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By Peter Zhang

Pumpable cribs are being increasingly used in longwall tailgate entries and bleeders for roof support under abutment pressure. Their high peak capacity and confinement-controlled yield characteristics from reinforced-bags make them relatively safe and reliable standing supports for highly productive longwall panels. The pumpability of crib material over a long distance from the surface greatly reduces material handling and makes the underground installation more efficient and flexible. However, the performance of pumpable cribs is dependent upon crib properties, crib pattern, quality of installation, and geological and mining conditions. For proper crib selection and safe support design, it is critical to understand the basic technical requirements and performance of pumpable cribs in longwall tailgate entries and bleeders. Based on more than 10 years of experience using pumpable cribs for longwall standing support, the requirements and performance of pumpable cribs in longwall tailgate entries and bleeders under different mining conditions are presented. Experience shows that the load-displacement curve from full-scale lab testing is a good indicator of pumpable crib suitability for certain applications. Crib peak capacity, stiffness, and residual characteristics define its applicability to certain gate entries and bleeders under different cover depth. Crib pattern and spacing can be determined by abutment pressure level, roof, floor, and rib conditions, local roof stability, and ventilation requirements. To achieve proper in-situ performance, quality control of crib installation is also important.

Jan 1, 2012

By Bill Stormes

This paper will discuss implementation of the Hilti OneStep rock anchor technology to improve face bolting operations during the May 2008 longwall move; increase rock anchor installation capacity; and improve safety by reducing employee exposure periods by utilizing the Hilti OneStep rock anchors.

Jan 1, 2008

By Kazem Oraee

Falls of ground have historically been the main cause for fatalities in underground mines. Although recent advances in technology have reduced the number of such accidents, when failures occur they usually result in severe consequences. Risks of rock falls, use of heavy machinery and electrical apparatus, entry to confined spaces, working in noisy and dusty environments, and working on unstable platforms are some of the hazards in ground control operations. Managing these risks requires a management regime involving strict adherence to operational codes of practice and an enshrined culture of safety. These should be subsequently reinforced through active participation of management, systematic training, and stringent internal and third party auditing. Accident reports show that the major contributing factor in most rock fall accidents is the failure to adequately manage known risks due to the lack of a systematic process. OHSAS 18001 (Occupational Health and Safety Assessment Series) is an international occupational health and safety management standard specification to develop occupational health and safety at the workplace. This standard is intended to help mine operators control occupational health and safety risks. Addressing the requirements of OHSAS 18001 can be a complex and demanding task. A comprehensive guide for efficient and accurate implementation of this standard is provided in this paper. The discipline can be used to establish an accountable management system foreground control activities in underground coal mines. The paper also provides guidelines for preparing necessary documents, devising safety policies, procedures, performing risk assessment, and handling instructions. Finally, the paper concludes by providing a sound analytical basis in terms of the creation of a robust safety management system foreground control operations in underground mines. Full benefits of implementing an effective and systematic operational health and safety management system are illustrated. The procedure adopted and prescribed in this paper can be used in all underground coal mines where lack of appropriate ground control practices can create deficiencies in both safety and productivity.

Jan 1, 2012

By Peter Fischer

Between 45,000 and 50,000 claims for subsidence are submitted each year to the German hard coal industry. In order to improve the service to claimants a quality management was established in 2005. This paper describes the process now used for handling these claims. A quality check has also been carried out by way of a customer survey, which was performed by an independent research agency. The results of the survey are presented in this paper. In Germany, research into mining subsidence mainly involves the prediction of ground movements caused by deep mining operations. Special areas of research are subsidence reducing aspects of the extraction panel planning, especially from the point of view of mining dynamics, as well as securing and restoration measures. This work seeks to prevent or minimize mining-induced damages close to the surface, for instance, buildings or infrastructure installations (such as pipelines), or underground mine openings (e.g. shafts). Roof collapses and surface fractures caused by shallow mine workings of an earlier era currently constitute a special area of investigation.

Jan 1, 2006

By Yi Luo

A longwall mining operation went under a tall, self-supporting and free-standing wireless communication transmission tower. In anticipation of that the ground subsidence process might affect the transmission tower, the mining company took a proactive approach to mitigate the expected subsidence influences. The efforts include prediction of surface subsidence process at the location of the tower, assessment of stability, structural integrity and functionality of the tower structures, and design and implementation of mitigation measures. The intension of this paper is to demonstrate the techniques that can be used to assess and mitigate the subsidence effects on tall self-supporting and free standing tower structures.

