Technical Sessions

The 2022 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: Program content current on 5/27/2022. Updates may occur.

 

Tuesday, July 26

Fundamental Ground Control Studies I

9:30 am
Ground-Fall Accident Trends in Mining: 2010 to 2019
Gamal Rashed; NIOSH, Pittsburgh, Pennsylvania, United States, Zoheir Khademian; NIOSH, Pittsburgh, Pennsylvania, United States, Yuting Xue; NIOSH, Pittsburgh, Pennsylvania, United States

Mining has been recognized among the most hazardous industries in the United States, despite the significant reduction in injury and fatality rates over the past century. This study focuses on the ground-fall incidents classified in the Mine Safety and Health Administration (MSHA) database as “fall of roof, back, or brow” and “fall of face, rib, pillar, side, or highwall”. The main objective of this study is to conduct a comprehensive statistical analysis of the reported ground-fall incidents from 2010 through 2019. The authors studied 8,445 ground-fall incident-narratives and classified them into one of the following types: roof fall, rib fall, face fall, highwall failure, and rock-burst. These ground-fall incidents resulted in 46 fatalities, 33 permanent disabilities, 3,082 injuries, 119,520 nonfatal days lost, and 12,433 days of restricted work activities. Since most of the ground-fall injuries and fatalities occurred in underground coal mines and were attributed to falls of roof and rib, additional analysis was directed to evaluate roof and rib fall incident trends in underground coal mines due to the mining method, coal-seam thickness, mine size, seasonal effects, and the experience of the injured/victim. Ground-fall injury rates in room-and-pillar mines are higher than that of longwall mines. Of the injuries occurring in longwall mines, 50% occurred during development. The fatality rate in room-and-pillar mines was 1.64 times that of longwall mines. The rib fall fatality rate was higher than the roof fatality rate for both room-and-pillar mines and longwall mines. Ground-fall rates were found to be higher between July and October possibly due to higher humidity. Studying the ground-fall incident trends and hazards will help identify areas where additional research is needed and where innovative solutions need to be developed to reduce these potentially severe occupational hazards.

 

9:55 am
A Discussion on Causation Mechanisms for Overburden Bumps as Distinct From Coal Bursts
Russell Frith; Mine Advice Pty Ltd, Beresfield, New South Wales, Australia

The entire subject area of micro-seismic events due to stored strain energy, as distinct from gas-driven coal outbursts, can be readily sub-divided into firstly events with their energy source from within the coal seam (termed “bursts”) and secondly, event with their energy source outside of the coal seam in either the overburden and/or floor strata. The reason for sub-dividing micro-seismic events in this manner is that if the causation mechanisms and associated geotechnical conditions are materially different, then effective pre- mining predictions and subsequent operational controls may also differ. Attempting to explain a multitude of micro-seismic event types without consideration of varying source mechanisms will inevitably lead to inadequate causal explanations and effective controls. The paper outlines several different causal mechanisms for bumps emanating from both the overburden and/or floor of a coal seam by reference to both theoretical treatments and known associated case histories. These include massive pillar collapses (including the Coalbrook disaster in 1960), large-scale shear slip along fault planes/other geological discontinuities, the compressive failure of thick and strong strata units and finally, multi-seam stress effects. The objective of the paper is to provide an initial “cause and effect” list of geological and geotechnical circumstances that can and indeed have resulted in large magnitude micro-seismic events during underground coal mining activities, being able to predict the likely propensity for such significant events prior to mining being the first requirement in an effective prevention or consequence mitigation process.

 

10:20 am
Forty Year History of Testing at NIOSH’s Mine Roof Simulator. What have we learned?
Timothy Batchler; NIOSH, Pittsburgh, Pennsylvania, United States, Daniel McElhinney; NIOSH, Pittsburgh, Pennsylvania, United States

The National Institute for Occupational Safety and Health’s (NIOSH) Mine Roof Simulator (MRS) at the Pittsburgh Mining Research Division was commissioned for operation in 1981. The MRS was originally designed for longwall shield research and was the only hydraulic press in the world with the capability to apply biaxial loads to these large structures. The results of this forty-year research program have contributed to improved performance and increased knowledge of longwall shields, mobile roof supports and hydraulic cylinders in the mining industry. Also, considerable research and development have been conducted to develop standing support technologies for the mining industry. The Support Technology Optimization Program (STOP) was developed to evaluate how these various technologies can impact and improve both ground control conditions and mine safety. Additional MRS research has included the performance of ventilation stopping walls including the development of a new testing protocol to evaluate the transverse loading capabilities under arching load conditions and evaluating the performance characteristics of welded wire screen as used in underground mine roof surface control.  The NIOSH Mine Roof Simulator has been a reliable and valuable tool for the past forty years for the evaluation of safety structures used in mining and this research has contributed significantly to the advancement toward safer mines. This paper will summarize those research results and provide a glimpse of planned future activities.

