Technical Sessions

The 42nd International Conference on Ground Control in Mining continues to bring you the leading research on ground control in mining. Hear from recognized industry innovators and specialists from around the world.

Note: Check back often for updates.


Tuesday, July 25 2023

Keynote Session

8:10 am
Roof Bolting and Roof Falls

Dr. Syd S. Peng

Founder of the 40-year-old International Conference on Ground Control in Mining, Syd Peng is a global leader who has advanced the science and technology of mining. Growing up in Taiwan, Peng worked in the Shinchu coal mine and moved to the US to study gas outbursts and safe mining practices at the South Dakota School of Mines and Technology. Peng received his PhD at Stanford University, where he married his wife, Felicia. Married and graduated, Peng’s first engineering job was at the Twin City Research Center until 1974, when he moved to West Virginia University and declared, “this is where mining is supposed to be.” Chairman of the WVU mining program for 28 years, Peng developed their PhD program and the US standards for longwall mining and surface subsidence damage control. As a consultant, Peng became known as a problem solver for industry and inspired safe mining practices in China coal mines. Peng has had a significant impact on the mining industry and continues to influence future generations of mining engineers through the Syd S. and Felicia F. Peng Ground Control in Mining Award.

There are two basic types of roof bolts used in US underground coal mines, tensioned and non-tensioned, based on suspension or beam building principles. The use of continuous miners and roof bolters for development of and installation of roof bolts results in a nearly standard roof bolting pattern: 4 x 4 ft in 20-ft wide entries, regardless of roof stratigraphic sequence. Consequently, the design of roof bolting is restricted to selection of bolt parameters such as type, diameter, and length. Under this system, roof falls are not uncommon including skin falls, large falls, massive falls and cutter roofs and occurred sometime after development and mostly at intersections. Remedial measures commonly adopted are briefly described. In terms of roof bolting theories, “suspension” has been most commonly used regardless of bolt type used. The vast variety of roof bolting patterns adopted in coal mines of various countries including Australia, Canada, China, Germany, Mexico, and South Africa presented at ICGCM in the past 40 years was shown and discussed - a complete lack of a commonly acceptable roof bolting theory or theories.

Mining Induced Seismicity

Chair: M. VanDyke, NIOSH, Pittsburgh, PA

8:30 am
Examination of Coal Burst Mechanics and Control Measures in Major United States Coal Fields

H. Maleki; Spokane, WA, United States

This paper reviews a study conducted for ACARP to assist Anglo-American operations with better understanding of coal burst mechanics and control based on U.S. practical experience, investigations and inspections in three major U.S. Coal Fields over the past 45 years. The study examined both nonviolent and burst prone case studies to identify contributing risk factors and addressed failure mechanisms for major US coal fields in Utah, Colorado, and Kentucky, which have diverse characteristics. To investigate the mechanics of coal burst, a multi-pronged approach is used. First, field measurements in three mines from East Mountain, Utah, are complemented with finite-difference stress analyses to address the importance of horizontal stress in coal pillar mechanics of violent failure. Second, the computational procedures for estimating seismicity resulting from slip along geological discontinuities are reviewed, as these measurements point to seismicity being the trigger mechanism for violent failure of marginally stable structures. This joint-slip seismicity mechanism is in agreement with the more recent improved re- examination of Mining-Induced Seismicity (MIS) data from the Crandall Canyon Mine, Utah by University of Utah (UOU). Third, a hybrid statistical-analytical methodology is applied for identifying significant factors affecting coal burst. It utilizes data from 30 case studies including those from Utah, Colorado, Kentucky, and other Eastern U.S. mines. The statistical approach reinforces field measurements and points to strata rigidity, joint spacings and horizontal stress field as significant factors affecting damage resulting from coal bursts. This provides practical capabilities for identifying operations of higher risk using a rigidity- cavability variable. Overall, the study provides valuable insights into the mechanics and risk factors associated with coal bursts in major U.S. coal fields. By utilizing a multi-pronged approach that incorporates both field measurements and statistical-analytical calculations, the study offers practical capabilities for identifying operations of higher risk and improving coal burst control.

8:52 am
Seismic Energy and Energy Released Due to Strata Failure During Longwall Caving Operations

Z. Agioutantis and C. Gerwig; University of Kentucky, Lexington, KY, United States, J. Wickline; Arch - Leer South, Grafton, WV, United States

In longwall mining, the progressive caving of the roof allows stored energy to dissipate over an extended time. However, in some cases, the roof strata above the longwall panel are unusually strong and do not break with the movement of the working face. Instead, the roof material bridges over the expanse of the longwall, forming an unstable beam. Later, when this beam breaks, the stored energy can release suddenly, causing a seismic event to propagate to surrounding mine workings. The focus of this paper is to estimate the size of beams that form over longwall panels to the seismic energy recorded by the mine. Analytical calculations are used to verify two-dimensional models of longwall roof beams to determine the strain energy and gravitational energy developed, that is then released upon strata failure to surrounding mine workings. Models are developed using RS2 to model the stress, strain, and deflection along the bottom of the longwall roof beam to calculate gravitational and strain energies. Gravitational energy is shown to be the main driving factor in energy release, except for the beams with low heights. Analyses allow the determination of beam characteristics based on seismic data. The analysis is refined using swelling factor to compare various beam drop heights. The models show an increase in seismic magnitude for higher drop heights. However, the actual data provided by Buchanan mine displays a decrease in seismic magnitude with increasing drop height, indicating that energy is dissipated into the broken gob material as drop height increases.

