By Tristan Jones, Ph. D., and Greg Molinda
A roof control plan, detailing support methods, is a requirement for all underground coal mines. These plans usually include changes to roof support when encountering “adverse” roof conditions. Adverse roof can be indicated by damage due to excess stress, geologic structures like clay veins and sandstone channels, or excessive water inflow. When roof bolters observe adverse roof on a development section, supplemental support can be installed as detailed in the roof control plan.
Adverse roof can also be caused by defects or discontinuities that are unseen in the roof. These include mud zones, coal streaks, bedding plane separations, joints, fractures, or cracks. These defects can only be identified by roof drilling.
Roof bolters are often relied upon to notice these defects. This is logical since they have the opportunity to observe the roof geology with every hole they drill. Many roof control plans call for increased primary or secondary support whenever defects are found within the bolted horizon. As a result, roof bolters are often the ones making the decision to change the support accordingly.
Roof bolters are also responsible for ensuring that a mine meets the criteria set out by the Mine Safety and Health Administration to ensure sound roof support. Sections 75.204(f)(1&2) require that regular test holes be drilled to at least 12 in (30 cm) above the anchorage horizon for mechanically tensioned roof bolts, to ensure competent strata. They also require that bolts be anchored at least 12 in (30 cm) into a stronger strata when those bolts are used to suspend the roof from that strata. It is up to the roof bolter to ensure that these regulations are met through the observations they make.
It is widely accepted in the underground coal industry that roof bolters are capable of identifying defects in the roof based on drilling cues like the rate of penetration, vibrations, dust from cuttings, and sounds. It is also accepted that this ability improves as the individual driller gains more experience. Nevertheless, new research conducted by the National Institute for Occupational Safety and Health (NIOSH) brings those assumptions into question.
NIOSH researchers recently visited six underground coal mines in the northern Appalachian basin to observe the drilling process of typical roof bolters. The goal was to document how accurate roof bolters were in detecting defects in the roof. For the purposes of the study, any roof feature that results in a rock void is referred to as a crack (Figure 1). In each case the bolters were requested to drill a hole as they usually would, noting the depth and estimated size of any cracks they believed they encountered. Researchers constructed drill logs at the site from this information while the hole was being drilled. Immediately after the drill steel was removed, a borehole camera was used to record actual cracks in the roof. The time between the drilling of the hole and inspection with the video was limited to less than one hour to prevent skewed results from time-dependent bed separation. Narration on the video record included geological and geotechnical observations as well as the measured height above the roof line of any defects found on the video. The logs created from the driller’s observations were later compared with the evidence found on the video record.
The analyzed drill holes were evenly spread across the six mines. In each mine, the drill holes were all completed within the same area, typically within the same entry, and always within the same section. In each mine, drillers with varying experience levels were asked to drill in the same location in order to reduce variation in roof quality for the various drillers. The lithology encountered at the six mines was remarkably similar, mainly consisting of mudrock and shale with interbedded coal streaks, and occasional limestone or sandstone interbeds.
Certain factors may influence the driller’s ability to sense defects in the roof. In this study, researchers recorded the aperture (size) and vertical position of the crack within the drill hole, as well as the years of roof bolting experience of the driller. Logically, larger apertures or greater experience were both expected to improve the likelihood that a roof defect would be accurately identified. A crack was considered to be accurately identified if the driller’s identification was within ±3 in (7 cm) of the location as found on the video record.
During the study, 21 drill holes were recorded, containing 29 confirmed cracks, drilled by 12 different roof bolters. Overall, the drillers successfully identified 16 of the 29 cracks, or 55%, while they missed or incorrectly identified the remaining 13 (45%). There were also 28 times when the driller identified a crack that was not verified by the video record, causing a false positive. These false positives made up 64% of all driller identifications.
The most common cause of false positive results was wrong interpretation of the drilling cues. This caused drillers to identify a roof defect when one did not exist. A less frequent event was improper height measurement by the driller. Accurately identifying roof defects is essential, but correctly identifying the location of a crack within the drill hole also plays an important role in choosing what type and length of secondary support to install. Rock bolts, especially those that are not fully grouted, require a good anchorage in order to operate best. Incorrect identification of a defect and its location may lead to selection of bolts that are too short for the conditions, resulting in anchoring into a zone that cannot support the load.
