Technical Sessions

The 2021 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 in this 2-day virtual event.

*All times listed are Eastern Daylight Time (+3 hours Pacific Time).

 

Tuesday, July 27 | 1:00 - 4:45 pm ET

 

Bumps, Design, Support

1:00 – 2:45 pm ET
Moderator: Brijes Mishra, West Virginia University, Morgantown, WV
MC: Michael Murphy, NIOSH, Pittsburgh, PA

1:00 - 1:15 pm ET
Investigation of Coal Burst Potential Using a Finite Element Method Approach

Zach Agioutantis, University of Kentucky, Lexington, KY; Cristian Cardenas Triana, University of Kentucky, Lexington, Kentucky, United States; Jesus Castillo, University of Kentucky, Lexington, Kentucky, United States

Coal bursts – a form of rock bursts - are typically characterized by a rapid failure of coal and rock around an underground coal entry or crosscut. Rock bursts have been related to underground mining for more than one hundred years in different regions of the US. They can be typically controlled by changing the stress regime and by mitigating the accumulation process of strain energy. In recent years, different researchers have developed several indicators to predict the rock burst tendency in underground mines, such as the strain energy storage index, the strain energy density, and the burst potential index.

Due to the complexity of such events, numerical modeling (which is capable of solving complex boundary value problems) is typically needed to determine the burst potential and damage locations in underground openings. The present work estimates the burst tendency of coal pillars in a longwall tailgate system, through the use of a combination of rock burst potential indices coupled with numerical modeling. This was accomplished in two phases. The first phase considers the material characterization and the calculation of rock burst potential indices for the particular layout using data available in the literature. The second phase consists of a finite element modeling approach to evaluate the strain energy storage rate change for two mining stages. This was accomplished by conducting a parametric analysis varying the pillar loading conditions and material properties to estimate the burst tendency through the calculation of the burst potential index.

The present work presents a first order approximation that demonstrates that if strain is allowed to build up in a pillar of a specific geometry then the burst potential increases unless steps are taken to mitigate the strain build-up. The calculated burst potential index was above 100% for a gateroad pillar that actually burst in front of a longwall face.

1:15 - 1:30 pm ET
A Study of a Limestone Pillar Stability in Multilevel Mining Conditions Using Numerical Models and In- Situ Monitoring

Gamal Rashed, NIOSH, Pittsburgh, Pennsylvania, United States; Brent Slaker, NIOSH, Pittsburgh, Pennsylvania, United States; Michael Murphy, NIOSH, Pittsburgh, Pennsylvania, United States

Pillar stability continues to be a significant concern in multiple-level mining conditions particularly for deep mines when pillars are not stacked or the thickness of interburden between mining levels is thin. The National Institute for Occupational Safety and Health (NIOSH) is currently conducting research to investigate the stability of pillars in multiple-level stone mines. In this study, FLAC3D models were created to investigate the effect of interburden thickens, the degree of pillar offset between mining levels, and in-situ stress conditions on pillar stability at various depths of cover. The FLAC3D models were validated through in-situ monitoring that was conducted at two multiple-level stone mines. The critical interburden thickness required to minimize the interaction between the mining levels on top-level pillar stability was explored. The model results showed that there is an interaction between numerous factors that control the stability of pillars in multiple-level conditions. A combination of these factors may lead to various degrees of pillar instabilities. The highest degree  of local  pillar instability occurred when pillar overlap ranges between 20 and 60 %. On contrary, the highest degree of stability occurred when the pillars are stacked. Generally, for depths of cover investigated in this study, the stability of pillars shallower than a 100 m (~ 330 ft) or with interburden thicknesses greater than 16 m (~ 53 ft) do not appear significantly impacted by pillar offset. The results of this study improve understanding of multilevel interactions and advances the ultimate goal of reducing the risk of pillar instability in underground stone mines.