9:14 am
Longwall Mining Generated Seismic Activity Buchanan County, Virginia

V. Scovazzo; John T. Boyd Company, Richland, WA, United States

During the winter months of late 1985 and early 1986 residents along Dry Fork near Grundy, Virginia experienced seismic events that they attributed to longwall mining at Island Creek Corporation's (Island Creek) Virginia Pocahontas Mine No. 3. Island Creek contacted STS D'Appolonia Ltd. (STSDL) on March 7, 1986, to investigate these events. An STSDL representative, Mr. Scovazzo, interviewed residents of Dry Fork on March 10, 1986. It was noted that only people within a residential structure felt the events they described as house shaking, vibrating walls, and ceramics falling from shelves. STSDL representative installed a blast monitor at a residence. Within two days, various events were recorded. Some could be attributed to truck and heavy equipment movement. Eight events had no obvious cause but appeared on resident’s written logs as earthquakes caused by mining.
STSDL contacted Utah Geophysical, Inc. who had developed a technique to determine the mode of rock failure using the seismic record. Utah Geophysical was engaged to install and monitor a microseismic network, locate the epicenter of the events, and work with STSDL in determining modes of failure.
The network was comprised of six geophones located at five stations. Data was recorded for 24 days from May 20 through June 13, 1986. The data was filtered to eliminate events determined to have an origin other than the longwall panels’ overburden and under-burden. The larger events from this filtered data set were selected and analyzed to determine their hypocenter locations, time of occurrence, magnitudes, and failure mode. Processed data was interpreted, tabulated, and plotted by STSDL to provide information relative to the sources of the seismic vibrations and their possible relationship to the mining operation.
The investigative project tasks and objectives included:

  • Determine the location and cause of the seismic events.
  • Determine the failure mode.
  • Determine whether there was a relationship between the vibrations and longwall mining at the VP-3 Mine.
  • Determine the adverse effects of these events on structures and determine the size of the damage zone.
  • Conduct a subsidence survey over two longwall panels and relate the movement, if possible, to the seismic events.
  • Complete in-mine observations and relate them, if possible, to the seismic events.
  • Determine the source of all seismic events recorded such as mine induced, surface coal mine blasts, truck movement, heavy equipment movement, lighting strikes, naturally occurring earthquakes.
  • Obtain subsurface samples from a geologic/geotechnical drill hole.
  • Complete a rock mechanics testing program.
  • Determine whether modifications to the mine plan are advisable in the interest of reducing the surface effects.
  • Redesign the mine to reduce the effects of the events or to eliminate the events.
  • Determine whether the damage to surface structures was directly related to the vibrations. This paper does not cover all the tasks and research completed for the project but presents the data and procedures used to determine the following:
  • Many seismic events were related to longwall mining.
  • The failure mode was horizontal tensile failure of shale located between thick quartz arenite layers. This occurred in an areas above active and previously mined longwall panels.
  • The bulk of the seismic energy was within the natural frequency of residential structures.

9:36 am
Control of High-Order Seismicity in Deep Western United States Mines Through Structural Mine Designs

H. Maleki, Spokane, WA, United States

Mine seismicity is influenced by the mechanical properties of both near-seam strata and overburden/underburden and the prudent selection of mining methods and mine layout designs utilizing site specific structural setting of the mines. The importance of adequate data collection, monitoring and prudent structural designs has long been recognized and utilized in many United States operations with adequate engineering resources. In this paper, two case studies of historic seismic events and contributing factors, in deep coal and trona mines with overburden thickness exceeding 1,500-ft, are reviewed using observations and analysis. By back analyzing the overburden response in a historic room-and-pillar coal mine, the overburden collapse mechanism associated with a 3.3 magnitude seismic event is reviewed as well as the need for improvements in designs including panel and barrier pillar widths. Similar high-order seismic events were also experienced in trona mines of the Green River Basin during 1990’s, interrupting room-and-pillar mining and creating ground control concerns. Efforts to improve structural mine designs are described in this paper leading to stable and productive operations for decades following the high-order seismic events in both coal and trona case studies. These efforts included improving room-and-pillar extraction layouts and utilizing properly- designed longwall mining technique that were verified using long-term geotechnical instrumentation. The study mines are located on the Wasatch Plateau (WP), Utah and in the Green River Basin (GRB), Wyoming. More recently, in Utah’s East Mountain alone, Energy West Mining Company (EWMC) has extracted some 165 million tons of coal without any significant incidents. Overall, favorable geologic conditions and prudent mine layout designs are key elements to this successful experience in both single and two-seam layouts. Utilization of conservatively designed longwall mining technique in the GRB has been geotechnically stable, using reported experience in fifteen panels and monitoring results in the NW District with measurable longwall-induced seismicity being rare. These studies demonstrate a considerable amount of progress which has been made within the last four decades in the application of longwall mining technique, integrated monitoring, and analyses for the verification of structural designs in mines in the United States.

Design Tools and Guidelines

Chair: D. Tuncay, West Virginia University, Morgantown, WV

10:30 am
Using UT3PC and LaModel to Aid the Mine Engineer in Evaluating Mine Layout Design

M. Larson and B. Kim; National Institute for Occupational Safety and Health, Spokane, WA, United States

Researchers at the National Institute for Occupational Safety and Health have tested the finite element code, UT3PC, with LaModel to evaluate their results using know case studies. The purpose of the testing was to demonstrate the usefulness and limitations of these programs in evaluating mine layout design. Both of these software packages are simple to use and free for public download. Their use by mine engineers, along with other tools, such as empirical methods, can enhance the assessment of a mine layout design. LaModel can model mining layout on a mine scale or, at least, on a multiple-panel scale. However, this software has limitations in calibrating to conditions at many coal mines in the western U.S. The UT3PC software models smaller parts of a mine, and is used to evaluate the design of pillars, shafts, or drifts, but the software calculates all components of the stress tensor and provides stresses, displacements, and safety factors throughout the model volume. The two codes are used to evaluate design of one known case, and UT3PC is used to evaluate another case. The results and approaches demonstrated in the testing are intended to aid mine engineers in evaluating site-specific problems and to show that these tools can provide useful information to the mine engineer in evaluating mine layout design.

10:52 pm
The Road to Zero: The Fifty-Year Effort to Eliminate Roof Fall Fatalities from US Underground Coal Mines

C. Mark and G. Rumbaugh; Mine Safety and Health Administration, Pittsburgh, PA, United States

Sixty years ago underground coal mining was the most hazardous job in the US. Roof falls were a big part of the problem. They killed about 100 miners every year, more than all other causes put together. Fast forward half a century to 2016, and the first year ever with zero roof fall fatalities. Just three miners were killed by roof falls during the following six years. How was this historic goal achieved? This paper starts with a modern analysis of the causes of the roof fall fatalities in 1968. Then it follows the reductions over time by category, using snapshots of the fatalities occurring in subsequent decades. Along the way it evaluates the influence of the regulatory environment, changing mining methods, and better ground control technology. The paper shows that in 1968 more than half of roof fall fatalities at large mines were attributable to an inadequate safety culture. The immediate effect of the 1969 Coal Mine Health and Safety Act was to reduce the riskiest activities, like needlessly going under unsupported roof. Other hazards, like large roof falls, required technological developments before they were brought under control. Roof Control Plans, which the US Bureau of Mines had been advocating since the 1920’s, played a significant role throughout the process.

