fatalities occurred in underground coal mines and were attributed to falls of roof and rib, an additional analysis was directed to evaluate roof- and rib-fall incident trends in underground coal mines associated with the mining method, coal-seam thickness, mine size, and the seasonal effects.
The authors studied 8,303 ground-fall incident-narratives from both coal and metal/non-metal mines and classified them into one of the following types: roof fall, rib fall, face fall, highwall failure, and rock burst. Classifying the ground-fall accidents into these five categories and determining injuries and fatalities associated with each category will help identify areas where additional research is needed and where innovative solutions need to be developed to reduce these potentially severe occupational hazards. These ground-fall incidents resulted in 46 fatalities, 33 permanent disabilities, 3,082 injuries, 119,520 non-fatal days lost, and 12,433 days of restricted work activities.
For the period between 2010 and 2019, the average ground-fall injury and fatality rates in room-and-pillar coal mines are higher than those of longwall mines. The average fatality rate in room-and-pillar mines was 2.36 times that of longwall mines. The average injury rate in room-and-pillar mines was 0.82, while it was 0.51 in longwall mines. Of the injuries occurring in longwall mines, 52% occurred during development. The rib-fall fatality rate was higher than the roof-fall fatality rate for both room-and-pillar mines and longwall mines. The severity of rib-fall injuries tends to be higher than that of roof-fall injuries. Ground-fall rates were found to be higher between July and October, reinforcing research findings suggesting this could happen as a result of increased humidity.
... During the 1990's an average of almost 2000 non-injury roof falls were reported annually, while studies found that roof falls were highly concentrated in a few mines with the weakest roof. Between 1995 and 2003, for example, just 20 underground mines (out of more than 400 producing more than 100,000 t per year) accounted for nearly one quarter of all roof falls in the USA [9]. ...
During the past 15 years, roof fall rates have fallen dramatically in US coal mines, particularly in regions where the roof is weakest. The remarkable reduction in the number of roof falls has been accomplished with more effective roof support systems. The purpose of this paper is to present a design methodology that builds on and quantifies the basic roof support concepts that have been successful in the USA. The methodology starts by defining three modes of roof support, based on the roof strength relative to the stress level: (1) suspension, where roof bolts mainly provide skin control for strong roof; (2) beam building, where moderate strength roof can be supported by roof bolts alone; and (3) supplemental support for weak roof. Next, a large database of roof fall histories at a number of mines is used to define the approximate boundaries of these three regimes based on the coal mine roof rating and the depth of cover. Finally, guidelines are presented for site-specific design of support systems within each regime. The new computer package, analysis of mine roof support (AMRS), implements the design methodology.
... The exposed layer was extremely moisture sensitive and weak and was not encountered in any of the nearby boreholes. Figure 10 shows that the roof fall locations have wet underground CMRR indices of 35 or less; these are usually referred to as critical zones, where supplemental supports and additional caution are required for mining purposes (Mark et al. 2004). ...
Among all United States underground coal fields, those in Illinois have the highest rate of roof fall events due to their weak and severely moisture sensitive roof rock units. Rockmass characterization is the key initial step in designing safe and economical roof control measures in underground coal mines. In this study, a performance-based roof rockmass characterization is investigated. The geologic conditions as well as underground mine geographic specifications, roof fall analysis, mining method, utilized supplemental roof control measures, and geotechnical properties of roof rock units were considered to link the roof performance to rockmass characterization. The coal mine roof rating (CMRR) rockmass characterization method was used to evaluate the roof conditions and roof support design for an underground coal mine located in the Illinois Coal Basin. The results of several mine visit mappings, laboratory test results, and geotechnical issues and concerns are presented and discussed. The roof support designs are analyzed based on the rockmass characterization and are compared with the observed performance. This study shows that (1) CMRR index is a reasonable method for characterizing roof rockmass; (2) moisture sensitivity and bedding strengths in the horizontal direction are essential parameters for roof support design in mines with weak roof conditions; and (3) the applicability of the analysis of roof bolt system for roof support design of the studied mine is questionable.