Jan 1, 2008

By John Hoelle

Coal mines in southern West Virginia, south-western Virginia and eastern Kentucky have experienced coal bumps at least since 1933. Most of the bumps have occurred due to high cover, strong roof and floor strata and stress concentrations due to the mining sequence. A longwall mine in eastern Kentucky first experienced coal bumps on the tailgate side of the longwall face in 1989. The bumps continued until 1996. The bumps were the result of: Thick overburden up to 2200 feet Strong roof and floor (strata strengths up to 25,600 psi UCS and elasticity modulus up to 4,800,000 psi Previous over-mining in places Sandstone channels Not all characteristics occurred simultaneously. The bumps produced seismic events recorded up to 4.3 (Richter scale magnitude), and damaged pillars that were up to 147 by 152 feet in size.

Jan 1, 2008

By Dennis R. Dolinar

Screen material in the form of welded wire mesh and geogrids are used in underground coal mines to prevent the fall of small pieces of rock from the roof between roof bolts. Further, if the screen is installed during the production cycle, roof fall injuries can be reduced significantly. Therefore, the National Institute for Occupational Safety and Health (NIOSH), because of the safety implications, conducted an evaluation of screen materials commonly used in U.S. coal mines to determine their support characteristics and identify the parameters that could affect their performance with respect to controlling the fall of rock from the roof surface. To evaluate the load and stiffness characteristics of the screen, a test frame was designed and installed in the Mine Roof Simulator (MRS) at the NIOSH Pittsburgh Research Laboratory (PRL). With this test set up, the screen is bolted at four corners of the frame and a center load is applied. The test set up allows for up to 20 in of screen deflection while the load and screen defection are continuously recorded. A series of tests were conducted to evaluate the effects of various parameters such as bolt tension, the type of load bearing surface and the size of bearing plates on welded screen performance. The most common screen material used in U. S. coal mines is an 8-gauge wire welded in a 4-by-4-in spacing or aperture. The peak screen load was normally limited by wire breakage. The conditions at the bearing plate influence the nature of the wire breakage. However, the screen stiffness was controlled by slippage at the bearing plates, and by weld and wire failure. The size of the bearing plate had a significant effect on the performance of the welded screen. Therefore, the design capacity of the 8-gauge screen is evaluated based on plate size. For a 6-by-6-in bearing plate the average peak load is 2,900 lb with a stiffness of 250 lb/in. For an 8-by-8-in bearing plate, the average peak load is 4,500 lb with a stiffness of 430 lb/in. These test results are based on a 4-by-4-ft bolt spacing. A geogrid mesh was also tested.

Jan 1, 2006

By D. Su

This paper presents the details and results of a preliminary investigation into the response of borehole pressure cells (BPCs) in soft coal. Two parallel approaches, laboratory calibration and nonlinear finite element simulation, were adopted for deter- mining the BPC K-factor, which was defined as the ratio of cell pressure change to the corresponding change in external pressure. Results from both the laboratory calibration and the nonlinear finite element simulation revealed a nonlinear response of cell internal fluid pressure to external loading. The K-fact& also is shown to increase with increasing setting pressure and decreasing medium modulus. -Depending on the setting pressure, the K-factor for coal having a Young's modulus of 1.6 x lo5 psi ranges from nearly 1.2 at 2,000 psi of external pressure to 1.485 at 8,400 psi of external pressure. The improved under- standing of BPC behavior leads to the more accurate calculation of BPC K-factor in coal which significantly improves the correlation between pillar pressures measured in underground coal mines and those predicted by nonlinear finite element simulations of these pillars.

Jan 1, 1990

By James Pile

Long, high capacity, highly tensioned, cable bolts (strand bolts), stiff enough to be installed with resin like a solid bolt, yet flexible enough to be used in any lengths, have been widely used in Australian coal mines since the early 1990?s. A brief history and update on recent developments of this technology is provided. The first major US trial of this Australian technology, has recently taken place at BHP Billiton?s San Juan longwall coal mine in New Mexico. Thirty-five foot long, high capacity, highly tensioned, post-grouted cable bolts are being used to provide active secondary support of intersections in areas where operator safety and life of mine considerations are paramount, and standing support cannot be installed for reasons of equipment access. The cable bolts are resin anchored into overlying sandstone channels, tensioned, and post-grouted. The installation procedure and support results for these highly tensioned cable bolts are given.