11:14 pm
NIOSH Gas Well Stability Research – Research to Practice

D. Su, P. Zhang and Z. Khademian; CDC/NIOSH/PMRD, Pittsburgh, PA, United States and B. Kim; CDC/NIOSH/SMRD, Spokane, WA, United States

Building upon the research results and knowledge obtained from the ongoing NIOSH Gas Well Stability Research, researchers from the National Institute for Occupational Safety and Health (NIOSH) provided detailed analyses on potential longwall mining and gas well casing interactions at an unconventional shale gas well pad in a West Virginia longwall mine. Following the recommendations from recent research findings, the 5-1/2” production casings in all five wells were left uncemented from 4,200 feet below the surface to the surface. Prior to the first longwall panel excavation, NIOSH researchers conducted sophisticated 3-dimensional numerical modeling to predict longwall-induced deformations and gas well casing stresses, which were compared with post-first panel and post-second panel mining 40-arm Caliper surveys. The pre-mining predictions and comparisons with post-mining 40-arm Caliper surveys were presented to all parties involved, including the coal and gas operators, Mine Safety and Health Administration (MSHA), Pennsylvania Department of Environmental Protection (PADEP), and West Virginia Department of Mines (WVDOM), prior to the re-entry operation on December 13, 2022. To date, the re-entry operation is a resounding success.

11:36 pm
Load Capacity and Stiffness Characteristics of Pumpable Standing Support System Used in Underground Coal Mines

T. Batchler, T. Klemetti and D. McElhinney; NIOSH, Pittsburgh, PA, United States

Unplanned tailgate collapses and difficult mining conditions in U.S. longwalls have necessitated the use of innovative tailgate support systems. Severe ground movement induced by longwall loading has caused operators to utilize secondary support systems that can match yield and strength requirements with expected ground reactions. Standing support performance requirements depend on the nature of the ground deformation behavior, and therefore require a good understanding of the interaction between the support and ground to achieve an optimum roof support system. Mining operations often employ standing support strategies that are unique to their mine. A support system that functions successfully in one situation could fail in another situation. It is important that the design and installation of the standing support systems provide the highest probability of preventing roof falls. Pumpable roof supports are currently being used to provide a safe working environment for longwall mining. A full understanding of the impact of these performance characteristics is necessary to optimize the support application and to provide a foundation for making further improvements in the support performance. Optimize of roof support applications and helps ensure the protection of mine workers by preventing roof falls due to inadequate support design. The load capacity and stiffness characteristics of 27-inch diameter pumpable support system were evaluated using National Institute of Occupational Safety and Health’s (NIOSH) Mine Roof Simulator (MRS). Previous tests to evaluate the performance characteristics of a single pumpable support. The goal of this research is to evaluate the performance characteristic effects of the pumpable system contrasting a single pumpable test result. This pumpable support system configuration simulates current installation practices in U.S. coal mines.

Numerical Modeling Applications in Ground Control I

Chair: B. Kim, NIOSH, Spokane, WA

1:00 pm
A New Strain-Softening Anisotropic Constitutive Model for Coal Mine Roof Simulation

D. Guner, T. Sherizadeh, K. Karadeniz, S.l Nowak and A. Kirmaci; Missouri University of Science and Technology, Rolla, MO, United States

Injuries and fatalities resulting from roof falls are still a major concern in U.S. coal mines, and many researchers are focused on developing meaningful approaches to minimize this hazard. In the U.S., coal mine roofs can vary widely in composition, geometry, and inherent heterogeneity, leading to a wide array of different roof fall failure types. Researchers frequently employ numerical modeling tools to reproduce this physical phenomenon and provide key guides to engineers regarding roof conditions and control as long as the input data and constitutive relations are appropriately selected. Simulating such complexity requires expertise and constitutive models that adequately simulate roof behaviors. However, the currently available constitutive models have shortcomings in capturing some of the roof fall failure types common in underground coal mines, such as guttering. A new constitutive model is developed to address the shortcomings of constitutive models commonly used for coal roof stability analysis. The constitutive model developed in this study combines the anisotropic-elasticity and nonlinear strain-softening responses. In addition, the developed constitutive model can simulate up to three different joint sets implicitly, allowing for use in both continuum and discontinuum settings, and can be used in both Lagrangian finite-volume and distinct element method codes of Itasca. First, the mathematical expressions of the model are provided.
Afterward, the validation phase of the developed model that covers comparisons with analytical solutions and those of existing models (e.g. anisotropic-elasticity, ubiquitous-joint, and bilinear strain-hardening/softening ubiquitous-joint plasticity) is presented. Finally, a hypothetical coal mine roadway facing high horizontal stress is simulated. The presented model successfully shows three distinct stages of a cutter roof in a shale roof: i) initiation of shear cracks at the rib-roof intersection, ii) extension of cracks deeper into the roof (i.e. above the roof bolt length horizon), and iii) cantilevered roof. It is demonstrated that the presented constitutive model has the potential to help future researchers in simulating various roof responses.

1:22 pm
Coal Pillar Rib Stability: A Regression Model Based on Distinct Element Method

K. Mohamed and M. Sears; NIOSH/CDC, Pittsburgh, PA, United States, D. Guner, T. Sherizadeh, A. Kirmaci and S. Nowak; Missouri University of Science and technology, Rolla, MO, United States

Coal ribs present a major safety hazard in underground mines, and there is currently no unified approach for their design in the United States. To tackle this issue, this study investigated the stability of underground coal pillar ribs under development load. Utilizing over a thousand 3DEC models, a framework for calculating Coal Pillar Rib Rating (CPRR) was developed. A detailed step-by-step method was outlined for computing the CPRR based on a 3DEC analysis, which was then validated with various 3DEC models, demonstrating a strong correlation between the two models. The results demonstrated that the Rib Factor of Safety (RibFOS) under development load is directly proportional to Coal Pillar Rib Rating (CPRR), Coal Mine Roof Rating (CMRR), and floor strength, and inversely proportional to the overburden depth and rib height. Moreover, a regression equation was formulated to predict the RibFOS based on the aforementioned parameters. Furthermore, empirical relationships were developed between the RibFOS and the applied Primary Rib Support Density (PRSD) in U.S. coal mines. To ease the calculations, a standalone application, Design of Rib Support (DORS), was developed. This application can be used to assess rib stability, calculate CPRR, RibFOS, and PRSD easily.