 

10:45 am
Longwall Gateroad Stability Analysis Based on Field Monitoring and Bonded Block Modeling Results
Zoheir Khademian; NIOSH, Pittsburgh, Pennsylvania, United States, Morgan Sears; NIOSH, Pittsburgh, Pennsylvania, United States

Based on the 2010-2019 MSHA accident report database, 89% of ground control accidents in the US longwall mines were caused by gateroad roof instability. This can be related to the significant changes in loading conditions that gateroads are subject to from the development to the longwall face advance phases, combined with the highly laminated roof structures, typical of longwall coal mine geology. One of the challenges in designing efficient support systems is to understand the roof instability mechanisms along with support-roof interactions under this complex condition. Field monitoring of roof displacement and stress changes coupled with detailed analyses of numerical models are well-documented techniques to study roof instabilities. However, traditional modeling techniques require additional calibration to simulate delamination and buckling of laminated shale roof, one of the main roof instability mechanisms in longwall mines. An alternative may be the UDEC Bonded Block Method (BBM) numerical modeling where laminated roof and support interactions can be explicitly captured. This paper presents the development of a BBM model of a longwall entry at a 180 m depth of cover with a three-entry gateroad layout along with roof extensometer monitoring results. The models are validated against the monitoring results, showing the potential for BBM numerical models to assist with optimizing longwall layouts and gateroad support systems.

 

11:10 am
LUNCH

 

Underground Stone Studies

12:30 pm
International Experience with Airblasts and its Relevance to Underground Stone Mines
Christopher Mark; Mine Safety and Health Administration, PITTSBURGH, Pennsylvania, United States

Recent pillar collapses have caused five large airblasts in underground US limestone mines. These events have injured three miners and put many others at risk. To better understand the hazard, MSHA conducted a comprehensive review of the international literature on airblasts. More than 40 airblast cases have been documented, in a wide variety of minerals. Experience clearly shows that miners that are in the direct path of the air as it moves towards the mine exits are at the greatest risk. The paper describes a risk management methodolgy that can be used to evaluate the level of hazard throughout the mine and aid in the selection of controls to reduce the risk.  The paper concludes with a case history of an actual pillar collapse and airblast.

12:55 pm
Pillar Strength Estimation of Underground Stone Mines with Numerical Simulations
Mustafa Can Suner; West Virginia University, Morgantown, West Virginia, United States

Synthetic Rock Mass (SRM) approach is a relatively new method to study the mechanical behavior of rock masses. Recently, a SRM approach was established to estimate pillar strength from laboratory scale to in-situ dimension using the two-dimensional Universal Distinct Element code (UDEC). In this study, the developed methodology is applied to the S-Pillar database to estimate pillar strength and failure mechanisms as a function of width-to-height. First, stone mine rocks are categorized into three as low-, medium-, and high-strength according to their uniaxial compressive strength (UCS). Later, laboratory-scale intact rocks are scaled-up to the average in-situ pillar dimensions by explicitly considering the naturally existing joint sets. It is found that the strength of the pillars with low UCS are over-estimated when compared to pillars with high UCS. This observation is attributed to the high joint density of the pillars with high UCS; so, they suffer more adverse effects of discrete discontinuities as also discussed in the literature. After obtaining in-situ pillar strength, the width-to- height ratios of the pillar models are varied from 0.5 to 2.0 to gain a robust understanding of pillar mechanical behavior. Tensile failure is found to govern the pillar behavior in a width-to-height ratio of 0.5 while the combination of tensile failure in rib elements and shear failure in the core of the pillar is observed with the hour-glass shape when width-to-height ratios are equal to 1.0, 1.5, and 2.0.

 

1:20 pm
Massive Pillar Collapses in U.S. Underground Limestone Mines: 2015-2021
Gregory Rumbaugh; Mine Safety and Health Administration, Pittsburgh, Pennsylvania, United States

During the years 2015-2021, five major pillar collapses occurred at four underground stone mines in the eastern United States. These events resulted in large surface subsidence features, affected underground travelways, and caused powerful airblasts that damaged mine infrastructure and resulted in three injuries. A pillar fails when it is compressed beyond its peak resistance and it sheds load. A massive pillar collapse occurs when an array of adjacent pillars suddenly fail simultaneously. Pillar collapses are a particular hazard for miners because they can occur with very little warning, and they can affect miners located far away from the collapse. Each event involved the sudden collapse of at least 12 benched pillars whose width-to-height ratio was 0.8 or less. It is not possible to predict when, or even whether, a particular array of pillars will collapse. However, experience has shown that certain factors are associated with an increased likelihood of a pillar collapse. This paper provides the relevant factors associated with the stability of the pillars involved in each of the massive pillar collapse events.

 

1:45 pm
Geologic Factors Affecting the Occurrence of Massive Strata Collapses in Limestone Mines within the Mauch Chunk Formation of Southwestern Pennsylvania
Nicole Evanek; NIOSH PMRD, Pittsburgh, Pennsylvania, United States, Thomas Anderson; University of Pittsburgh, Pittsburgh, Pennsylvania, United States

Four massive strata collapses have occurred in US underground limestone mines in the last two years. Preliminary investigations have shown that pillar size and shape in bench areas are leading causes. But the influences of the strength characteristic and thickness of the overlying strata have yet to be analyzed. This paper reviews the characteristics of strata above several underground Loyalhanna limestone quarries and evaluates factors that may contribute to massive strata collapses. There are eleven underground Loyalhanna limestone quarries that comprise the study area in southwestern Pennsylvania.  Two of these eleven are known to have had experienced massive strata collapses. This study focuses on the Mauch Chuck Formation directly above the Loyalhanna limestone and identifies stratigraphic and structural factors that may have contributed to the occurrence of the massive strata collapses. The strength characteristics of weak layers within the Mauch Chunk Formation and the occurrence and position of the low-angle faults are postulated to be two significant factors. This report will also examine how these geologic factors might be used in combination with room-and-pillar layouts to mitigate the occurrence of massive strata collapses in this mining district.