Although it could be argued that false positives would lead to an increased amount of support, consistent false positives may lead to complacency and the failure of a roof bolter to react when actual cracks are encountered. Additionally, incorrect crack identification may lead the bolter to install additional roof support where it would not otherwise be required, increasing cost and overall bolt installation time. These cost savings may serve as a useful incentive for mines to improve the crack identification success rate of their roof bolters, coincidentally insuring that extra roof support is placed where it is needed most, resulting in improved safety.
The effect of crack aperture on identification success was an important question addressed by this study. Cracks identified by drillers ranged in size from “hairline” cracks to openings 1.5 in (3.8 cm) wide, as measured from the video record. As expected, roof bolters were more successful at identifying larger aperture cracks (Figure 2). Crack identification rates ranged from 53% for cracks 1/16 in (0.06 cm) open to 75% for cracks 1 in (2.5 cm) open or greater. However, even though roof bolters were more successful at identifying larger cracks, three roof cracks larger than ½ in (1.3 cm) wide were still missed, including one that was 1.5 in (3.8 cm) open. This data suggests that drillers may not be finding as many of the large cracks as operators may expect. While large cracks may be most hazardous, small cracks cannot be overlooked from a safety standpoint. No research has been completed on the persistence of cracks of varying sizes occurring in coal measure rocks, or on the frequency of falls caused by cracks of various sizes. It is also unknown how long it takes small cracks to develop into larger cracks.
Roof Bolter Experience
It is taken for granted that a miner with years of drilling experience will be more likely to find cracks than a new miner. With this in mind, the data was analyzed with respect to driller experience. Roof bolters in the study ranged in drilling experience from six months to 15 years. These bolters were divided into three groups: Inexperienced (two years or less), Intermediate (two to five years), and Experienced (more than five years).
An unequal number of large cracks were encountered by each experience group, introducing a bias to the data. In order to standardize the results, the success rate for crack identification was limited to small cracks of ¼ in (0.64 cm) or less. Eliminating the larger cracks from this analysis was enlightening because small cracks are those most likely to be missed and thus they should draw greater attention.
Figure 3 shows a link between the experience of the roof bolters and the success that those drillers had in identifying small cracks. The success rates increased steadily from 33% for the inexperienced group up through 50% for the experienced group. As expected, experience improves a roof bolter’s chances of identifying roof cracks of ¼ in (0.64 cm) or less. However, a success rate ranging between 33 and 50% is still relatively low, indicating that these cracks are still hard to identify.
Once the above success rates were established, the overall success rate in crack identification (including cracks of all sizes) was compared to the percentage of small cracks encountered by each experience group (Figure 4). It was found that as the percentage of small cracks identified (≤ ¼ in) decreased, the overall success rate increased, regardless of the roof bolter’s experience. This indicates that though more experience does help roof bolters identify the smallest roof cracks (Figure 3),the percentage of cracks encountered that are small has a greater influence on the overall success rate—i.e., the overall success rate increases only when the percentage of small cracks decreases. Therefore, the experienced bolter’s overall success rate of 73% (Figure 4) is inflated, because that group encountered a smaller percentage of small, hard-to-identify cracks. Regardless, the overall crack identification success rate for all bolters was still only 55%.
Correctly Diagnosing Roof Defects May Be Harder Than We Think
Surface exploration drilling usually produces both a rock core and a geophysical log as evidence of rock composition and defects. However, when the roof is drilled underground during bolting, operators rely on the roof bolters’ interpretation of drilling cues to locate defects in the roof. With a success rate of only 55% in crack identification, the reliability of information provided by roof bolters may be in question. Roof bolters are typically trained on the job, with the training focused mostly on machine operation and the roof bolting cycle. Most new miners receive little training on roof geology and the nature of roof defects. The results reported here show that experience has only a limited benefit to improving roof crack identification. Based on conversations with engineers and employees at each of the study mines, it can be said that very few experienced roof bolters have ever been provided with evidence of the accuracy of their roof crack identification. As a result, they have no way to verify their ability to diagnose roof cracks. This limited experience is then passed on to new roof bolters, perpetuating a line of potential misinformation.
As mentioned, the drilling cues available to roof bolters include changes in penetration rate, vibration, dust from cuttings, and drilling noise. The contrast between drilling hard and soft rock formations can also result in changes to the above parameters similar to those found during crack interception, causing false positive results.