1:44 pm
Stability Analysis of Underground Excavations in Limestone Under Dynamic Loading

B. Kim and M. Larson; CDC/NIOSH, Spokane, WA, United States

Fault-slip bursts induced by mining activities can result in severe damage to nearby mine development. In general, engineers designing rock support in burst-prone mines not only need to consider stress redistribution resulting from excavation, but also dynamic loading that results from seismic waves generated by large mining-induced fault-slip events. In this study, we simulated dynamic waves in sedimentary rock from a blasting source and from a mining-induced-seismicity source using a Universal Distinct Element Code (UDEC) model with Discrete Fracture Networks (DFNs). Our purpose was to investigate the stability of openings and the adequacy of existing support systems at an underground limestone mine in South Korea. A baseline model with only static loading was also simulated, which verified observed stability under such conditions. The results indicate that the current support system of shotcrete lining is likely to work well under dynamic loading caused by blasting. However, when the dynamic source was a far-field fault-slip event, the dynamic loading exceeded the support capacity of the shotcrete lining. Thus, if a mine is operating in ground with an elevated risk of mining-induced fault slip, mine engineers should consider seismic wave propagation and ground motion distribution from a remote, mining-induced fault-slip event to determine a suitable support design.

2:06 pm
A Study of the Leading Factors Associated with a Massive Ground Collapse in an Underground Limestone Mine in Southwestern Pennsylvania

G. Rashed, N. Evanek and B. Slaker; NIOSH, Pittsburgh, PA, United States

In the past decade, there have been several incidents of massive ground collapses in underground stone mines in the Eastern United States. Fortunately, these events have not resulted in any fatalities. However, three mine workers were injured and many others were at risk due to the hazard from the fall of ground and/or the air- blast associated with these massive collapses. The National Institute for Occupational Safety and Health (NIOSH) initiated a project to better understand the causes of these massive ground collapses to help prevent their future occurrence. The main objective of this study is to investigate the main factor(s) that initiated one of these massive ground collapses. The authors relied on several tools including 3D LiDAR scans for the surface subsidence feature and the underground mine workings around the collapsed area, borescope data at multiple locations around the collapse, S-Pillar analysis for average and unfavorable conditions, and numerical models. The emphasis has been placed on the impact of the following limestone pillar stability parameters: pillar width-to-height (w/h) ratio, benching, the existence of karst features, weak roof, and geological discontinuities. 3D LiDAR scans were utilized to show the shape and the extent of karst cavities observed within stable pillars around the perimeter of the collapsed area. Together, this coupling of field measurements and observations and modeled systems suggests the potential causes of the collapse.

2:28 pm
Prediction of Dynamic Subsidence in the Proximity of Longwall Panel Boundaries – The Influence of the Edge Effect Offset

Z. Agioutantis and J. Romero; University of Kentucky, Lexington, KY, United States

Reliable prediction of dynamic deformations is important when planning to undermine important structures that cannot tolerate large relative deformations and or large horizontal strains. This paper presents examples of dynamic deformation profiles that include subsidence, horizontal displacements, and horizontal strain for a number of points over a retreating longwall panel in the eastern US. The prediction points are located at different distances from the excavation rib and therefore present a different response in terms of dynamic movements. The formulation is based on the influence function method as implemented in the Surface Deformation Prediction System (SDPS). Results show that the subsidence development curve may be reliably predicted for surface points in the middle of the panel. However, in cases where points may be affected by the magnitude of the edge effect offset, subsidence predictions may be very conservative, i.e., higher than actual movements.

Stone Mining Case Studies & Technology Application

Chair: N. Evanek, NIOSH, Pittsburgh, PA

3:20 pm
Utilizing 3D LiDAR to Map a Massive Ground Collapse

R. Clister; Lehigh Hanson, Connellsville, PA, United States, N. Evanek and G. Rashed; NIOSH, Pittsburgh, PA, United States

In October 2020 a massive ground collapse occurred in southwestern Pennsylvania. In order to develop a complete picture of the dimensions of the collapse, 3D LiDAR scanning was utilized on the surface to map the subsidence feature as well as in the underground workings around the collapse. From the underground point clouds, researchers were able to map prominent joint sets and compare them to in mine measurements, as well as monitor changes to the collapse area over time. The underground point clouds were then projected below the surface subsidence feature point clouds. The LiDAR scan data for the surface subsidence feature confirmed that the massive ground collapse initiated due to pillar failure. The use of both underground and surface LiDAR scans in conjunction helped to create a comprehensive three dimensional picture of the massive ground collapse and allowed for a better understanding of the dynamics associated with this event.

3:40 pm
A Case Study of Principle Horizontal Stress Direction Changes at the Subtropolis Mine

T. Miller; East Fairfield Stone Company, North Lima, OH, United States, G. Rashed and N. Evanek; NIOSH, Pittsburgh, PA

The stability of underground excavation in stone mines are greatly affected by the in-situ stress. The high horizontal stress can impact the stability of the roof and the floor stability. The detrimental effect of the high horizontal stress could be roof cutter and floor heave. To avoid these effects, the orientation of the high horizontal stress with respect to the heading/crosscut directions should be considered particularly when the rockmass is weak or the thickness of the cap rock is small. Past research on horizontal stress show that most mines experiencing damaging effects from horizontal stress conclude that the principle stress direction is the same throughout the mine and the region. At the Subtropolis mine in Petersburg, Ohio, the principle horizontal stress direction appears to change as conditions change and differs from the regional horizontal stress, particularly under a previously mined strip mine. The areas of differing principle horizontal stress directions were analyzed using geologic mapping, 3D LiDAR scanning and numerical modeling. 3D LiDAR scans were used to monitor the impact of the high horizontal stress when the cap rock is small (around four feet). 3D numerical models were conducted to investigate the effect of high horizontal stress on roof stability.