1:30 - 1:45 pm ET
Injectional Anchorage System for PC- Strand Cable Bolts in Underground Coal Mine

Steve Tadolini, Minova Americas, Lakewood, Colorado, United States; Cody Hildreth, Minova
USA, Georgetown, Kentucky, United States; Colton Cook, Arch Coal, Inc., Sharples, West Virginia, United States

High capacity PC-strand cable bolts have been anchored with traditional polyester resins for underground support applications since introduced in 1994. These systems, extremely effective in controlling roofs that are subjected to large amounts of uncontrollable convergence, are installed in gateroads, intersections, bleeder entries and belt lines. When 3 entries were being driven from the Alma Seam to the 2 Gas Seam, approximately 80 ft of interburden, unique ground conditions presented themselves which required a distinctive cable bolt application in the brow areas created during the development of the ramps. The immediate roof, consisting of interbedded shales, was overlain by a massive sandstone layer – which was perfect for developing adequate cable anchorage. The key to supporting these brow areas for the life of mine, was to stiffen the immediate roof with primary bolts and fully grouted cable support systems. The cables were post-tensioned to minimize downward roof deflections and maintain massive roof elements. Cables post injected with Carbothix, a two- component resin, was completed with a recently developed PEX injection system to ensure top-to-bottom injection ensuring full column anchorage. This paper describes the unique ground control requirements and innovative solutions that were implemented to support and evaluate these critical entries.

1:45 - 2:00 pm ET
Investigating the Factors Affecting the Stability of Coal Ribs with In- Seam Rock Parting Through Numerical Simulations

Yuting Xue, NIOSH, Pittsburgh, Pennsylvania, United States; Khaled Mohamed, NIOSH, Pittsburgh, Pennsylvania, United States

Researchers from the National Institute for Occupational Safety and Health (NIOSH) have been working on the development of an engineering-based rib control method through the developed coal mass numerical model and strength reduction technique. The Coal Pillar Rib Rating (CPRR) was originally introduced to characterize the quality of solid coal ribs. In this study, the CPRR technique is updated to address coal ribs with in-seam rock partings. The rib factor of safety is a key parameter in calculating the CPRR. Therefore, a parametric study was conducted to determine the factor of safety of ribs with an in-seam rock parting under different mining scenarios and rib conditions. Statistical analysis was carried out on 1,842 cases to investigate the influence of various factors on the stability of coal ribs with in-seam rock partings. The results show that overburden depth, bedding condition, minimum coal strength, coal unit thickness, and parting strength have observable influence on rib stability, while the parting thickness and location were found to have negligible influence on rib stability. The statistical analysis of the data can be helpful for understanding the logic behind the CPRR technique.

2:00 - 2:15 pm ET
Assessment of Floor Heaves Associated with Bumps in a Longwall Mine Using the Discrete Element Method

Bo-Hyun Kim, CDC/NIOSH/SMRD/MSB, Spokane, Washington, United States; Mark Larson, CDC/NIOSH/SMRD/MSB, Spokane, Washington, United States

This study was developed as part of an effort by the National Institute for Occupational Safety and Health (NIOSH) to better understand rock-mass behavior in longwall coal mines in highly stressed, bump-prone ground. The floor-heave and no-floor-heave phenomena at a western U.S. coal mine cannot be properly simulated in numerical models using conventional shear-dominant failure criteria (i.e., Mohr-Coulomb or Hoek-Brown failure criterion). Kim and Larson (2019) demonstrated these phenomena using a user-defined model of the s-shaped brittle failure criterion in conjunction with a spalling process in FLAC3D. The results of the FLAC3D modeling agreed with the observations of the relative amounts of heave from each gate-road system. However, the FLAC3D model adopted many assumptions and simplifications that were not very realistic from a physical or mechanical perspective. In order to overcome the limitations of the FLAC3D model, 3DEC modeling in conjunction with the Discrete Fracture Network (DFN) technique was performed to better understand the true behavior of floor heave associated with underground mining in an anisotropic stress field. The effect of stress rotation in the mining-induced stress field was considered by using a different geometry of rock fractures in the coal seam. The heterogeneity of the engineering properties (i.e., cohesion and tensile strength) were also considered by using Monte Carlo simulations. Consequently, the 3DEC models using the DFN technique resulted in predictions of floor heave that agreed with observations of the relative amounts of heave from each gate-road system, but the cause of heave was mainly related to the degree of anisotropy instead of the size of the pillar.