Jan 1, 2007

In this paper, the authors attempt to apply a simplified non-homogeneous void diffusion model (NHVDM) to predict surface subsidence for longwall mining in Illinois. A generalized error function subsidence profile is derived from this model. The model validation indicates that the NHVDM approach appears to fit the observed subsidence data as well or better than the error and the trigonometric functions, particularly around the trough edge areas.

Jan 1, 1990

By Daniel Barkley

Underground coal mining in Illinois has a lengthy history dating back to the early 1800?s. High extraction underground mining has played a significant role in production through out this time. Modern mechanized longwall mining methods were introduced in the 1960?s and have been in continual use ever since. Regulations governing subsidence from underground coal extraction and the mitigation of resulting impacts to surface structures and surface land initiated in 1983. Despite the history of experience with subsidence, recent plans to longwall in Illinois have stirred public emotions and strong opposition. Anti and pro longwall groups formed and divided a community over the potential economic benefits new mining ventures could bring verses the potential problems associated with longwall mining in productive flat farmland. The debate culminated in a county wide referendum vote on the use of longwall mining. This paper provides an overview of the history of underground mining in Illinois, the controversy that has risen in central Illinois regarding longwall mining, and the efforts to educate the public about regulations that govern longwall subsidence.

Jan 1, 2007

By Robert B. Francini

Longwall mining and pipelines have coexisted since longwall mining was introduced into the United States. How the pipeline operator reacts to longwall mining under their line depends on many factors. This paper outlines the process the pipeline operator goes through to determine the appropriate response when their pipeline is being undermined. Typical mitigation procedures are reviewed. Finally, the paper looks at current design practices for new pipelines that are expected to be undermined during their lifetime.

Jan 1, 2011

By A. Wahab Khair

Coal mining has evolved into extensive use of continuous mining machines for development and production. This has resulted in an increase in dependence on the versatile continuous miners. The present-day continuous miners are powerful in terms of rated power but are still plagued by wear of bits which hinders development and production as the bits have to be continuously changed at regular intervals. Previous attempts to analyze bit wear has been either restricted to the theoretical approach and or laboratory testing. This paper discusses about the overall aspect of bit and drum design for different conditions. Using the laboratory model set up, numerical models and field testing, a new bit and drum design was analyzed to evaluate their performance under different conditions. The results proved that a newer bit design and drum design were successful in limiting the amount of wear as well as improving the performance of the continuous miner and production.

Jan 1, 2008

By Yajie Wang

Traditionally, roof falls are usually attributed to bad roof strata and high vertical stress related to the overburden depth. However, roof falls under shallow overburden depth are oflen caused by a high horizontal stress field. Recent studies have proven that the high horizontal stress field, if existing, plays a major role in roof stability. Also the roof conditions are related to the entry diredions under a horizontal stress field. Questions are: How to orient the entry to a favorable direction to prevent the roof falls? If the entry cannot be oriented, how to support it? Through a case study in mine A, this paper analyzes the horizontal stress effect on roof stability. The guideline for the entry orientation is given based on the results of analysis. Additionally, suggestions are made for those entries that cannot be oriented to a favorable direction.

Jan 1, 1998

By Hamid Maleki

Having developed a methodology for estimating mining-induced seismicity resulting from slip along geological discontinuities in 2003, in this paper, the importance of measurements are high-lighted in the study of caving and seismic source mechanisms as applied to mine designs. Geotechnical data is analyzed from four case studies to address cave condition, load transfer, and potential effects on stability and resource recovery in four Utah mines using measurements and observations conducted over the last three decades. Cave conditions are evaluated on the basis of measurements in the gob of a western U.S. operation mining under the massive Castlegate Sandstone and indirectly using the results of measurements and boundary-element modeling at panel boundaries at a mine where strata behave elastically. Together with subsidence measurements and microseismic monitoring, regional caving and load transfer in multiple-panel extractions are addressed while the effectiveness of barrier pillars in reducing stress and seismic response where mining under massive stratigraphic units is analyzed using measurements and three-dimensional computer modeling.