4:00 pm
An Examination of the Loyalhanna Limestone’s Structural and Stratigraphic Features and Their Impact on Mining and Ground Control Practices

A. Iannacchione, and T. Anderson; University of Pittsburgh, Pittsburgh, PA, United States

Loyalhanna Limestone, an excellent source of aggregate, crops out along Chestnut Ridge in Fayette and Westmoreland Counties where it is extensively quarried and mined. Although seemingly characterized by rather simple stratigraphy and structures, previous studies have shown that ground conditions are impacted by thrust faults, some with significant dips, local through-going brittle and ductile deformation zones, weathered joints, solution cavities, and prominent stratigraphic cross-bedding features. In places, extensive, localized faulting, observed within underground openings, also may contribute to instability. These stratigraphic and structural complexities, when encountered in underground workings, have resulted in unstable ground conditions that can impact miner safety. The structural character and stratigraphic variations in the vicinity of nine underground mining operations were examined along Chestnut Ridge. The analyses, adjacent to underground mining operations, are based upon data contained in mine permit files maintained by the PA Department of Environmental Protect, Bureau of Deep Mine Safety. Additionally, geospatial techniques have been utilized in order to further examine conditions along the Chestnut Ridge Anticline, especially in the vicinity of structural domes, where extensional features dominate, and saddles, where high horizontal stress features are observed. Along Chestnut Ridge, the elevation of Loyalhanna Limestone can rise from a low of 500-ft below sea-level to over 2700-ft above sea-level. The information has been incorporated into ArcGIS to assist with the analysis and description of prominent structural and stratigraphic features. The authors hope that these examinations will improve understanding the geologic setting of the Loyalhanna Limestone occurring along Chestnut Ridge and further, contribute to development of engineering practices that can be implemented by mine planners, regulators and safety officials with the goal of lowering risks to miners.

4:20 pm
Monitoring Floor and Roof Displacements for Underground Stone Operations

C. Newman; Appalachian Mining & Engineering, Lexington, KY, United States

Injuries and fatalities related to “ground/rock” falls for underground stone mines continue to increase as operations are developed under deeper cover, with steeper dips, and in more geologically complex conditions. Following a series of multiple pillar collapse events occurring in underground limestone operations, MSHA launched the “Pillar Collapse Initiative” in 2019, providing the industry with resources to raise awareness of potential ground instability within “legacy” areas where previous floor benching has occurred. Although a lot of emphasis has been placed on engineering considerations and the design of tall (+60-foot), slender (width-to-height ? 0.8) pillars there has been less focus on the impact of header span for single and multiple level stability.
Through the deployment of a ground movement monitoring system, real time displacement measurements in the immediate roof are used verify potential areas of instability as well as the effect of efforts utilized in mitigating ground movement. This presentation outlines some of the engineering considerations used to identify areas of potential instability as well as long-term monitoring of “active” and “legacy” mining areas.


Wednesday, July 26 2023

Operators Presentations: Experiences and Solutions in Coal Mining

Chair: G. Hasenfus, Barr Engineering Co., Sewickley, PA

8:30 am
Operators, Opportunities and Practical Solutions

G. Hausenfus; Barr Engineering Co., Sewickley, PA, United States

9:00 am
Scale Effects in Coal Mine Ground Control and Implications for Practical Decision Making

M. Gadde, Peabody Energy, St. Louis, MO, United States

9:30 am
Longwall Operations Co-existing with Unconventional Gas Wells

M. Robb; Alliance Coal, LLC, Oakland, MD, United States

10:00 am
Three Fork Creek Crossing – From Geotechnical Investigation to Mining

K. Melvin; Arch Resources, Leer Mine, Philippi, WV, United States

10:30 am
Geological Factors that Impact the Ground Control in Pittsburgh Seam Longwall Mining

J. Lu; CONSOL Energy, Canonsburg, PA, United States

11:00 am
Examples of Complexities in Multiple Seam Mining

P. Worley; Worley & Sons Engineering, Bolt, WV, United States

11:30 am
Ground Control Challenges in Low Seam Northern App Mining

J. Zelanko; Rosebud Mining Co., Kittanning, PA, United States


Thursday, July 27 2023

Mine Case Studies I

Chair: Z. Agioutantis, University of Kentucky, Lexington, KY

8:30 am
Pillar Recovery in the Illinois Basin

C. Mark and T. Gardner; Mine Safety and Health Administration, Pittsburgh, PA, United States

Coal mines in the Illinois Basin have not engaged in pillar recovery for many decades. In large part this is due to the exceptionally weak nature of much of the roof in the region. But, as room-and-pillar mining is conducted at greater depths, and larger pillars become necessary, recovering them becomes more attractive.
Recently a Western Kentucky coal mine became the first in recent times to successfully employ full pillar recovery. This paper describes the many challenges that were overcome during the process. From the beginning, the focus was on protecting the miners from roof instability, using cable bolts, mesh, mobile roof supports (MRS), and engineered final stumps. Two short test panels were extracted first, and the results were used to adjust the support pattern and mining sequence. Further adjustments were made based on subsequent experience. To date, seven panels have been recovered without a ground fall injury.

8:52 am
Geotechnical and Hydrogeological Data Collection for the Mining Industry – Tales from the Field

K. Andrews, S. Stansfield and S. Keim; Marshall Miller & Associates, Blacksburg, VA, United States

Geotechnical, geological, and hydrogeological fieldwork and data collection for the mining industry is often conducted by younger, less-experienced staff. In many cases, fieldwork personnel are recent college graduates or are otherwise relatively new to the mining industry. Due to busy schedules, staffing shortages, project budgets, and in some cases, poor management, new fieldwork hires often do not receive thorough training. In the past and the present, many workers in the mining industry learn(ed) by “doing”. While this is still considered to be the most practical approach and the easiest way for a company to determine the skill set of an employee, there is always some risk involved in the “feet to the fire” method. The reality is that field personnel are often collecting data upon which major operational decisions will be made or upon which regulators will base enforcement decisions. Inadequately trained or inexperienced field personnel may collect inaccurate data or fail to recognize key conditions that may influence decisions made by regulators, design engineers, mine managers, and corporate leaders. Even the best engineers may come to incorrect conclusions if they are starting with poor input data. Decisions based on inaccurate data often result in delays, budget issues, unsafe conditions, or even worse consequences. This paper provides numerous mining industry examples of incidences where inaccurately collected field information did, or could have, resulted in major adverse consequences. The paper also offers some simple approaches to minimizing the potential for such problems.