2:10 pm
Coffee Break

 

Case Studies I

2:30 pm
Evaluation of Seismic Potential in a Longwall Mine with Massive Sandstone Roof Under Deep Overburden: An Update
Mark Van Dyke; NIOSH, Pittsburgh, Pennsylvania, United States, Zoheir Khademian; NIOSH, Pittsburgh, Pennsylvania, United States, Joe Wickline; Coronado Global Resources Inc., Beckley, West Virginia, United States, Jake Beale; Pilot Geophysical LLC., Riner, Virginia, United States

In 2016 a 3.7ML magnitude event occurred at longwall mine in southwestern Virginia which was recorded by the United States Geological Survey (USGS). The event was the largest of its kind since the global mine design change in (date). Mine management requested researchers from the National Institute for Occupational Safety and Health (NIOSH) to access geological data and determine what parameters could possibly lead to events of a magnitude 1.0ML or greater. Working together using 2152 geostatistical data points and modeling revealed three major geological factors in common with the majority of the 181 recorded +1.0ML events since 2009. The three geological factors in increasing seismic potential when overlapped are:

  • Low potential (1.0+ ML): overburden greater than 579.12 m.
  • Moderate potential (1.5+ ML): overburden greater than 5.79m and 6.1-12.2 m of sandstone within 15.24 m above the coal seam.
  • Elevated potential (3.0+ ML): overburden greater than 5.79m, 6.1-12.2 m of sandstone within 15.24 m above the coal seam, and caving height of less than 4.572 m above the coal seam.

The three factors were used to create seismic forecast maps that produced and accuracy within 74%-89% of 1.0 ML or greater events, 72% accuracy of 1.5 ML or greater events, and 100% accuracy of events 3.0ML or greater based on seismic history (Van Dyke et al., 2018). The map was created to not only show how geological data can be combined to understand why a seismic event occurred in a particular area, but how it could be used to forecast potential seismic areas in future mining. This paper is an update to see how well the seismic forecasting map worked in areas mined since the map was originally published and how the map has helped miner safety and health based on its implementation. Since the implementation of the forecasting map, the map has correctly forecasted 56-72% of 116 total 1.0-1.49 ML events of 74-90% of 50 total 1.5-1.99 ML events and of 89% of 9 total 2.0 ML or above events

 

2:55 pm
Installation and Preliminary Results of a Novel Fiber Optic Distributed Strain Monitoring System in an Active Underground Room and Pillar Metal Mine
Samuel Nowak; Missouri S&T, Houston, Missouri, United States, Paul Brooks; Nyrstar, New Market, Tennessee, United States, Dogukan Guner; Missouri University of Science & Technology, Rolla, Missouri, United States

In this work, we present a novel fiber optic cable which is designed for distributed strain sensing in grouted boreholes where highly localized displacements are likely to occur. Laboratory results demonstrate that the novel fiber optic cable is capable of monitoring the opening and shearing along cracks or discontinuities in cemented boreholes where cables specifically designed for strain sensing cannot. An installation method for the DOFS system is demonstrated via a field scale application in an active underground room and pillar metal mine. The installation efforts themselves are described and shown to be simple and effective for use by the mining industry. Through the developed technology, a large stoping area was instrumented with a single, low-cost fiber optic cable measuring over 1 km in total length across 11 grouted boreholes. The preliminary results of the long-term monitoring campaign are presented, along with future intended uses of the collected data.

 

3:20 pm
Parametric Analysis on Inclined Pillar Stability
Zach Agioutantis; University of Kentucky, Lexington, Kentucky, United States, Kostas Kaklis; Botswana International University of Science & Technology, Palapye, Botswana

Pillars are used as the primary support structures for underground mining to maintain stability by supporting the over laying strata. In the case of horizontal seams, the pillars are typically subjected to axial loading due to the weight of the overburden and/or abutment stresses, while in the case of inclined seams they are subjected to oblique loading due to both the vertical and horizontal in-situ stress. Over the years numerous studies have been completed on square and rectangular pillars in horizontal seams that have resulted in numerous pillar stability equations and criteria. However very few studies are available with respect to pillars in inclined seams. Inclined pillars are subject not only to high normal stresses, but they are also subject to higher shear stresses that depend on the inclination of the seam, the ratio of the horizontal to vertical in-situ stress as well as the physical and mechanical properties of the material. This paper presents a parametric numerical investigation of pillars in inclined seams using the finite element method by evaluating different geometries such as seam inclination, width to height ratio of pillars, width to length pillar ratios, etc.

