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By R. D. Yancich

A case study of longwall mining surface subsidence was performed at a mine in Southwestern Pennsylvania. The mine operated in the Pittsburgh coal seam which averaged 70-72 inches with 6-8 inches draw slate in the longwall location. The average overburden depth ranged from 625 8eet to 660 feet. Three adjacent longwall panels were studied and classified as Panel 1, Panel 2, and Panel 3. The following is information on each and can be seen on the General Mine Map (Fig. 1).

Jan 1, 1984

By Luis Giraldo

The installation procedure of fully grouted rebar bolts requires that the bolt be pushed through the resin grout until it is fully inserted in the borehole. Resin grout is a highly viscous fluid that is forced to flow through the annulus between the bolt and the borehole as the bolt advances. Variables such as bolt insertion speed, bolt vs. borehole diameter and grout viscosity determine the magnitude of the pressure generated ahead of the bolt tip. Under weak rock conditions, the magnitude of the pressure of bolt insertion is often sufficient to initiate hydraulic fracturing along joints or bedding planes that allow injection of grout into the strata leading to significant loss of bolt encapsulation. The grout pressure front has also been linked to gloving, a bolt installation problem that reduces bolt effectiveness. A series of tests were conducted to characterize the pressure of bolt insertion during bolt installation under various conditions. Analysis of the data shows three distinct stages of pressure build up during bolt installation. Understanding of the mechanisms involved may allow development of strategies for controlling the pressure buildup and its negative impact during roof bolting under weak rock conditions.

Jan 1, 2006

By Ming Ju Liu

China has been suffering from the most serious coal and gas outburst accidents. The situation has become worse than ever with the mining of deeper coal seams at higher production rates in a more complex underground mining environment, in order to meet the huge national demand for coal-fuelled power plants. In the year 2007 alone, more than 30 outburst accidents occurred in underground coal mines with a death toll of 258, and in the year 2008, as of May 22, nine outbursts have already occurred in Henan, Hubei and Chongqing, killing 68 workers. At the same time, China also experienced new trends of coal and gas outburst accidents since its coal production boom in the year 2001. These trends are of great importance for government agencies, coal mine owners and operators to take initiative and effective measures to prevent and control the occurrence of coal and gas outbursts in China?s widely spread outburst prone mining areas. This paper presents outburst data of China coal mines since 2001 and describes new trends of outburst occurrence in recent years, which include outbursts coupled with shocks or rock bursts of competent roof, outbursts in mines under development, outbursts in new mines and mines that have never suffered outbursts before, outbursts in mines that are believed to be of low gas content as well as outbursts in more long-wall faces. Based on the observation of outburst occurrence in China in the period from 2001 to 2008 as well as mining and geological conditions, detailed analysis of the possible causes that led to those new trends of outburst accidents is conducted. The analysis helps shed light on possible future remedies and the necessary measures to alleviate or eliminate outburst accidents in underground coal mines. Suggestions are put forward on future studies that could be of interest to national and provincial government agencies regarding strategic policies, proper technical management practice, identification of outburst prone coal seams, online monitoring of impending outbursts as well as prevention and control measures.

Jan 1, 2008

By Tex M. Kubacki

In 2007, the University of Utah Seismograph Stations (UUSS) temporarily expanded its seismic monitoring network in Emery County, Utah, in response to the August 6 Crandall Canyon mine collapse that trapped six workers. In spite of this additional coverage, no seismic events were located in the area during a six-day period (gap) from August 8 to August 13. The Mine Safety and Health Administration (MSHA) ?Command Center Log Books? however, contain references to 16 instances of underground ?bumping? and ?bouncing? that occurred during the gap. When ground truth information provided by the log books is compared to the raw seismic data recorded by the UUSS network, the correlation offers insight into the chronology of mininginduced seismicity (MIS) in the aftermath of the collapse. The new research focuses on identifying and locating seismic events that occurred during the six-day gap using double-difference relocation techniques and correlating them with log book ground truth. Ten seismic events were identified in the vicinity of the mine during the six-day gap, ranging in magnitude from coda magnitude (MC) 0.5 to 1.4. Three of these events were amenable to location by double-difference techniques. The location of one of these events is directly confirmed by ground truth information from the log books. As a result of this new research, the accuracy of Crandall Canyon double-difference locations has been improved, and the number of events correlated with ground truth information has increased. The seismic events occurring in and around the collapsed area appear in two distinct sections of the mine?east and west of the center of collapse. It was originally reported that the post-collapse events occurred in episodic clusters. The first two episodes occurred to the east, followed in time by a cluster on the west, and then by a third episode on the east. With the addition of new events to the data set, this pattern is no longer as clear, although the events still occur in clusters. Only one MIS cluster was found to occur within the bounds of the minimum estimated collapse area (Pechmann et al., 2008).