9:14 am
Field Instrumentation and Data Analysis of Ground Movement and Pillar Performance at the Maple Eagle Mine in Southern West Virginia

M. Sears, C. Compton, M. Mazzella and T. Minoski; NIOSH, Pittsburgh, PA, United States, M. Morris and J. Bright; Blackhawk Mining, LLC, South Charleston, WV, United States

To promote the safety and health of U.S. mineworkers, researchers with the National Institute for Occupational Safety and Health (NIOSH) are currently involved in gathering additional field data from the Nation’s underground coal mines. The purpose of this field study was to monitor ground movement and changes in pressure during mining where conditions include a thick parting in the coal seam. To accomplish this, three arrays of field instrumentation were installed in the wrap-around bleeder of a room-and-pillar panel at the Maple Eagle Mine located in Southern West Virginia. Borehole pressure cells (BPCs) were used to monitor the change in pressure in pillars located adjacent to the gob. Roof extensometers were installed to measure roof sag at various horizons in the immediate roof, while rib extensometers were installed to measure displacement of the in-seam parting as mining progressed. Additionally, visual observations and manual measurements were taken during subsequent site visits. The repeatability of results, important from a scientific perspective, was accomplished by successfully obtaining field measurements from two nearly identical instrumentation sites. The data collected indicated that the front abutment extent at the edge of the panel was approximately 200 ft, corresponding to a distance of 9–10 times the square root of the depth. Yielding of the slabbed pillar began when the pillar line was approximately 290 ft. outby the instrumentation sites. This was followed by load shedding onto the abutments and the measurement of small amounts of roof sag in the #1 entry. Instrumentation data, as presented in this paper, can be used to better understand stress redistribution, pillar performance, and roof/rib displacements. Additionally, this data is suitable for assisting in the calibration of numerical models. Insight gained from applying these models, can be used to make better engineering-based judgments for conditions which could potentially be encountered in the future.

9:36 pm
Automation of Roof Bolting in Underground Coal Mines through Electro-Hydraulic Control

P. Willaman ,Franklin, PA, United States, K. Saeler; Komatsu, Franklin, PA, United States, J. Leeming; Komatsu, Warrendale, PA, United States

Utilizing automation is one way to improve competitiveness. Roof and rib bolting in underground coal mines has not changed significantly for over forty years. Most roof bolting is still a largely manual process involving high levels of manpower carrying out repetitive work. Roof bolting statistically has one of the highest levels of underground accidents, including finger, hand, and arm injuries. Operators are exposed to falling objects and becoming entangled in roof bolting apparatus while manhandling awkward six-foot drill steels and roof bolts. It is estimated that over 25 million roof bolts are installed annually in the US underground coal market with well over 1,500 operators involved in roof bolting on a day-to-day basis. Recently electro-hydraulic roof bolting rigs have been installed to offer partial automation of the drilling cycle and the bolting cycle. The next stage is to provide a storage and loading system to allow full automation of full column resin roof bolts. This paper will review the latest electro-hydraulic roof bolting equipment and discuss the testing of a fully automated drill rig. Full electronic control allows each step of the roof bolting process to be recorded providing a digital record of each installation, including hole depth, spin time, gel time, torque, and cycle time. Other benefits include monitoring of water pressure and flow, used to prevent blocked drill steels as well as digital fault finding. A fully automated roof bolting machine greatly improves operator safety and gives high quality repeatable roof bolting with reduced cycle times.

Tools for Monitoring of Deterioration and Subsidence

Chair: S. Baker, Rosebud Mining Co., Kittanning, PA

10:30 am
Empirical Correlation of Geotechnical and Hydrogeological Characteristics of Overburden Rock in Stream Valleys Overlying Central Appalachian Underground Coal Mines

S. Ghaychi Afrouz, Blacksburg, VA, United States, K. Andrews; Marshall Miller & Associates, Blacksburg, VA, United States

Ground control analysis and groundwater inflow assessments for underground coal mines require in-depth field investigations including, but not limited to, geological and geotechnical core logging, downhole geophysical logging, laboratory rock strength testing, and packer (Lugeon) testing. Field and laboratory data collection are time consuming and expensive, and an initial understanding of the expected trends amongst various geotechnical and hydrogeological parameters can assist with the planning and optimization of geotechnical characterization studies and grouting plans. In this study, a comprehensive database is analyzed, comprised of important rock mass characteristics (hydraulic conductivity, Rock Quality Designation (RQD), and lithology) for coal-bearing rocks in the Central Appalachian Basin. Statistical analysis of the data is completed to establish an enhanced understanding of geotechnical and hydrogeological conditions expected to be encountered as underground coal mining passes near and beneath streams and rivers. The results of the study provide valuable reference material for the coal mining industry for completion of shaft installations, mine stream crossings, environmental assessments, and grouting and mine dewatering designs.

10:52 pm
Highwall Stabilization and Ground Control – Underground Mine Design

C. McCannon; Respec, Lexington, KY, United States

Carolyn McCannon (RESPEC) will present on highwall stabilization options for beginning underground mines from a highwall. The presentation will focus on highwall hazards and stabilization options, implementation and construction of cost-effective highwall stability measures, and design of a final highwall to minimize needs for highwall stability reinforcement materials.

11:14 am
Use of Structure from Motion Photogrammetry for Monitoring of Underground Storage Facilities

M. Jan Michael Gurgel and A. Preusse; Institute for Mine Surveying, Mining Subsidence Engineering and Geophysics in Mining, RWTH Aachen University, Aachen, North Rhine-Westphalia, Germany

The planning, construction, and monitoring of underground storage facilities is a key factor in meeting upcoming challenges as part of the energy transition from fossil fuels to renewables. Therefore, the many years of knowledge and expertise in ground control gained from coal mining are highly valuable for new mining and storage operations. For such applications, our previous work examined cavity registration combining structure from motion and lidar through a case study in a former lead ore mine. Building on this, the results of this recent case study are discussed regarding the possible application in sites for nuclear waste disposal in Germany including a view on the situation in the United States. Photogrammetric methods are particularly suitable as part of long-term monitoring, which relies on both precise survey results and a large variety of detailed visualization. In order to gain a better understanding of the resulting application potential for holistic monitoring, both existing repositories sites in Germany and ones yet to be found are examined. This search and selection process for high-level nuclear waste disposal is studied in detail together with rules resulting from the (German) mining law, regulations, and other guidelines. Hereby, key requirements of photogrammetric products like oriented images, point clouds, and 3- D models are identified for different case studies. The findings on accuracy and integrity of the generated models are not only of interest for future repository sites or underground pumped storage power plants, but also offers various possibilities for underground mining of coal and other commodities.