3:45 pm
Hydrogeologic Characterization of Bedrock Units for Mining and Environmental Purposes Using Downhole Geophysical Logging Technology

Kevin Andrews; Marshall Miller & Associates, Blacksburg, Virginia, United States, Andrew Karpa; Marshall Miller & Associates, Blacksburg, Virginia, United States

Groundwater flow in bedrock units is an important factor to be considered for most mining situations, both underground and open pit excavations. Characterization of the hydrogeologic system into which a mine will advance is an important factor for slope and entry stability, mine dewatering design, and potential environmental considerations including stream loss and water well damage. In addition, hydrogeologic characterization of bedrock is important for locating water wells and for groundwater contamination mitigation. Hydrogeologic characterization for a site begins with a desktop study and often evolves to include drilling, geologic logging, monitoring well installation, groundwater sampling, pump testing, and packer (Lugeon) testing. In addition to these methods of hydrogeologic data collection, downhole geophysical logging of exploration and monitoring well boreholes provides detailed information associated specifically with in-situ conditions of each hole. The basic suite of geophysical logging tools used for hydrogeologic characterization often includes Optical or Acoustic Televiewer (O/ATV), full wave sonic, gamma-density, fluid temperature, resistivity, specific conductance, and flowmeter. For many hydrogeologic settings, groundwater flow in the bedrock is dominated by discontinuities, or fractures, in the rock. O/ATV logging allows for determination of the frequency (clustering), orientation, and aperture width of in-situ fractures, while sonic logging allows for estimation of intact rock strength. Fluid temperature logging and flowmeter measurements provide insight with regard to potential groundwater inflow zones, and other tools provide information for geologic unit contacts. This paper outlines the process and benefits of using downhole geophysical logging for hydrogeologic characterization and provides general examples from industry experience.

 

Wednesday, July 27

Design Tools and Guidance

9:30 am
Engineering Guidelines Derived from NIOSH Gas Well Stability Research
Daniel Su; CDC/NIOSH/PMRD, Jefferson Hill, Pennsylvania, United States, Heather Dougherty; NIOSH, Jefferson Hill, Pennsylvania, United States, Mark Van Dyke; NIOSH, Jefferson Hill, Pennsylvania, United States

This paper presents the critical scientific data acquisition, data interpretation, sophisticated 3-dimensional modeling, and preliminary engineering guidelines derived from the current NIOSH gas well stability research. Results from the NIOSH field instrumentation programs and the parallel 3-dimensional numerical modeling programs indicate that under shallow cover, the measured horizontal displacements within the abutment pillar are one order of magnitude higher than those measured under deep cover. Cementing alternatives are found to have significant impacts on longwall-induced casing stresses and deformations.
Engineering guidelines on longwall-induced deformations, casing and cementing alternatives, gas well setback distances, as well as risk assessment strategy are proposed.

 

9:55 am
How to Bridge the Gap Between Geotechnical Research and Credible Mine-Site Geotechnical Design Tools
Mark Colwell; Geotechnical Software Services Pty Ltd, Caloundra, Queensland, Australia

As per most other earth science engineering problems, the underground coal geotechnical environment and the way in which roof and rib support interacts with the rock mass are complex issues. While the problem should not be oversimplified (i.e. the dominant failure mechanisms or critical data input parameters should not be ignored), without question judicious simplification is at the heart of all good engineering design. Engineers play a vital role in supplying the world with solutions and products. But it is crucial that engineers make genuine contributions. One way to ensure this is to employ simplification in the engineering process. In recent decades, many rock mechanics scientists/researchers have become heavily reliant on desktop numerical modelling/simulation for their understanding of coal mine strata behaviour, as the collection of information and the formulation of industry-wide databases suitable for analysis of rock mass response and ground support performance is viewed as costly and time consuming. Unfortunately, in the field of coal mine strata control it is now evident that some cause-and-effect research is being founded in mathematical   models with selective observations or measurements being used to justify the findings. This is an exact reversal of the Scientific Method that has served mankind well for centuries and if allowed to proliferate must inevitably  lead to a lessening rather than improvement in our fundamental understanding of the real world. Pioneers in the field of geotechnical research have stated that marked progress in the field of mining rock mechanics requires the careful collection, analysis and presentation of field/mine experience and that the advantage of empirical modelling is its firm links to actual experience. Thus, if it is judiciously applied, it can hardly result in a totally wrong answer. As this paper explains, over the last 25 years extensive databases specific to various geotechnical design issues associated with the Australian underground coal industry have been formulated with the subsequent analyses resulting in several empirical/analytical design techniques that are based on a sound mechanistic understanding of the geotechnical environment. Subsequently, these design techniques have been incorporated in sophisticated (yet easy to use) windows based software and have become the dominant/preferred design methodologies used by over 70% of Australian collieries. Common sense suggested that by providing mine-site geotechnical engineers with credible and readily useable design tools, improvements to safety and productivity would inevitably result. The research funding associated with the development of the design techniques was provided on that basis and that is exactly what was achieved. This Australian experience provides a “template” to the worldwide mining geotechnical community in how to conduct research with a far greater chance of success in transferring the knowledge gained to the mine-site user. Yes, developing an industry-wide database of information with several (rather than one or two) monitoring sites can be time consuming and costly. However, the initial outlay of monies can result in extremely cost-effective software that readily solves industry-wide problems as opposed to continuing costly site-by-site monitoring/calibration of a consultant’s numerical model.