... In some cases of very weak mine roof, such as shale with a CMRR of less than 35, the orientation might not matter because the minimum principal stress is still great enough to damage the rock. Trends in the past have shown that a mine can be oriented properly with the major horizontal stress, but can still experience roof falls during entry development (Mark and Mucho 1994;Mark et al. 2004). In these cases, the mine engineer must use other methods to mitigate the hazard beyond reorienting the mining direction, including additional rock reinforcement, as discussed below. ...
... In the roof that had been gunited, three roof falls were experienced over the span of 24 crosscuts. In the adjacent entry, the roof had not been gunited and 13 major roof falls occurred over a 6-year period (Mark et al. 2004). In a separate study, a spray-on polymer sealant was found to successfully reduce the amount of measured rock fall into the coal mine entry compared to areas with an unsprayed roof (Klemetti et al. 2009). ...
Ground control research in underground coal mines has been ongoing for over 50 years. One of the most problematic issues in underground coal mines is roof failures associated with weak shale. This paper will present a historical narrative on the research the National Institute for Occupational Safety and Health has conducted in relation to rock mechanics and shale. This paper begins by first discussing how shale is classified in relation to coal mining. Characterizing and planning for weak roof sequences is an important step in developing an engineering solution to prevent roof failures. Next, the failure mechanics associated with the weak characteristics of shale will be discussed. Understanding these failure mechanics also aids in applying the correct engineering solutions. The various solutions that have been implemented in the underground coal mining industry to control the different modes of failure will be summarized. Finally, a discussion on current and future research relating to rock mechanics and shale is presented. The overall goal of the paper is to share the collective ground control experience of controlling roof structures dominated by shale rock in underground coal mining.
... In recent years, it is not uncommon to attribute entry stability and roof control problems to the existence of high horizontal stress [7][8][9][10][11][12][13][14]. In fact, high horizontal stresses, referring to those high above that calculated from pure elastic Poisson's effect, is a norm for underground mines that are far below drainage. ...
Ground control is one of the four subsystems of underground mining. It covers not only roof control, but also rib control, floor control, pillar design, shield design, overburden failures and subsidence. In the past three decades, ground control has made a tremendous advancement and many case studies have demonstrated its important role in the daily mining operations. However, there are plenty of room for improvements. This paper discusses the research needs in 12 subject areas including research approach, rock property, geology, computer modeling, in-situ stresses, roof bolting, coal pillars, field instrumentation, failures, surface subsidence, shield supports and coal bumps.
... Other troubling trends from this analysis were the exceptionally high (greater than 100% higher than the national rate) noninjury roof fall rates associated with 44% of the room-and-pillar production in the northern Appalachian region, specifically the Upper Kittanning, Sewickley and Upper Freeport coalbeds (Table 4a). Weaker geology is the most likely explanation (Mark et al., 2004). The data also confirms that, despite the weak roof, longwall mines in the northern Appalachian region (Pittsburgh coalbed) maintained an admirable record, with noninjury roof fall rates significantly lower than the rest of the country for 95% of the longwall production in that region. ...
From 1999 through 2008, groundfall events resulted in 75 fatalities, 5,941 injuries and 13,774 noninjury roof falls in U.S. underground coal mines, according to the U.S. Mine Safety and Health Administration (MSHA). A comprehensive analysis of these events was conducted to better understand where and why these incidents occurred. The first segment of the study used the MSHA database to evaluate ground-fall injury rate trends related to mining method, year, locality, seam thickness, mine size, coalbeds and seasonal effects. In the past decade, roof fall injury rates have dropped 50%, but rib fall injury rates have remained constant. Room-and-pillar mines had a roof fall injury rate that was nearly 2.5 times the longwall rate. High groundfall rates occurred in the Illinois Basin, a finding that correlates with other studies that have shown that roof rock in the Illinois Basin is weaker than that in the central Appalachian coalfields. Finally, noninjury roof fall rates in the eastern and central United States were found to be 50% higher from July through September, possibly because of the changes in humidity levels that may cause shale mine roofs to deteriorate. The second segment of the study evaluated the root causes of failure by reviewing all MSHA groundfall fatality reports for 1999-2008. Pillar recovery was the leading cause of groundfall fatalities (19%), followed by rib falls, roof skin falls and traveling under unsupported roof (16% each). Defining prominent ground control incident trends and hazards will identify areas where additional study is needed and where innovative solutions need to be developed to reduce these severe occupational hazards.