Jan 1, 2012

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 John Popovich

The MICON RokLok binder system including component and consolidation processes and safety and its first aide are described. The implementation procedures and selection of injection parameters are discussed. Finally a case example of recovering a 30-ft. long roof fall in mainline haulage entry is used to illustrate the economic potential of preventive measures by RokLok injection.

Jan 1, 2001

By Anil K. Ray

This paper discusses in situ studies conducted at an Illinois Basin room-and-pillar coal mine to evaluate the performances of three different types of bolts under similar geologic conditions. The bolt types considered were the fully grouted passive rebar (FGPR), the resin-assisted mechanical anchor (RMAB), and the fully grouted tension rebar (FGTR). During the evaluations, the three types of bolts were installed in a single entry covering a span of three contiguous pillars with almost identical geologic conditions. The performances of the three types of bolts were investigated by instrumentting the bolts prior to installation as primary supports on cycle. Additional data on roof behavior was collected with extensometers and shear meters placed near the instrumented bolts. Excluding some time lapse right after installation, the performance evaluation of the three bolt systems was based on the data collected for over a year. Based on this study, it was found that the axial loads measured in the different bolt types did not show any identifiable patterns under the prevailing geological conditions at the mine. Therefore, the assumption that active bolts must be superior to passive ones is not always valid. Even though the studied mine did not have any thinly laminated, weak rocks in the immediate roof, the passive bolts performed as well as the active systems.

Jan 1, 2012

By Clive Chadbourne

Investigations of world wide coal face strata control practice, the study of theoretical calculations and detailed engineering analysis have been combined to design and select a powered roof support suitable for use in conjunction with the extraction of 4.5-m (14.7- ft) of the 7.5-m (24.6-ft) thick Warwickshire Thick Coal Seam. The interface between geotechnical criteria and engineering design has been closely considered along with the following 3 important factors: Firstly, the need to further improve strata control and operational characteristics whilst producing a wall integrated overall face design. Secondly, to enable the supports to be rapidly transported from face to face in roadways driven at the minimum size for operational requirements. Finally, to provide hydraulic power, automatic control and monitoring systems which will improve operations reduce manpower requirements and improve management information. Clear design requirements were determined in order to allow selection on a commercial basis. A method of tendering is described with the subsequent analysis and selection process. In the event oversupply of coal in the UK resulted in the order not being placed. Brief consideration has been given to supports suitable for reserves to be mined In a few year's time where there is potential for extraction well in excess of 4.5-m (1 4.74.

Jan 1, 1994

By Andrew P. Schissler

The Citronen Zinc-Lead Project is one of the world?s largest undeveloped zinc-lead resources, containing more than 13 billion pounds of zinc and lead metal. The project is located in northern Greenland, a self-governed region of the Kingdom of Denmark (Figure 1). Citronen is the northern-most mine in the world, operating at a latitude of 83o 3? N. In excess of 67,000 meters of diamond drilling have been completed to delineate the resource. As of January 2012, the measured, indicated, and inferred reserves for the Citronen project totaled 70.8 million tonnes, consisting of 5.7% zinc + lead (3.5% zinc cut-off) (Ironbark, 2012). For the underground mine plan, room-and-pillar and long holestoping mining methods were selected. The beneficial application of structural fill was considered for increased recovery, efficient tailings disposal, and ground control. Three types of fill were compared - paste fill, cemented rock fill, and iced tailings. An environmental and economic feasibility study concluded that iced tailings was the preferred alternative provided it met the ground control requirements. The purpose of this paper is to explain the investigatory steps that led to the possibility for the use of iced tailings as structural fi