 

2:15 - 2:45 pm ET
Q&A/Break

 

Design Software

2:45 – 4:45 pm ET
Moderator: Brijes Mishra, West Virginia University, Morgantown, WV
MC: Michael Murphy, NIOSH, Pittsburgh, PA

2:45 - 3:00 pm ET
Analyzing Rib Stability and Support Using A Coal Pillar Rib Rating

Khaled M. Mohamed, NIOSH, Pittsburgh Mining Research Division, Pittsburgh, Pennsylvania, United States; Yuting Xue, NISOH, Pittsburgh, Pennsylvania, United States; Gamal Rashed, NISOH, Pittsburgh, Pennsylvania, United States; Robert Kimutis, NISOH, Pittsburgh, Pennsylvania, United States

Researchers from the National Institute for Occupational Safety and Health (NIOSH) are developing a methodology for assessing the stability of coal pillar ribs. The developed methodology is a hybrid numerical-empirical technique that provides ground control engineers with more insight into primary rib support in underground coal mines. The Coal Pillar Rib Rating (CPRR), Primary Rib Support Density (PRSD), and overburden depth are the key elements in the developed rib assessment methodology. The CPRR was developed based on numerical models of different rib compositions. The Unsupported Rib Factor of Safety (RibFOS) for development loading of mining is defined using the CPRR and overburden depth. A database of 118 coal pillar rib cases was collected by NIOSH from active underground coal mines in the United States, MSHA rib fatality reports, and published mining reports in Australia. It was found that all ribs in the longwall mines database are supported. A RibFOS threshold of 1.5 could be used to delineate the boundary between supported and unsupported ribs in room-and-pillar mines. Relationships between the applied PRSD and calculated RibFOS for rib cases were developed. Currently, additional research is underway with the goal of investigating the effects of rock brows on rib stability and support density.

CPRR calculations were conducted for 99 surveyed coal ribs from 20 underground coal mines. A preliminary minimum primary rib support density (PRSD) line was obtained from these surveyed cases. Based on this study, the rib performance was classified into four performance categories; (1) supported ribs with spalling, (2) supported ribs without spalling, (3) unsupported ribs with spalling, and (4) unsupported ribs without spalling. The minimum PRSD and the range of RibFOS for each group was defined.

3:00 - 3:15 pm ET
Development of the Analysis of Mine Roof Support (AMRS) Design Methodology

Christopher Mark, Mine Safety and Health Administration, PITTSBURGH, Pennsylvania, United States; Zach Agioutantis, University of Kentucky, Lexington, KY, United States; Ryan Stephan, Mine Health and Safety Administration, Pittsburgh, PA

More effective roof support systems have dramatically reduced roof fall rates in U.S. coal mines over the past 15 years. The design methodology developed in this paper builds on and quantifies this success. 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. Statistical analysis of a large database of roof fall histories at a number of mines helped to define the approximate boundaries of these three regimes. A new computer package, Analysis of Mine Roof Support (AMRS), implements the design methodology.

3:15 - 3:30 pm ET
Evaluation of Standing Supports for Longwall Tailgate Entry Using NIOSH Support Technology Optimization Program (STOP)

Timothy Batchler, NIOSH, Pittsburgh, Pennsylvania, United States; Timothy Matthews, NIOSH, Pittsburgh, Pennsylvania, United States; Ihsan Berk Tulu, West Virginia University, Morgantown, West Virginia, United States; Haochen Zhao, West Virginia University, Morgantown, West Virginia, 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. In 2020, the National Institute for Occupational Safety and Health (NIOSH) updated the Support Technology Optimization Program (STOP) in order to better evaluate these secondary support systems by providing mine operators with a simple and practical tool to make engineering decisions about the selection and placement of various secondary support systems. This update implements important changes to STOP’s user interface and functionality and significantly enhances the use of ground reaction curve design criteria. The paper provides examples of using the 2020 version of STOP to evaluate a current standing support system followed by using the ground reaction curve design criteria at this case study mine.

3:30 - 3:45 pm ET
Introduction of Hard Rock Influence Factor (HRIF) into Abutment Load Estimation for Underground Coal Mines

Deniz Tuncay, West Virginia University, Morgantown, West Virginia, United States; Ihsan Berk Tulu, West Virginia University, Morgantown, West Virginia, United States; Haochen Zhao, West Virginia University, Morgantown, West Virginia, United States