11:36 am
Lineament Analysis of Shuttle Radar Topography Mission (SRTM) for Fracture System Identification

S. Phillipson, Triadelphia, WV, United States

The Mine Safety and Health Administration’s Roof Control Division evaluated the use of Shuttle Radar Topography Mission data to identify fracture traces on the Earth’s surface. Results were compared to the analysis method employed by the Division for 30 years, which relies on LANDSAT-TM Band 4. Shuttle Radar Topography Mission elevation models are expected to provide a direct representation of bedrock surface, while LANDSAT-TM Band 4 represents reflected near-infrared radiation as an indication of moisture content and is only a proxy for possible structures. Three areas in southwestern Pennsylvania were chosen for ground-truthing lineaments derived from both methods, following a series of desktop comparisons in six areas of the contiguous U.S. Field studies identified numerous instances in which SRTM lineaments corresponded with joints at the projected location, and were oriented within 10 degrees of joint strike, indicating that those SRTM lineaments represented the structural grain of the area. In some cases, lineaments were associated with individual joint zones. Photo-lineaments derived from the existing LANDSAT-TM methodology appear to offer a higher coincidence with expected trends of joints on a regional scale when performing the desktop comparisons, while field observations indicated that SRTM-derived lineaments had a higher association with individual geologic structures in the case study field area. It appears that both methods can identify the structural grain of an area, and that both tend to identify linear segments of valleys where erosion has preferentially occurred along joint zones or other structures.

Mine Case Studies II

Chair: K. Mohamed, NIOSH, Bridgeville, PA

1:30 pm
A Novel Hybrid Cementitious Material for Pumpable Roof Supports: From Laboratory-Scale Investigation to Full-Scale Production

A. Nikvar Hassani, Rochester, MN, United States, L. Zhang; University of Arizona, Tucson, AZ, United States

To address the drawbacks of the conventional Portland cement/fly ash (PC/FA) based cementitious material currently used in practice to construct pumpable roof supports (PRSs), a novel cementitious material has been developed based on the geopolymerization technology. The novel cementitious material is comprised of two pumpable grout streams: stream 1 is a slurry composed of pozzolanic material and high calcium additive and stream 2 is an alkaline solution comprised of a gelling agent. Biopolymer has been added to the second stream to provide ductility to the geopolymer cementitious material. In the laboratory scale, systematic experiments have been carried out to investigate the viscosity, setting time, and uniaxial compressive strength (UCS) of the new cementitious material and develop the right recipe for field applications. By collaborating with industry and NIOSH, full-scale PRSs have been constructed and tested. The results show that (i) the novel cementitious material based PRSs has much higher peak UCS and similar or higher residual strength than the conventional PC/FA based PRSs, and (ii) unlike the PC/FA based cementitious material, the novel cementitious material shows good long-term durability with no-cracking/disintegration when exposed to air.

1:52 pm
Technical System of Intelligent Ground Control in Close-Multiple Coal Seams

Y. Li, N. Wang and J. Wang; China University of Mining and Technology Beijing, Beijing, Beijing, China (Mainland)

The close-multiple coal seam is an important component of China's coal resources and plays a key role in total coal reserves. However, the overburden movement in the mining process of close-multiple coal seams is usually extensive, which is seriously affected by overmining. It is therefore essential to study the mechanism of roof breakage, the relationship between the shield and the surrounding rock, and the mechanism of overburden movement to ensure the safe operation and the efficient exploitation of close-multiple coal seams. This work first proposes a technical route for intelligent ground control. Secondly, the principle of cooperative control between the shield and overburden roof is revealed and the shield support capacity calculation method is summarized. Thirdly, the quantitative criterion for disturbance degree of key stratum (KSDD) was proposed based on the key strata theory. Finally, several intelligent evaluation and analysis systems have been developed in terms of overburden breakage and shield support capacity, multi-information analysis of ground control, and disturbance degree of overburden. The research results may provide an essential reference for the development of intelligent ground control in the mining process of close-multiple coal seams.

2:14 pm
Effect of Casing Cementing Alternative on Shale Gas Well Casing Deformations Induced by Longwall Mining

P. Zhang and D. Su; NIOSH, Pittsburgh Mining Research Division, Pittsburgh, PA, United States, B. Hyun Kim; NIOSH, Spokane Mining Research Division, Spokane, WA, United States

Shale gas wells drilled in longwall chain pillars are influenced by longwall mining. Longwall mining on each side of the chain pillars can create overburden movements and induce deformations in gas well casings. If the deformations result in a casing breach, intrusive shale gas could leak into the longwall mine with serious safety consequences. This study concerns five shale gas wells in the chain pillars between two adjacent longwall panels in the Pittsburgh coal seam under a cover depth of 670 ft and a mining height of 7 ft. The gas wells were constructed with typical casing design, but a casing cementing alternative with uncemented production casings was practiced. Four production casings were uncemented, and one production casing was unintentionally cemented with good quality cement to a 535-ft depth and low-quality cement stringers from a 444-ft to a 535-ft depth. The longwall faces passed by the shale gas wells as each adjacent longwall panel retreated. Production casings were surveyed by a multi-finger caliper after each panel was mined. No deformation was detected in the four uncemented production casings, but small deformations were detected in the partially cemented production casing. The maximum measured deformation in the cemented production casing was 0.05 in after first panel mining and 0.06 in after second panel mining. The casing deformations were also predicted by the FLAC3D model set up on site-specific overburden geology. The predicted deformations were in good agreement with the measurements in terms of deformation locations and magnitudes. All five shale gas wells resumed normal production after being temporarily plugged during the period of mine-by. This study provides a successful demonstration of the effectiveness of a casing cementing alternative for decoupling production and intermediate casing, and thus minimizing the influence of longwall mining in the shale gas wells drilled through longwall chain pillars.