 

10:20 am
Analysis and Design of Faceroad Roof Support (Adfrs)
Mark Colwell; Geotechnical Software Services Pty Ltd, Caloundra, Queensland, Australia

This paper summarises the results of a research project whose goal was to provide the Australian coal  industry with a longwall installation roadway design methodology that could be utilised by suitably qualified colliery staff. This goal has been achieved and the design methodology is referred to as Analysis and Design of Faceroad Roof Support (ADFRS). The intended benefits to underground operations, in the provision of this information and resource, are a safer and more productive workplace. In relation to other strata control issues, such as coal mine pillar and roof support design (for standard roadway widths i.e. < 5.5m wide), there has been comparatively very little research undertaken in relation to the geotechnical design and management of longwall installation (i.e. wide) roadways. As a result, in terms  of a satisfactory outcome, faceroads within Australia had been quite problematic and by way of example of the 207 cases associated with the two-pass dataset, 40 resulted in an unsatisfactory outcome involving the use of standing support, PUR and/or high levels of remedial tendon support with two faceroads “lost” and having to be re-driven due to major roof falls. The principal reason for such a failure rate was the clear absence of suitable design equations that relate the required levels/type of roof support as a function of the competency of the roof and the horizontal stress acting across the roadway. ADFRS now fills the gaping void that existed in the Australian underground coal industry with respect to the geotechnical design and management of longwall installation roadways and to the best of the author’s knowledge ADFRS is the first systematic faceroad design technique to be developed for any country’s underground coal industry.

 

10:45 am
User-friendly Finite Element Design of Shafts and Tunnels
Mark Larson; Spokane Mining Research Division, NIOSH, Spokane, Washington, United States, Heather E. Lawson; CDC NIOSH, Spokane, Washington, United States

Rational design based on engineering fundamentals is essential for the layout of safe, stable shafts and tunnels. Shafts and tunnels including winzes, raises, adits and cross-cuts are lifelines to the underground and must be secure.  The well-known finite element method of analysis provides an easy avenue to rational design by providing distributions of stress, strain and displacement induced by a proposed shaft or tunnel layout. Of particular importance is the extent of yielding, if any, about the proposed excavation. While some yielding may be tolerated during excavation, extensive yielding is generally unacceptable and indicative of a need for redesign. Redesign may require a change in section shape, orientation or spacing in case of multiple openings.  This contribution describes a significant advance in existing software previously developed for five important design problems in strata-bound mines such as coal, trona, salt, potash and some large, lead- zinc mines. This software allows analysis to proceed in three easy steps: (1) preparation of a strata property files, (2) generation of the finite element model, a mesh, and (3) finite element program execution. Circular, elliptical and rectangular shaft sections are available for a single shaft, twin shafts and shafts in a row.
Spacing of twin shafts and shafts in a row is a free parameter. Tunnel shapes are the same but include the conventional arched back section. Dipping strata of variable thickness, depth and orientation relative to the excavation of choice are allowed. To be sure, the analysis is three-dimensional. An elastic-plastic material model based on associated rules of flow and an N-type yield condition for isotropic, transversely isotropic or orthotropic rock are used for all strata. Example analyses of shafts, winzes, raises, adits and cross-cuts from several hardrock and softrock mines illustrate the three step process including mesh plotting and plotting of selected results. The goal is to advance mine design technology for improved mine health and safety. A User Manual, software and other documentation are available at the website UT3PC.net.

11:10 am
Coffee Break

11:20 am
Numerical Modeling and Measurement of Casing Deformations of Shale Gas Wells in Longwall Chain Pillars under Deep Cover
Peter Zhang; National Institute of Occupational Safety and Health, Pittsburgh, Pennsylvania, United States, Bo Hyun Kim; NIOSH, Pittsburgh Mining Research Division, Pittsburgh, Pennsylvania, United States, Mark Van Dyke; NIOSH, Pittsburgh Mining Research Division, Pittsburgh, Pennsylvania, United States

The integrity of shale gas wells in longwall chain pillars under the influence of longwall mining is a safety concern for both coal and gas operators. Previous modeling of gas wells under shallow depth has shown that FLAC3D model is capable of predicting longwall-induced stresses and deformations in gas well casings with reasonable accuracy. The predicted casing deformations can be verified by multi-finger caliper surveys. This study concerns the influence of longwall mining on eight shale gas wells in a shale gas well pad located in the chain pillars between two adjacent longwall panels under deep cover greater than 1,000 ft. FLAC3D modeling was performed to predict longwall-induced deformations in gas well casings based on site-specific mining and geological conditions. Small deformations less than 0.5 in were predicted in the intermediate casing, and very small casing deformations less than 0.15 in were predicted below 600 ft depth due to high abutment pressure and high frictional resistance along the weak bedding interfaces. After the first panel was mined, a 56-arm caliper survey was performed on one of the wells to measure the casing deformations in the intermediate casing. The predicted casing deformations were then compared with the measured deformations. The comparison indicated that the prediction was generally in good agreement with the measurement in terms of deformation locations and magnitudes. Findings from this study provide valuable information not only for stability assessment of shale gas wells in longwall chain pillars and but also for developing engineering guidelines to preserve casing integrity through adequate pillar and casing design.