Jan 1, 2006

By Mikhail P. Nesterov

The paper presents the results of the research that can forecast the development of subsidences and deformations of the earth surface with time in the permanent marginal areas of the movement troughs arising when mining sylvinite beds in the Upper Kama deposit of potash ores where the rock formation support is made by means of interchamber (rib) pillars of various yieldability.

Jan 1, 1997

By Ry Stone

To maintain longwall continuity, NRE Wongawilli Colliery extracted the approximately 20-year-old Thompson Mains and in doing so, 20 roadways were holed into during longwall retreat. The orientation of these roadways with regard to the longwall face varied between (i) a parallel alignment where the entire roadway was simultaneously subjected to a significant increase in vertical stress on holing and (ii) a more favorable high angle of alignment, meaning that only a portion of the roadway was subjected to an increase in vertical stress on holing. In addition, the longwall holed into and was recovered in one of the existing roadways. The geotechnical environment presented several additional challenges during longwall retreat. The orientation of the stress field resulted in a major horizontal stress notch on the maingate and an elevation in the major horizontal stress in front of the longwall face during the retreat up to an adjacent goaf edge. The presence of geological structure zones also resulted in both swings in the major horizontal stress direction and increased levels of roof deformation. The gate road support designs were based on commonly accepted strata management techniques and principles. The support designs in the hole-through roadways were based on empirical design models generated from (i) aconceptual geomechanical model of pre-driven hole-throughand recovery roads and (ii) a worldwide database on pre-driven hole-throughand recovery roads. Imperative to the success of extracting the panel was a sitespecific strata management process. In this regard, consideration was given to the following: roof displacement monitoring in the gate roads (in particular, Longwall (LW) Acceleration Position), roof-to-floor convergence monitoring in the hole-through roadways, the ability of the mine to limit downtime prior to and during holing by utilizing an Authority to Mine (ATM) type process, and the use of Trigger Action Response Plans (TARP).

Jan 1, 2012

By Andrzej Walentek

This paper presents the results of numerical modeling of the fractured zone around longwall gateroads. The numerical computations have been based on the results of underground measurements of the fractured zone around longwall gateroads, which were performed for various geological and mining conditions and measured with the use of the endoscopic method with a digital camera. Having results of the underground investigations, trials of duplicating them by the Finite Element Method using software PHASE and the Hoek-Brown failure criterion were performed. In order that the fractured zone predicted by the model-based computations closely simulate that measured underground, some selected parameters in the Hoek-Brown failure criterion were modified based on the earlier determined relations used for prediction of the roof-to-floor convergence in longwall gateroads. As a result, the model-based computations produce results very close to those obtained from the underground measurements. Determination of the fractured-rock zone around the mine roads is of great importance to mining practice, especially in planning and designing supports for longwall gateroads. The influence of the front abutment pressure causes the fractured rock zone around the roads to increase and results in an increase of support load. Moreover, proper determination of the fractured rock zone is one of basic elements in the process of planning roof bolting for mine workings.

Jan 1, 2009

By Brian R. A. Wood

Any defect activity resulting from an applied stimulus on a structure will generate a stress wave which travels in the medium under the laws of acoustics, thus the name of acoustic emission (AE). AE techniques can therefore provide information on the deformation of both surface and below ground structures, Two operating coal mines were monitored from the surface, and both mining activity and post mining microseismic activity were detected. The nature of the detected AE signals demonstrated that there were a range of deformation mechanisms in operation and that some predictive capabilities were possible with the technique.

Jan 1, 1992

By Rob Thomas

Pre-driven recovery roads are used to speed up the longwall recovery process by reducing bolt-up requirements and by improving ground conditions on the face during the removal of the powered supports. To lower the geotechnical risks associated with the use of pre-driven recovery roads, and so improve its overall acceptance in the industry as a viable alternative to more conventional, potentially difficult and costly longwall recoveries, this paper outlines (i) a geomechanical model for the use and design of pre-driven recovery roads, (ii) an updated version of the worldwide database on pre-driven recovery roads, (iii) a summary of the various methods of holing a longwall into a pre-driven recovery road and (iv) a series of relevant ground support and strata management guidelines.

Jan 1, 2008