In one study mine, one inexperienced driller and two drillers with intermediate experience were tested for accuracy. Collectively they identified six cracks, all false positives. Further, they did not locate the three real cracks that were found on the video record, one of which was 1.5 in (3.8 cm) wide. The borehole camera confirmed that all six of their false positives could be attributed to coal streaks found at the same locations where they had identified a crack. The coal streaks involved soft drilling and produced similar drilling cues to roof cracks. In this case, the random coal streaks were not roof defects and posed no roof hazard, yet they triggered the installation of supplemental support.
The Path Forward
Recalling the critical importance of accurate roof crack identification, consider these statements:
• mines routinely rely on their roof bolters to install additional roof support where necessary in order to ensure the safety of everyone in the mine;
• roof bolters may only successfully identify 55% of all cracks that they encounter.
The inherent danger in the relationship between these two ideas becomes obvious.
The simplest way to mitigate the risks will be by improving the training given to miners. Changing roof geology presents a challenge in diagnosing defective roof in real time. Roof bolters are relied upon by most underground coal mines to take an active role in providing evidence of hazardous roof conditions, and it is likely to remain that way for some time. Roof bolters would thus benefit from several levels of improved training to overcome the sizeable challenges that face them.
Basic information on roof geology, including pictures or drawings of “cracks” of all types, could be used to introduce the hazards to new roof bolters and help deepen their understanding of the importance of their task in diagnosing roof hazards. Also, task training by experienced roof bolters should include roof drilling in entries that have known roof defects, which can then be matched to predictive drilling cues. Later, instant verification with a borehole camera would tie the drilling cues experienced by the bolter to a visual confirmation of the defect, giving a mental association between the two.
Beyond training, mine operators can use borehole cameras to inspect drill holes on a regular basis. While borehole inspection provides the best roof diagnostic data, it may become lengthy or disruptive. In such cases borehole cameras should be used whenever a major change in geology is noted by mine personnel, or for periodic checks to help ensure high-quality crack identification by the drillers. Though they are not permissible, current borehole cameras are digital, low-cost, and user-friendly. They offer high image quality and are unparalleled as a method of verifying roof lithology in situ. This method of roof surveying would help to map geologic trends and defects, bringing a greater understanding of roof competence.
The value of relying on roof bolter operators to identify roof defects is that they provide real time results, a benefit that cannot be matched though borehole camera inspection. Roof bolting provides a great amount of data for roof defect diagnostics. This is the basis of “smart bolting.” The Smart Bolter is a roof bolting machine that has been outfitted with electromechanical sensors designed and calibrated to “feel” changes in roof strata. The concept has been investigated for decades, and versions of this technology have been developed around the world. The most successful “smart bolting” machines are now outfitted for use in limestone mines where they have shown good results by identifying cracks and mud bands, in real time, during drilling. The machines are also suitable for use in coal mines, and have performed well in experimental situations, though mechanical difficulties have prevented their successful introduction into coal mines. As these problems are resolved, the machines promise to be useful for defect detection, and could become the future standard for roof bolting.
The practice of using roof bolters to identify defects is firmly established within the underground coal mining industry, and is unlikely to change anytime soon. The wealth of data concerning roof conditions that is accessible to the roof bolter, and the advantages of acquiring it and finding defects in real time, will help ensure that bolters remain the primary source of roof sampling in the near future. However, the potential consequences of misdiagnosing cracks, and the potential cost savings of accurately identifying cracks, warrant considering the matter further. Given the results of this study, and until technology offers a viable alternative in coal mines, mine operators should assess their training programs, both new and refresher, to ensure that bolters have the skills needed to identify roof hazards with greater accuracy.
The authors would like to acknowledge the assistance of the six cooperating mines. Their help in providing drill sites and access to roof bolters was invaluable, and shows their interest in the accurate diagnosis of roof defects and ensuring roof safety.
The findings and conclusions in this report have not been formally disseminated by the National Institute for Occupational Safety and Health and should not be construed to represent any agency determination or policy.
About the Authors
Jones is an associate service fellow, ground control, and Molinda is a lead research scientist for NIOSH’s Office of Mine Safety and Health Research in Pittsburgh, Pa. Jones can be reached at 412-386-6536 (E-mail: TJones9@cdc.gov); Molinda can be reached at 412-386-6890 (E-mail: firstname.lastname@example.org.)