 

11:45 am
Analysis of Coal Rib Fracture Depth Using Numerical Modeling And Artificial Neural Network
Yuting Xue; NIOSH, Pittsburgh, Pennsylvania, United States

The failure of coal ribs is a major hazard in underground coal mines. Over the past decade, rib falls resulted in over 50% of the ground-fall fatalities in U.S. underground coal mines. Researchers from the National Institute for Occupational Safety and Health (NIOSH) have been working on the development of an engineering-based rib control method. A user-friendly standalone application, called Design of Rib Support (DORS), is being developed to ease the calculations of rib support. DORS can estimate the Primary Rib Support Density (PRSD, ton/ft2) for development loading, but it is still missing a key factor in rib support design which is bolt length. The bolt length can be estimated from the rib fracture/softening depth and this study focuses on the coal rib fracture depth analysis with numerical modeling and artificial neural network. Hundreds of FLAC3D simulations with different mining scenarios and rib conditions were conducted. The simulation results were compared with the data collected from field and acceptable results were obtained. In addition, machine learning (ML) technique was used to predict coal rib fracture depth under different conditions. Based on the numerical simulations, the parameters for mining scenario and rib condition were collected as features for ML and coal rib fracture/softening depths were collected as target to predict.
Promising results were obtained with artificial neural network. The results from this study will improve DORS application as a ground control tool for mining engineers.

 

Thursday, July 28

Fundamental Ground Control Studies II

9:30 am
A First-Principles Causation Hypothesis for Pillar Bursts in Underground Coal Mines
Russell Frith; Mine Advice Pty Ltd, Beresfield, New South Wales, Australia

A review of published literature reveals case histories whereby the entire periphery of a coal pillar has “burst” out as a single event during first workings, the associated energy and material release causing significant damage to adjacent roadways and any equipment/infrastructure located within said roadways. Such events are distinct from coal burst events during first workings such as that at the Austar Mine in 2014, or those linked to overburden bumps related to either horizontal stress-driven slip along major faults and/or thick, massive strata units in the overburden or floor of the coal seam. The paper considers as to how the necessary “unstable” conditions for a pillar burst event could conceivably be generated, based on established coal pillar mechanics, specific pillar loading conditions and the shear- restraint of horizontal planes according to both cohesion and friction. The associated hypothesis is applied to a published example to test its veracity. The longer-term objective of this type of back-analysis is to provide a “cause and effect” list of geological, geotechnical and mine layout circumstances that can and indeed have resulted in entire coal pillar bursts during underground coal mining activities, being able to predict the likely propensity for such events prior to mining being a mandatory requirement in an effective prevention or consequence mitigation process.

 

9:55 am
Energy Considerations Related to Strata Failure During Caving Operations
Zach Agioutantis; University of Kentucky, Lexington, Kentucky, United States

Mining operations that involve caving of the overlying strata are designed so that caving will occur in a controlled manner by progressive overburden failure. However, there are many instances where the overlying strata will bridge over the caved area. As the caved area expands due to panel advance, the overburden may fail suddenly releasing large amounts of energy during these failure events. The released energy consists of both the energy released due to the bending failure and other failure of the strata, as well as the dynamic energy due to the material movement as it drops into the gob area. This paper will discuss considerations and present calculations related to the energy released due to overburden failure in caving operations such as longwall mining. Numerical modelling will also be used to estimate the elastic and other forms of energy released in idealized caving geometries.

 

10:20 am
Influence of Mineralogical Compositions on Anisotropic Burst-Prone Coal Strength
Bo-Hyun Kim; CDC/NIOSH/SMRD/MSB, Spokane, Washington, United States, Gabriel Walton; Colorado School of Mines, Golden, Colorado, United States, Heather Lawson; CDC/NIOSH/SMRD/MSB, Spokane, Washington, United States

In this study, we evaluated the effects of different shapes and distribution densities of mineral grains in coal on failure mechanics using the numerical software 3DEC. The main aim of this study was to identify possible failure mechanisms because of mineral habit and frequency in coal. Exploring differences in failure mechanics associated with the mineral grains helped to determine the role of mineral character as a possible contributor to characterize burst-prone coals. To achieve the goal of this study, a series of numerical specimens were prepared in the 3DEC model as follows. Firstly, the 3DEC modeling in conjunction with the Discrete Fracture Networks (DFNs) technique was performed to explicitly generate the discontinuities (i.e., cleats and bedding planes) in the numerical specimens based on the results of laboratory analyses. Then, the different realizations of various mineral grains were embedded in the 3DEC model to simulate an unconfined compressive strength (UCS) test to assess the influence of the mineralogical characteristics on the UCS. As a result, although the UCS of the coal was highly anisotropic depending the orientations of the cleats, the shapes and distribution densities of mineral grains in the coal affected not only the strength but also the failure mechanism of the coal.