Mining-induced stresses in underground coal mines play a significant role in pillar and support design, hence in the safety of mining operations. Adequate design of pillars and roof control plans rely highly on the accurate assessment of the mining-induced loads, as well as the load-bearing capacities of the supports. In commonly used pillar design tools for underground coal mines, overburden loading is estimated by simple geometric rules such as the tributary area theory and the abutment angle concept. These methods overlook important mechanical responses such as specific overburden mechanics, structural competence of the overburden strata, geology, in-situ stresses, and overburden/pillar interactions. The site-specific overburden geology is one of the major influencing factors for these responses. To include the effect of geology in the load estimations, a new parameter called Hard Rock Influence Factor (HRIF) that will represent the strength and stiffness of the overburden is proposed. This parameter is a function of the strength of the overburden layers, thicknesses of strong beds, relative locations of the strong beds in the overburden, and also the panel width and overburden depth. 2D numerical models of 13 case studies from 12 different U.S. longwall mines with different overburden geologies are used in this study. The 2D models have been verified against field measurements such as surface subsidence, and stress measurements and these models are used to estimate the side abutment loads and gob loads. Using these results, regression analysis is conducted for an abutment loading model with the HRIF as a variable, and successful results have been achieved.

3:45 - 4:00 pm ET
Development of Time- Dependent Rock- Fluid- Geomechanics Model and Software Suites: Application for Time- Dependent Shaly Roof Stability Analysis

Shimin Liu, The Pennsylvania State University, state college, Pennsylvania, United States; Ang Liu, The Pennsylvania State University, state college, Pennsylvania, United States

Roof instability is a pervasive hazard that may cause injuries and fatalities in underground coal mines. Variations in air humidity and the duration of roof exposure are known to significantly influence the stability of shale roof strata. Moisture-sensitive shales progressively deteriorate over time due to humidity-interaction and matrix swelling, which can trigger skin failure and roof falls. Quantifying the evolutions of dynamic mechanical and petrophysical properties of shale roof requires a comprehensive understanding of the retention behavior of multiphase fluids in shale and the associated air-water-shale/coal interactions and induced alterations. This study presents an approach to systematically develop a mechanism-based framework to model the deformation and failure behaviors of shale roof and coal pillars by considering the coupled effects of water vapor flow, elastic deformation and damage. The Time-Dependent Rock-Fluid-Geomechanics Model (TD-RFG Model) was developed to evaluate time-dependent rock structure dynamics. The interactions between elastic deformation, transient fluid transport and discontinuous damage are analyzed and modeled through TD-RFG. The established TD-RFG platform provides a pathway for multi-field and time-dependent rock structure stability assessment. A case study was reported for shaly roof stability analysis in the Illinois Basin. The time- and location- dependent damage area propagates to create connected channels for moisture intake. Thus, the damage-induced increases in permeabilities will facilitate the moisture transport process and the seasonal relative humidity variations in mine opening will be quickly reflected at the interior locations in shale roof, coal pillar and ground floor. These results illustrate a novel approach for quantifying and evaluating the water retention behavior and suction potential in unsaturated air-water-shale/coal system that can be used to define strategies for ground control.

4:00 - 4:15 pm ET
Concept of an Integrated Capture and Evaluation Approach for Cavities Supporting Underground Knowledge

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

The present state of underground utilization is dominated by fundamental transformation processes. Besides conventional mining, new applications like underground pumped storage power plants (UPSPP) or the final storage of highly radioactive waste may emerge in the future. The previous contribution highlighted opportunities and challenges with regard to subsurface spatial planning and to associated ground control issues.

Based on the outlined possible use of structure from motion and photogrammetry, this paper presents an integrated capture and evaluation approach for underground cavities in detail. The combination of different existing (open source) software libraries for the purpose of aligning unstructured image sets and subsequent processing steps is evaluated with special emphasis. Such an approach is meant to address the fact that profound knowledge of underground space is necessary for future exploration and monitoring. This method should simultaneously enable tasks of surveying, monitoring, and specifically visualization. There may be significant synergies resulting from innovative evaluation algorithms in conjunction with more powerful computing hardware and high-resolution camera sensors performing well in low lighting conditions.

Image-based visualization methods are particularly suitable for ensuring transparency when conducting large- scale underground projects. For some years now, it has become increasingly important to obtain and maintain the so-called social license to operate. This is notably true for projects that attract considerable public attention and are the focus of distinct debate. The presented approach is intended to provide better understanding of planned underground activities in general. In this way, these tasks fit into ongoing technical and social challenges of ground control.