... Roof falls occur in mines when horizontal stress causes roof strata to buckle, rupture, and fall [1]. Roof falls are a hazard in mining that can lead to injuries, damaged equipment, blocked escape routes, and disrupted resource production in the mine [2]. Studies by the Mine Safety and Health Administration (MSHA) show that mines producing 20% of underground coal in the US resulted in 60% of roof falls, mostly located in Illinois and the Appalachian coal basins [3]. ...
Microseismic monitoring involves placing geophones on the rock surfaces of a mine to record seismic activity. Classification of microseismic mine data can be used to predict seismic events in a mine to mitigate mining hazards, such as roof falls, where properly bolting and bracing the roof is often an insufficient method of preventing weak roofs from destabilizing. In this study, six months of recorded acoustic waveforms from microseismic monitoring in a Pennsylvania limestone mine were analyzed using classification techniques to predict roof falls. Fuzzy classification using features selected for computational ease was applied on the mine data. Both large roof fall events could be predicted using a Roof Fall Index (RFI) metric calculated from the results of the fuzzy classification. RFI was successfully used to resolve the two significant roof fall events and predicted both events by at least 15 hours before visual signs of the roof falls were evident.
... It also facilitates predicting permeability and fluid flow in coalbed methane (CBM) gas reservoir (Tyler et al., 1997;Peters, 2000;Bell and Bachu, 2003;Shi and Durucan, 2005;Bell, 2006;Colmenares and Zoback, 2007). Vertical stress/overburden load data are very useful for numerical simulation of faulted zones, rock stiffness as well as rock falls (Coolen, 2003;Wu et al., 2004;Mark et al., 2004;Nemcik et al., 2006;Karacan et al., 2008). The measurement of vertical stress/overburden load would help in calculating area extraction ratio in pillar designing. ...
Well log analysis has been carried out to evaluate resistivity and density log data of about seven wells distributed over 3.5 sq. km area of Rangamati, Raniganj coalfield, India. Geological logs of these boreholes have recorded occurrence of four major consistent coal seams (A, B, C and D) at different depths, varying between 52 to 200 m in the area. Cleat volume/porosity ranges from 1% to about 3% in this area. Vertical stress magnitude ranges from 0.55 to 4.59 MPa from 52 to 202 m and increases with depth. The gradient of vertical stress magnitude decreases within coal seams compared to the stress gradient at roof and floor of the same seams. A methodology is proposed for estimation of permeability for a macro-cleat system of coal from well log derived porosity and from known cleat spacing from Rangamati area. Permeability value ranges from 0.5 md at a depth of 193–200 m to 18 md at a shallower depth of 45–53 m while vertical stress decreases from 4.566 MPa to 1.254 MPa respectively. Permeability values of four coal seams have been correlated with effective horizontal stress to infer the maximum horizontal stress orientation.
... Many times reorienting the mines to minimize drivage in the N-S direction has provided some relief. In other cases, roof rock is so weak that even minimum stress magnitudes are enough to cause roof damage (Mark et al., 2004). ...
Some of the most difficult coal mine roof in the United States can be found in the Illinois Basin. Factors contributing to the high roof fall rate include weak, moisture-sensitive roof rock; high horizontal stress; and limited longwall mining. The depth of cover ranges from 27 to 300 m (90 to 1,000 ft), and roof dam- age from horizontal stress can be severe. Moisture-sensitive roof rock, which contributes to roof skin deterioration and roof fall, is common above the Springfield-Harrisburg Herrin #5 and #6 seams in the Illinois Basin. The roof fall rate increases significantly in the humid summer months. Using laboratory and field studies, the National Institute for Occupational Safety and Health (NIOSH) has shown that highly moisture-sensitive roof rock can be directly correlated to poor roof conditions. Controlling the skin is the key to reducing rock fall injuries, and roof screening is, by far, the best remedy. Illinois Basin coal operators have been successful in reducing the number of rock fall injuries in recent years. NIOSH has documented best practices for screen installation, which has resulted in safe, efficient operations. Other solutions to skin failure include the use of denser five bolts/row patterns to reduce spans between bolts; systematic supplemental support in intersections; straps and large pans to protect operators; and air conditioning to remove moisture from the intake air.