 

10:45 am
Characteristics of Abutment Pressures and Bolt Loads in Longwall System: A Review on Stress/Load Measurements
Gaobo Zhao; West Virginia university, Morgantown, West Virginia, United States

Longwall mine instrumentation for measuring abutment stress distribution, mining induced immediate roof stress change and deformation, and primary/secondary support loads and deformations provides critical guidance for the design of gateroad chain pillars and roof support. Field measurement results are also recognized as the best way to calibrate numerical models that have been used to study the failure and support mechanisms. International Conference on Ground Control in Mining (ICGCM) started in 1981 and since then, there were 32 papers published in the proceedings discussing the stress and load measurements using various field instrumentation techniques. The focus of this paper is on summarizing the analyses of the data published in these papers. Stress/load change monitoring included the following categories: gateroad chain pillar, abutment pressure, bolt load, and pre-driven recovery room. The data shows that the magnitude of stress changes as the face advances varies considerably from panel to panel and mine to mine. However, regardless of types of stress changes, pillar stress, abutment pressure, and bolt load, there exists a rough trend of stress increase as the face advances, generally consisting of three stages, i.e., stress/load (1) initially increases steady or slowly, (2) increases rapidly, and (3) increases reaching the peaks and dropping down slowly or sharply. The stress/load increases start earlier during the second panel mining and are larger than the first panel mining. This review makes it easy for the interested persons to find valuable data for other similar projects.

11:10 am
LUNCH

 

Monitoring and Analysis Studies

12:30 pm
Upwards Surface Movement Above Deep Coal Mines After Closure And Flooding: Analytical Model Results
Andre Vervoort; KU Leuven, 3001 Leuven, Leuven, Belgium

At the 2017 ICGCM-conference in Morgantown, a case study was presented with measurements of the upwards surface movements or uplift after the closure of two neighboring coal mines (Winterslag and Zwartberg, Belgium). One was closed in the sixties, while the other closed in 1988. The study was based on satellite data (Radar-interferometry or INSAR). The analyses showed a significant variation of uplift values above the mined area and its surrounding zones, as well as a variation of the uplift rate as a function of time. In other words, the phenomenon of uplift is complex. At that time, it also was clear that more research was needed to better understand this phenomenon, i.e., an identification of the various mechanisms that are relevant for the process of uplift, and the quantification of the various impacts. A systematic analysis of all relevant parameters resulted in a framework, which forms the basis for analytical calculations of uplift values. The main impact is due to a change in hydrostatic pressure. However, not only the expansion of the softest parts, i.e., the collapsed or gob material, is important, but the expansion of the non-collapsed strata layers due to a change in pore pressure must also be integrated into the model. That also is the reason why the shape of uplift values along a linear transect is different from the shape of the initial subsidence values. Overall, a very good correlation is observed between the INSAR-measurements and the calculated values along north-south transects.

 

12:55 pm
Advantages of Structure from Motion Compared to Laser Scanning for Registration of Underground Cavities Using Mobile Devices
Malte Jan Michael Gurgel; Institute for Mine Surveying, Mining Subsidence Engineering and Geophysics in Mining, RWTH Aachen University, Aachen, North Rhine-Westphalia, Germany

Recent three-dimensional surveying techniques have opened up new opportunities for the registration and monitoring of underground cavities. There are essentially two main methods available: Laser scanning is used both above surface and increasingly underground. But photogrammetry is also experiencing a renaissance with techniques like structure from motion through the algorithm-based orientation of high-resolution  images. Both methods provide the capability to reconstruct reality by various 3-D products like points clouds and meshed models. However, they differ significantly in the steps of registration and calculation as well as in the resulting evaluation options. A basic comparison is conducted regarding underground application, considering monitoring of dynamic changes over time, surveying capabilities as well as sophisticated three-dimensional visualization as a basis for further application of virtual reality (VR) and augmented reality (AR). Findings from case studies carried out in simulated and real mines in Germany will be incorporated into this comparison. In particular, mobile acquisition methods using smartphones, tablets, and mirrorless cameras are addressed. These devices lower the entry hurdle for an integrated underground registration significantly and are therefore of a main research interest. Finally, essential advantages of an image-based acquisition and evaluation by means of structure from motion for underground use are outlined. Open challenges and future fields of application are discussed, including the area of active mining but also new use cases like underground storage of energy, commodities or residual materials.

 

1:20 pm
Distinct Element Analysis for the Effectiveness of Preliminary Coal Pillar Rib Support Systems Based on the Strength Reduction Method
Dogukan Guner; M?ssouri S&T, Rolla, Missouri, United States

Despite numerous empirical, numerical and hybrid studies conducted on the subject of coal rib failure, mine fatalities related rib failure continue to occur. To date, a standard methodology for the design of coal rib supports which can accommodate the wide range of unique conditions found in coal mines in the United States has not been developed, forcing mine operators to rely on the trial-and-error process or to utilize industrial legacy practices. In this study, to better understand the support systems' effectiveness, various rib bolting scenarios were investigated. A Distinct Element Method (DEM) coal mass constitutive model is proposed, and rib factor-of-safety (RibFOS) analyses were performed by applying a strength-reduction for the coal matrix and explicitly modeled face cleats. Ribs with various overburden depths and mining heights composed of solid Banded-Bright coal (BBC) were considered in parametric studies. Different scenarios such as rib bolt length, bolt location, and the number of bolts were studied. As a result of the unsupported coal rib analyses, a decaying exponential relationship is proposed between the mining height-overburden depth and the RibFOS. Results reveal that the DEM strength reduction with the coal-mass model can be used as a rib support design tool. The developed coal rib models clearly showed the coal-rib bolt interaction, and illustrated how the addition of rib bolts restricts the movement of the coal rib and prevents tensile failures. An unsupported RibFOS cutoff is proposed, beyond which the addition of primary support cannot stabilize the failed rib structure.