Jan 1, 2012

By Thomas A. Bailey

Since the advent of longwall mining, various methods and materials have been used during relocation to control the intrusion of the gob in the shield/panline area. (figure 1) Controlling this gob is critical to ensuring a safe, speedy, and productive longwall move. An uncontrolled gob can cause a number of problems, from increased worker exposure to entrapment of equipment to lost production. With an eye toward worker safety and increased productivity, several different methods have been developed to minimize gob intrusion. This presentation will take a look at the history and development of longwall ?screens? and ?mesh?, from chain-link fencing to high-modulus polyester geogrids. The benefits and advantages of these developments will be defined. Since 1994, longwall coal mines have used polymer geogrids during longwall recovery to control intrusion of gob material into the shield/pan line area. As described by Brandon and Collins (1996) in their paper presented at the 15th Ground Control Conference in Golden, CO., mines had used various forms of support, such as welded wire mesh, chain-link fencing, wooden cribbing, props, and steel cable to accomplish this. These methods worked, but were labor-intensive, slow, and provided varying degrees of gob stoppage. Although a few mines still use chain-ink fencing to control the gob, longwall coal mines have generally settled on polyester geogrids to control gob and roof and these mines have been instrumental in providing input in order to develop and improve these geogrids products. he trend during the past twelve years has been to incorporate stronger grids and to eliminate auxiliary reinforcements, such as steel cables and nylon or polyester webbing. A job that once required heavy steel cables, props, and fencing is now accomplished with strong, lightweight, high-modulus polyester grids. When we discuss high-modulus grids, we refer to the ratio between load and strain, as illustrated in figure 2. These grids are woven or knitted from yarns that have been pre-stressed. When these grids come under load, they pick up the load more quickly. This allows less deformation in, in this case, the mine roof. The same principle applies when these grids are used to control the ribs. With less elongation, the grids will hold rib material tighter, thereby keeping travel ways more open.

Jan 1, 2006

By Ray Brandon

The recent introduction of polymer geogrids for ground control purposes in underground mines allows mine operators more flexibility in choosing improved products for several applications. This is evidenced most clearly in longwall recovery operations. Since November, 1994, many mine operators have chosen Fortrac geogrid assemblies for the purpose of securing longwall recovery areas prior to shield removal. Fortrac is a lightweight, strong, and flexible polyester geogrid available in many strengths. Utilizing the "diamond" seaming method, different strengths of Fortrac geogrids are easily fastened together so stronger geogrids may be placed in the most critical areas and lighter geogrids in less critical areas.

Jan 1, 1996

By N. P. Kripakov

Results of a complex three-seam interaction analysis in a western U.S. coal mine are presented. The physical situation simulated by the models represents the development of a set of entries underneath two previously mined seams with all three seams in proximity to each other. The layout and the geometry of the remnant pillars in the upper seams were nonuniform and not always directly superimposed over each other, creating a very challenging modeling problem. The U.S. Bureau of Mines' MULSIM/PC computer code was upgraded and enhanced to perform the required three-seam interaction calculations. Analysis was limited to simple linear-elastic assumptions as only peak stresses were required. Results clearly indicated that stress concentrations in the lower seam increased approximately 100 percent when compared to cases simulating no extraction in the two upper seams. These stress concentrations generally occurred in zones directly below points of highest abutment pressures in the upper seams. Overall, the model results compared favorably with actual documented observed behavior where roof failures were known to have occurred.

Jan 1, 1996

By A. T. Iannacchione

Underground stone mining represents an emerging sector for the U.S. mining industry. As this expansion takes mines under deeper cover and as more efficient mining methods are utilized, adequate stone pillar design methods will become more important. It is the purpose of this paper to examine current design practices and to discuss issues for safe mine layouts so that a rational first approach towards balancing the demands for increased production can be weighed against increased risk to worker safety from rib instabilities and pillar failures. Seventy-two stone mine pillar designs were examined. Pillars with width-to- height ratios below 1.5 and subjected to excessive stress levels appear more likely to fail. When width-to- height ratios fall below 1 .0, defects in the pillars, such as through-going discontinuities, can have a significant influence on stability. Discontinuity persistence, dip, material properties, and orientation are important factors controlling pillar strength. The influence of discontinuity dip, a characteristic easily identified in the field, was examined so that its presence could be accounted for in developing generalized guidelines for pillar design.

Jan 1, 1999

By Thomas H. Wilson

Common offset and refraction seismic data have been collected over four longwall panels at two different locations within West Virginia. Surveys prior, during and after mining were made in both areas. The velocity of seismic wave propagation in the overburden rocks is observed to decrease in response to panel emplacement. Variations in the magnitude of this reduction, apparent as an additional first arrival time delay in the after mining common offset surveys, do not appear to correlate directly to the amount of subsidence that has occurred. Velocity reduction and signal absorption zones are also produced in the cover beyond the panel face over the adjacent unmined areas. Signal absorption and velocity reduction zones have also been observed over room and pillar entry areas between mined panels. Velocity reduction zones have been- observed beyond the panel face in both shallow and deep mines, but the signal absorption zone has not been observed over the deeper mine. Changes in the compressional wave velocities of the cover rocks are associated with changes in their elastic moduli. Differences between borehole derived and surface seismic derived interval velocities, are related to differences in the properties of the bulk volume sampled by each method.