2:36 pm
Design Methodology of Pumpable J-Crib Stopping Wall and Jennmar High-Strength Stopping Panel Wall

X. Li; Keystone Mining Services, LLC, Pittsburgh, PA, United States, B. Mirabile; Jennmar Corporaton, Pittsburgh, PA, United States, R. Adasiak; Jennchem LLC, Pittsburgh, PA, United States

Pumpable J-Crib stopping walls and Jennmar high-strength (HS) stopping panel walls are widely used for resisting high ventilation air pressures and roof support in underground mines due to their installation efficiency, convergence allowance, and better performance than traditional concrete block stopping walls. Based on the different entry dimensions and required maximum ventilation air pressures of up to 36-inch water gauge, 24”-, 27”-, 30”-, and 36”-diameter Pumpable J-Crib stopping walls, or 18”- 30” thick Jennmar HS stopping panel walls can be designed, and their support capacities and convergence allowances are evaluated for better roof control. Pumpable J-Crib stopping walls consist of 0.187”-diameter steel wire reinforced, J-Crib material filled bags, and non-wire-reinforced filler bags in-between. Jennmar HS stopping panel walls consist of two, 20 gauge (0.04”-thick), galvanized, cold-formed steel panels with J-Seal materials pumped in-between. Based on the fundamental structural design principles and specifications of ACI 318- 19(22), the compression and flexural strength, and shear resistance of the walls can be checked to be compliant with the codes and specifications following the Load and Resistance Factor Design (LRFD) method. Specifically, the 0.187”-diameter steel wires in the Pumpable J-Crib walls and 20 gauge steel panels in the HS stopping walls are integral parts of the walls, and are considered to effectively confine the J-Crib or J-Seal materials. These steel elements play a significant role in reinforcing and improving the strength or support capacity of the stopping walls from a structural design perspective. Due to the high ductility of low- density J-Crib/J-Seal materials and steel elements, these walls are good at accommodating or absorbing the roof convergence (? 5% entry height), which can’t be matched by a traditional concrete block stopping wall. Based on the successful design and application of these stopping walls in different mines for improving roof stability and avoiding the air leakage, the authors have formulated the design methodology and verified independently by the Finite Element Analysis (FEA) simulation technique. This design methodology combines the fundamental structural design principles with specific wall dimensions and material properties, facilitating mine engineers and government agencies to understand the in-depth working mechanism of these stopping walls.

Numerical Modeling Applications in Ground Control II

Chair: M. Sears; NIOSH, Pittsburgh, PA

3:30 pm
Gateroad Layout for Deep Cover, Dipping Longwall Recovery: A Case Study in the Western United States

C. Newman and D. Newman; Appalachian Mining and Engineering, Inc., Lexington, KY, United States

The ability to assess, address, and predict mining induced stress distribution is paramount in maintaining a safe and efficient underground operation as coal mines continue to develop at deeper depths and in more geologically complex reserves. This is especially true for longwall mining operations given their limited flexibility in panel layout and orientation. This paper focuses on ground behaviors and conditions observed at two deep cover, steeply dipping longwall mining operations in Utah’s Unita coal basin. This region is susceptible to coal pillar bump/bounce events as a result of high depth of cover, high extraction mining, and the close proximity of a thick massive stiff sandstone stratum to active mine works. The successful prediction of mining induced coal bump events for Mine A and Mine B operating in the Unita basin has prompted mine management to develop a rock mechanics testing database for reserve calculations. The database is applicable to mine planning as well as the development of multiple numerical models to proactively identify and address adverse ground conditions using input parameters calibrated to site specific behaviors and conditions. For both Mine A and Mine B, the most critical question relates to longwall development and recovery for “extreme” deep cover longwall panels at overburden depths greater than 600- meters. This paper highlights the impact of high stress concentrations associated with deep cover longwall mining as well as the methods applied to mitigate stress impacts on miner safety and longwall production.

3:52 pm
Stress Distribution in Pillars as a Function of Opening and Pillar Geometry and Material Properties

Z. Wedding and Z. Agioutantis; University of Kentucky, Lexington, KY, United States

Excavation of underground openings results in the redistribution of the insitu stress into the rock adjacent to the opening. Stress redistribution depends on opening geometry, pillar geometry, material properties, possible confinement and constitutive models describing the behavior of adjacent rock. This paper will present typical pillar stress distributions for elastic and yielding pillars as a function of opening geometry, material properties and material constitutive models and will highlight the differences between them. Stress distributions will be compared with empirical strength profiles that have been developed for coal pillars as well as actual pillar geometries documented for stone pillars.

4:14 pm
Surface and Highway Subsidence Characteristics due to Longwall Mining Based on a Long-term Field Monitoring

G. Zhao; West Virginia University , Morgantown, WV, United States, W. Guo, J. Li, X. Li and L. Li; Henan Polytechnic University, Jiaozuo, Henan, China (Mainland)

Surface subsidence caused by underground longwall mining is a very complicated process in time and space, influencing building/structures on the surface of the mining area, including houses, piplines, highway, and so on. The field monitoring data of surface subsidence are essential for protecting them. This study investigated the dynamic and residual subsidence characteristics of both the surface and the highway over a 730-day monitoring period on a longwall panel. Three monitoring lines (1-3) were established to monitor the subsidence process, and three stages were identified: the initial stage of surface subsidence, the active period of surface subsidence, and the residual subsidence deformation stage. The maximum subsidence value on the highway (1786 mm) was smaller than that on the surface (2094 mm), and the maximum subsidence velocity on the highway (41.7-50.8 mm/day) was also lower than that on the surface (66.7 mm/day). During the residual subsidence deformation stage, both the surface and highway subsidence velocities decreased to below 0.6 mm/day. Furthermore, differences in subsidence characteristics between the surface and the highway were analyzed, with the subsidence value on the highway always being less than that on the surface, while the subsidence velocity during the residual deformation stage was not always lower. This could be attributed to the influence of dynamic loads from vehicles, which could induce additional residual subsidence on the highway. Further investigation into this phenomenon will be conducted in future research.