 

1:45 pm
Study on Pillar Stability in Underground Coal Mine under Weak Geological Condition in Indonesia
Takashi Sasaoka; Kyushu University, Fukuoka, Fukuoka, Japan, Akihiro Hamanaka; Kyushu University, Fukuoka, Fukuoka, Japan, Hideki Shimada; Kyushu University, Fukuoka, Fukuoka, Japan, Jiro Oya; Mitsui Matsuhima Holdings Co., Ltd, Fukuoka, Fukuoka, Japan

Indonesia is one of the world's leading coal-producing countries. More than 99% of coal mining in Indonesia is operating with open-cut mining. However, the underground coal mining method began to be adopted to extend coal production due to the increasing demand for coal and limited coal reserves that are suitable for surface mining. It is estimated that the guidelines of longwall mining from other major coal mine countries are not applicable for Indonesia because the mechanical properties of coal and its surrounding rock in Indonesia are very weak comparing with other countries. Therefore, it is necessary to research stability control of underground longwall mining under weak geological conditions. Previous studies indicated that there is higher stress acting around goaf and longwall face. A suitable design of the pillar is necessary to protect the gate-entry of the unmined panel from these effects. This study discusses that the change in stability of pillar with progressing longwall face by using FLAC3D.

2:10 pm
Break

 

Case Studies II

2:20 pm
The Design and Use of Grout Pillars for the Purpose of Holing Longwalls into Pre-driven Roadways
Rob Thomas; Strata2, Edgeworth, New South Wales, Australia

It is generally agreed that standing support should be installed in roadways that will for various reasons, be holed through with a longwall. Considering the magnitude of the associated abutment loading in conjunction with the need to ensure that the material used is cuttable, some form of cementitious support is often used. By far the most common type of standing support used in pre-driven roadways is some form of fibre-crete block; although a number of operations have used pumpable grout filled cribs, cement blocks or backfill; where in the case of the latter, the roadway is completely filled with either a cement-flyash mix or cellular concrete. There are however a number of significant deficiencies associated with cementitious supports; namely the strength and yielding ability of pumpable cribs, the need to ensure that fibre-crete or cement based supports are softened with timber such that they are able to yield in a controlled manner when subjected to an abutment load, the slenderness of the supports such that they are able to withstand any out-of-plane loading that may result on holing, and the cost and downstream impact on the conveying equipment associated with the removal of large volumes of backfill material. The above said, this assessment has addressed the design and use of grout filled pillars, which not only offer a high capacity and more squat form of standing support, but also do not necessitate the use of timber and the large volume of material and the associated infrastructure typically associated with the use of backfill. Similarly, the placement of the grout pillar against the inbye rib, confines and in doing so, maximises the strength of the resulting fender of coal on holing, and the strength and yielding ability of the grout pillar can be further increased through the use of mesh and/or fibre-glass bolts. Several case studies are discussed and the associated monitoring results demonstrate that compared to traditional cementitious standing supports and backfill, grout filled pillars offer an alternative and, in some cases, a more effective means of supporting pre-driven roadways.

 

2:45 pm
Mining-Induced Subsidence and Deformation Characteristics of Strata with Thick Unconsolidated Formation
Jinwang Zhang; China University of Mining and Technology(Beijing), Beijing, Beijing , China (Mainland), Dapeng JIANG; China University of Mining and Technology(Beijing), Beijing, Beijing, China (Mainland), Yipeng ZHANG; China University of Mining and Technology(Beijing), Beijing, Beijing, China (Mainland)

The subsidence and deformation characteristics of strata with thick unconsolidated formation is quite different from that with consolidated formation. In this paper, based on the engineering background of large mining height mining of No.21 coal seam in Zhaogu No.2 Coal Mine, Henan Province, China, the PFC 2D numerical simulation and surface subsidence field observation was conducted to study the subsidence characteristics of strata with thick unconsolidated formation (680m). Numerical model with dimensions of 2000m×750m was established and mining process with different working face lengths (150 m,200 m,250 m,300 m) and different working face advancing distances (40 m,80 m,120 m,160 m,200 m,240 m,280 m,320 m,360 m,400 m,440 m,480 m) were simulated. The surface subsidence characteristics and displacement field variation of rock strata under different working face advancing distances were observed and analyzed. The results show that with the advance of working face, the structure of subsidence-like basin will be formed in each rock stratum, and the influence range of mining on rock stratum is “bowl-shaped”, and the influence increases with the increase of advancing distance; the form of surface subsidence movement is an obvious “basin” phenomenon, which has little effect far from the working face. The unconsolidated layer has a buffer effect on the mining-induced subsidence, and the buffer effect is realized by the expansion of the subsidence area. The movement law of strata under different working face lengths is similar to that under general mining, and the structure of subsidence-like basins will also be formed in each stratum. The influenced area of strata after mining is 'bowl-shaped', and with the increasing of working face length, the influence extent is greater. The internal subsidence of the unconsolidated layer is more intense and complex than that of the rock layer; the surface subsidence movement shows an obvious 'basin' phenomenon. The field measured data of surface subsidence also confirm the above results.