4:15 - 4:45 pm ET
Q&A

 

Wednesday, July, 28 | 1:00 – 4:30 pm ET

 

Gas-Geotechnical

1:00 - 2:45 pm ET
Moderators: Heather Lawson, NIOSH Spokane Research Lab Library NIOSH Spokane Research Lab Library, Spokane, WA
Kyle Perry, Missouri University of Science & Technology, Rolla, MO
MC: Ted Klemetti, NIOSH, Pittsburgh, PA

 

1:00 - 1:15 pm ET
Acquisition and Interpretation of Critical Scientific Data for Shale Gas Wells Influenced By Longwall Mining

Daniel Su, CDC/NIOSH/PMRD, Jefferson Hill, Pennsylvania, United States; Peter Zhang, CDC/NIOSH/PMRD, Jefferson Hill, Pennsylvania, United States; Heather Dougherty, CDC/NIOSH/PMRD, Jefferson Hill, Pennsylvania, United States; Mark Van Dyke, CDC/NIOSH/PMRD, Jefferson Hill, Pennsylvania, United States

This paper presents the summary of critical scientific data acquisition and interpretation by NIOSH regarding the stability of unconventional shale gas wells influenced by longwall mining. Prior to the NIOSH gas well stability research, there was only one such coordinated research initiative, which was carried out by the coal and gas industries in 2013 and 2014. Currently, the National Institute for Occupational Safety and Health (NIOSH) is the only organization in the United States conducting such fundamental and impact-oriented research. Results from NIOSH’s instrumentation and modeling efforts indicate that under shallow and medium covers, the measured horizontal displacements within the abutment pillar are one order of magnitude higher than those measured under deep cover. Effects of casing couplers are evaluated which indicate that, although they have little effect on longwall-induced casing stresses, the couplers may aggravate longwall-induced casing deformation if the longwall-induced deformations coincide with the coupler locations. Cementing alternatives are found to have significant impact on longwall-induced casing stresses and deformations. Casing deformations measured by 60-arm caliper logging are in good agreement with numerical modeling predictions. Preliminary engineering guidelines are proposed on longwall-induced deformations, casing and cementing alternatives, gas well setback distances under chain pillar and barrier pillar scenarios, as well as risk assessment strategies.

1:15 - 1:30 pm ET
Geomechanical Modeling of Mining-Induced Permeability: Implications for Potential Gas Inflow From a Sheared Gas Well

Zoheir Khademian, National Institute for Occupational Safety and Health (NIOSH), Pennsylvania, United States; Kayode M. Ajayi, National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania, United States; Daniel W. H. Su, National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania, United States; Gabriel S. Esterhuizen, National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania, United States; Steven J. Schatzel, National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania, United States

Stability and integrity of unconventional shale gas wells drilled through current and future coal reserves can be compromised by ground deformations induced by nearby longwall mining operations. Due to the enhancement of the surrounding rockmass permeability by longwall mining, the high-pressure gas from a damaged, breached well may reach mine workings and overwhelm ventilation system capability to dilute the hazardous atmosphere. This paper describes a set of in-situ measurements coupled with a modeling procedure that uses geomechanical analyses for estimating longwall-induced rockmass permeability and estimates potential inflow to the mining area by a fracture flow analysis. A two-panel longwall model is constructed to represent a shallow 147 m-deep mining site in the southwestern Pennsylvania coal region. A Distinct Element Code (3DEC) is used to explicitly model the rockmass by a Discrete Fracture Network (DFN) logic. Field observation, surface subsidence measurement, underground coal pillar stress measurements, and in-situ permeability measurements are used for calibrating DFN geometries and initial aperture values. The fracture aperture distribution across the model is used for predicting potential shale gas inflow to the mine using a Fracture Flow Code (FFC) developed for this study. Several realizations are simulated to cover worst-case scenarios for the maximum inflow into the mining area in a comparative analysis. The presented modeling approach will help investigate the potential hazards of a shale gas well for mine safety should the well casing integrity be compromised by mining operations.