Jan 1, 1988

By Theresa M. Bodus

The strength of roof shales is, in part, a function of the preferred orientation of clay minerals within them. Therefore, analysis of clay fabric under both air-dried and hydrated conditions should be helpful in understanding roof shale failure. Core samples were collected from the Energy, Anna, and Lawson shales, which are locally present as roof shales of the Herrin No. 6 coal of southern Illinois. The Clay Fabric Index (CFI) was measured for thirty core samples, using X-ray diffractomtry, to investigate the relationship between clay fabric and roof collapse in coal mines. Greater CFI values indicate a weaker preferred orientation among clay minerals, which generally results in a weaker inter-grain bond. Average CFI values for air-dried samples were: 0.15 for Anna shale; 0.20 for Energy shale; and 0.37 for Lawson shale, .After the introduction of water, average CFI values increased to: 0.28 for Anna shale; 0.22 for Energy shale; and 0.43 for Lawson shale. The laboratory results indicate the following: [l] under air-dried conditions, Anna shale is the most stable, followed by Energy and Lawson shales; and 121 under hydrous (mine) conditions, Energy shale is the most stable, followed by Anna and Lawson shales. In mine environments where roof failures involving Energy shale have occurred, planar trends delineated by a relatively high density of small (0.2-4 cm diameter) iron concretions have been reported along the failure surface. Radial cracks have been observed around unsealed anchor bolt holes in the Anna shale. Instability of the Lawson shale is ubiquitous in coal mines. In the laboratory, similar physical changes were noted in the shale samples after the introduction of water, including [l] increased hydration around the perimeter of iron concretions in Energy shale, [2] radial extension cracks in Anna shale, and 131 extreme slaking of Lawson shale. Measured increases in the CFI reflect the observed physical changes of the samples upon the introduction of water, which probably reflect changes in strength that may result in roof failures in hydrated mine environments.

Jan 1, 1989

By W. Smith

The U.S. Bureau of Mines (USBM) is using laboratory and computer-based techniques to investigate the post- failure behavior of yielding pillars and pillar reinforcement techniques to control entry deformation. Poorly dimensioned yield pillars, can cause adverse ground conditions. Laboratory studies to evaluate various reinforcement techniques for controlling yield pillar deformation were conducted. A cracking and crushing model is used to evaluate potential tensile and compressive failure planes within pillars of various widths to assess the degree of yielding and stress redistribution. The use of an elasto-plastic model to evaluate the effect of various reinforcement on yield pillars is also discussed. These procedures effectively account for the residual pillar strength, important in yield pillar design.

Jan 1, 1995

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

By Nic Slatalla

Resin anchored rock bolts are an effective tool to control the deformation of an underground opening because they combine high support capacity with fast installation. However, there are still numerous shortcomings concerning the understanding and interaction of the system bolt-resin-rock. Field and laboratory test series have been conducted for evaluating the load capacity and bond strength of resin grouted rock bolts in different host rocks under axial load. This paper describes that the behavior and shear strength of the system bolt-resin-rock depends on the rock type and discusses a simple correlation between peak shear strength and shear stiffness in the ratio 1:1000 which may moreover be helpful for estimating rock bolt capacity after installation.

Jan 1, 2006

By Rob Thomas

Strata management is a critical and now necessary requirement in the New South Wales underground coal mining industry. As a formal and structured process, strata management allows ground support and mining methods to be tailored to the anticipated and encountered geotechnical environment in a controlled manner. In its application, strata management therefore enables operators to identify and control geotechnical hazards and the risks they pose to the mining operation, including risks to personnel safety and mine production. The paper considers recent developments in strata management in the New South Wales underground coal mining industry. Points covered include the statutory requirements stipulated in the Coal Mine Health and Safety Regulations, Geotechnical Hazard Plans, tiered support plans, the Authority to Mine concept, Trigger Action Response Plans and monitoring. Several successful and unsuccessful case studies are included which make specific reference to roadway development and longwall retreat and in doing so, the unique demands imposed by these two different aspects of the mining operation on strata management.

Jan 1, 2007

By R. Kanniganti

The primary problems of pillar design in seams subject to interaction are due not only to difficulties in predicting loading, but also to the often unsymmetrical nature of the stress fields. The effects of uneven stress fields on failure patterns within a pillar were studied wing a two dimensional finite element method. Lower seams, including ultra- close mining situations were evaluated considering conventional pillars. Pillar case study data for under mining were analyzed to determine if empirical trends in stable pillar sizing could be derived. Recommendations for multi-seam pillar design are made, based on anticipated loading conditions within a pressure bulb. Applications of the research results are demonstrated using case studies

Jan 1, 1995