1:30 - 1:45 pm ET
Observed Trends in Geotechnical and Hydrogeological Data for Appalachian Underground Coal Mines

Kevin Andrews, Marshall Miller & Associates, Blacksburg, Virginia, United States; Steve Keim, Marshall Miller & Associates, Blacksburg, Virginia, United States

The geotechnical and hydrogeological characteristics of coal seams and rock strata above and below mineable coal beds are often studied to assess potential groundwater inflow to a mine, to evaluate the effect of groundwater inflow on mine stability, and to assess the potential for mining to adversely affect aquifers and streams.  Geotechnical and hydrogeological characterization of coal and rock in the roof and floor of a mine also provides vital information for ground control design and mitigation measures such as grout and resin injection.  Important characteristics include hydraulic conductivity, Rock Quality Designation (RQD), and
lithology, among many others. Field investigations often include drilling, geological and geotechnical core logging, downhole geophysical logging, laboratory rock strength testing, and packer (Lugeon) testing. Analysis of geotechnical and hydrogeological data collected over the last 30 years provides a means to better understand the typical range of values encountered in Appalachian coalfields and to identify empirical relationships amongst the main parameters. Coal seams and sandstone units are generally considered to be better aquifers relative to other lithologic units, but quite often the most prominent aquifer in coal-bearing regions is the shallow Stress- Relief Fracture Zone, an aquifer with its hydraulic conductivity predominantly created by the frequency and nature of fractures in the rock. By analyzing the relationships amongst the main geological, geotechnical, and hydrogeological factors associated with mineable coal seams and mine roof and floor material, the current research provides a valuable reference for the coal mining industry in Appalachia.

1:45 - 2:00 pm ET
Stochastic Sampling on Synthetic Rock Mass to Study the Effects of Natural Joints on Pillar Mechanics

Mustafa Can Suner, West Virginia University, Morgantown, West Virginia, United States; Ihsan Berk Tulu, West Virginia University, Morgantown, West Virginia, United States

Synthetic Rock Mass (SRM) approach and Discrete Fracture Network (DFN) models, generated from the field surveys, are novel methodologies to study fractured rock mass behavior. In this paper, SRM and DFN methodologies used to understand the influence of natural joints on the mechanical response and failure mechanisms of stone mine pillars. First, a multi-stage up-scaling procedure with the homogenization process is established. Later, a stochastic sampling process on the SRM model to have further insight on the limestone mine pillar mechanics is carried out. Two-dimensional Universal Distinct Element Code (UDEC) is utilized to represent intact rock with the Trigon Discretized blocks and to capture fractured rock mass behavior. The laboratory size limestone mine rock specimens are systematically up-scaled to the average width of the field size limestone mine pillars. Then, the DFN model, generated from the field data, is sampled in a stochastic manner to construct the SRM of the field size pillars. By doing so, the effect of spatial location and persistency of discrete discontinuities on pillar stability is studied by assessing the pillar strengths in various width-to-height ratios.

2:00 - 2:15 pm ET
Application of Local Geology Dependent Ground Response Curve for Evaluating Standing Support Performance in Gateroad Entries

Haochen Zhao, West Virginia University, Morgantown, West Virginia, United States; Ihsan Berk Tulu, West Virginia University, Morgantown, West Virginia, United States

Longwall gateroad entries are generally subjected to severe mining induced stress changes and deformations; and they are prone to large roof falls, floor heaves and rib sloughing. To keep the integrity of gateroad entries, standing support system needs to be designed to compensate for the mining induced deformation of the surrounding rock mass. Local geology, together with the stress state, influences the ground response and plays a major role in the performance of a standing support design. Ground reaction curve (GRC) that is a function of geology and stress state can be used to assess the influence of local geology and stress state on standing support performance, and recently a modelling methodology was introduced by WVU that can be used for deriving the geology and stress state dependent GRC. In the US, the Support Technology Optimization Program (STOP) and its’ GRC design criteria have been widely applied to evaluate standing support performance. Geology and stress state dependent GRC can also be integrated in STOP for improving standing support design. In this paper, this modeling methodology is demonstrated, and the results obtained by the authors are verified with published roof deformation and support load measurements from a Northern Appalachian coal mine.

2:15 - 2:45 pm ET
Q&A/Break

 

Case Studies

2:45 PM - 4:30 PM

Moderators: Heather Lawson, NIOSH Spokane Research Lab Library NIOSH Spokane Research Lab Library, Spokane, WA
Kyle Perry, Missouri University of Science & Technology, Rolla, MO
MC: Ted Klemetti, NIOSH, Pittsburgh, PA

2:45 - 3:00 pm ET
The Evolution of Pre-Consolidation Techniques at Broadmeadow Mine

Robert Coutts, BHP, Moranbah, Queensland, Australia; Daniel Lynch, BMA Broadmeadow Mine, Moranbah, Queensland, Australia; Matthew Martin, BMA Broadmeadow Mine, Moranbah, Queensland, Australia

Ground pre-consolidation has been used throughout the underground coal industry as a method to improve productivity and safety when mining in areas of geological structure. Broadmeadow Mine has used a range of industry accepted consolidation techniques including grout and resin injection delivered from both surface and in-seam. Broadmeadow Mine has also developed an innovative fault consolidation method using underground in-seam directional drilling (traditionally used for gas drainage) with hole lengths up to 210m. The products injected include ultrafine cement, polyurethane (PUR), phenolic and urea silicate resins.
Successful consolidation of geological structures in Longwall (LW) 8 aided the decision to mine through large normal fault and dyke systems in LW12 and 13. This was despite relatively low hydraulic conductivity determined by Lugeon measurements.

Analysis of the various consolidation methodologies enabled conclusions to be drawn on the effectiveness, via observations, monitoring and LW retreat rates. It was found that spile rods injected with polyurethane resin produced the best outcome. Non- directional drilling was found to be too inaccurate at greater than 50m from the gate roads. Directional drilling was required deeper into the LW block allowing placement of spiles close to the intended roof horizon. The injection success rate dropped in hole lengths greater than 60m due to packing and sealing difficulties.

Extraction rates through the consolidated areas in LW 12 and 13 exceeded forecasts and ultimately contributed to the mine recovering an additional 4.3 million tonnes of raw coal through a fault zone which may otherwise have been sterilized.

3:00 - 3:15 pm ET
Modeling the Subsidence Basins Formed by the Tunnel Ridge Longwall under Interstate 70 in Washington County, Pennsylvania

Anthony Iannacchione, University of Pittsburgh, Pittsburgh, Pennsylvania, United States; Robert Winn, University of Pittsburgh, Pittsburgh, Pennsylvania, United States; Emily Adelshon, University of Pittsburgh, Pittsburgh, Pennsylvania, United States; Julie Vandenbossche, University of Pittsburgh, Pittsburgh, Pennsylvania, United States; Roy Painter, PA Department of Transportation, Uniontown, Pennsylvania, United States

Subsidence impacts to surface features as a result of longwall coal mining in the Pittsburgh Coalbed have been well documented. Impacts to interstate highways are less well known. Identifying the strengths and weaknesses of different predictive techniques is important so that impacts to roadway assets can be assessed and planned for. To that end, Pennsylvania Department of Transportation (PennDOT) contracted the University of Pittsburgh to analyze information collected during the undermining of I-70 by the Tunnel Ridge Mine in western Washington County. In addition, knowledge gained during this undermining activity would be used to project potential impacts to areas of future longwall mining under I-70.

Two subsidence prediction methods are used. One is the Profile Function method and relies on established empirical subsidence relationships for Pittsburgh Coalbed longwall panels. The Profile Function method was most useful in making general assessments of the subsidence characteristics. The second method used was the SDPS model. Here changes to the input parameters and the use of the topography option produced good agreement with measured surveys. The SDPS subsidence prediction technique was used prior to mining, then compared with actual survey data. Adjustments were made, producing an adequate match between predicted and measured subsidence. These improved predictive models are then used to characterize potential subsidence basins along a section of I-70 in western Washington County, PA.

3:15 - 3:30 pm ET
Roof Stability and Support Strategies Associated with Longwall-Induced Horizontal Stress Changes in Belt Entries

Peter Zhang, National Institute of Occupational Safety and Health, Pittsburgh, Pennsylvania, United States; Essie Esterhuizen, NIOSH, Office of Mine Safety and Health Research, Pittsburgh, Pennsylvania, United States; Morgan Sears, NIOSH, Office of Mine Safety and Health Research, Pittsburgh, Pennsylvania, United States; Jack Trackemas, NIOSH, Office of Mine Safety and Health Research, Pittsburgh, Pennsylvania, United States; Nick Hlopick, Iron Senergy

Longwall mining is one of the primary coal mining methods used in the United States. For safe and continuous production of coal in longwall faces, the stability of belt entries must be maintained during longwall retreat. Past experience of longwall mining in the U.S. has shown that the occurrence of roof falls in belt entries was largely associated with high horizontal stress. Therefore, it is important to understand how roof stability is affected by longwall-induced horizontal stress changes in belt entries and to strategically install supplementary support to prevent any potential roof falls. Researchers from the National Institute for Occupational Safety and Health (NIOSH) measured horizontal stress changes in the immediate roof in the belt entries for two Pittsburgh seam longwall panels oriented unfavorably to high horizontal stress. Hollow Inclusion Cells (HICells) were installed in the immediate roof at the intersection in each of the belt entries, and stress changes were monitored as the longwall face was approaching and passing the intersection. Numerical models were set up to calculate the longwall-induced horizontal stress changes in the roof at the monitoring sites. With the calibrated model, investigations were made of how horizontal stress is concentrated and relieved in the belt entry roof near the face under different panel orientations. The study demonstrated that longwall-induced differential horizontal stress can be used as an indication for the degree of horizontal stress concentration. Both measurements and modeling results showed that high horizontal stress is concentrated in the roof in the belt entry within about 50 ft outby the face when a longwall panel is unfavorably oriented to the major horizontal stress. Roof support strategies are discussed on how to maintain belt entry stability under such panel orientations.

3:30 - 3:45 pm ET
Analysis of Sea Floor Subsidence and a Back Analysis of the Lingan Mine - Phalen Mine Break Through

David Newman, Appalachian Mining & Engineering, Inc., Lexington, Kentucky, United States; David Forrester, AECOM, Sydney, Nova Scotia, Canada

Subsea longwall mining has been practiced in Nova Scotia since 1877. Subsidence and the potential for breakthrough continue to be a critical safety concern. These concerns are further exacerbated when considering multiple seam longwall mining conditions. Therefore, longwall mining of the two vertically adjacent seams should be conducted such that seams are mined from the top-down and longwall panels are columnized to transfer mining induced stress through the barrier pillar with longwall production occurring within the stress shadow of the overlying panel.

The Phalen mine operated in the Phalen seam in the 1980s and 1990s, undermining the abandoned workings of the Lingan mine and the No. 26 mine in the overlying Harbour seam. Two flooding events occurred when mining induced strains attributable to longwall mining in the Phalen seam propagated through the Lower Sandstone Unit in the Phalen-Harbour seam interburden and connected with flooded mine workings in the overlying Harbor seam. The Lower Sandstone Unit contains connate saline water. It is recharged and under pressurized by the flooded mine workings of the overlying Harbor seam.

The first event occurred in 1988 where the Phalen 1 East panel connected with room-and-pillar mining in the Lingan "A" panel. The second event was significantly more serious and occurred in 1992 when the Phalen 5 East panel was undermining the Lingan 2 East Panel (Forrester, et al 1999). This paper focuses on the second event and outlines an engineering approach in which empirical subsidence parameters were obtained through the back calculation of available data from the seafloor using the Surface Deformation Prediction System (SDPS). These empirical parameters were then implemented for the analysis of the Lingan-Phalen flooding event to determine the strain magnitude that initiated the failures. The strain for the Lingan-Phalen flooding was compared with historical and established strain thresholds to prevent breakthroughs.

3:45 - 4:00 pm ET
Rail Road Bridge Movements Above a Deep Longwall Mine in the Eastern US

Zach Agioutantis, University of Kentucky, Lexington, KY, United States; Steve Hicks, Coronado Coal Co, Buchanan Mine, Oakwood, VA, Virginia, United States

The prediction and control of ground deformations due to underground mining are important considerations in the permitting, planning, and monitoring of coal mining operations. Such movements mainly include subsidence, horizontal displacement and horizontal and/or ground strain. Predictions can be accomplished using regional subsidence parameters and site-specific data. Surface topography should be considered in ground deformation predictions, especially in cases where there are abrupt changes in the slope and elevation of the ground surface.

This paper discusses movements related to a railroad bridge which is undermined by a longwall panel. The bridge is transversely located above the panel which is part of a longwall mining operation in the eastern US. The paper presents initial vertical and horizontal movement estimates for bridge abutments, the mitigation measures implemented to decrease ground strain on bridge alignment as well as monitoring results for the actual movements monitored when the bridge was undermined.

Monitoring covers both bridge abutments as well as the ground area around the bridge. Actual movements are critically compared to predicted movements especially with respect to ground strain along the bridge alignment.

4:00 - 4:30 pm ET
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