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By Tingkan Lu

To understand the reinforcing mechanisms of the fully grouted resin bolt and clarify its performance in weak roof strata, the field monitoring investigation using instrumented bolts has been conducted in a coal mine. The total time of monitoring was around ten months. In the first 8 month period, the rock bolt performance relates to the initial development of the gateroad and subsequent time dependent strata response. After 8 months, the rock bolt performance relates to the effect of the approaching longwall mining operation, up to the point where the measuring station was destroyed due to the mining operation.

Jan 1, 1998

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 Xingtian Hui

Due to variation in the geological natures of underground coal mines, especially when the entry?s roof/floor are soft and weak, it is vital to select an appropriate roof bolting system to ensure entry stability, coal production, and miner?s safety. No single type of roof bolting system perfectly fits for various types of openings, even within a mine. From the results of continuous research on roof bolting in the past 10 years, the authors used a novel concept of ?Self-screwed? mechanism to develop five types of roof bolting under a patented Self-screwed Tube Bolting System (STBS)©. This roof tube bolting system can be applied to meet different timing-strength requirements under varying mining conditions to increases the stability of mine workings. This paper outlines the structures and mechanisms of the five types of self-screwed roof bolting systems along with their unique characteristics, advantages and disadvantages, and limitations. Two case studies using a Self-screwed Tube Drilling Bolting (STDB)©, one of the five types of roof tube bolting systems, show the principles of mechanics, suitability and parameters for a mine entry and a subway?s construction/ supports in the surrounding soil layers and soft sands, respectively are presented. The results confirmed the time dependent anchorage capacity between predictions and on-site measurements. Currently, other types of the self-screwed tube bolting systems are also being applied to different coal mines in China to increase entry stability and meet safety requirements.

Jan 1, 2009

By Francis Kendorski

The Skyline #3 Mine of Canyon Fuel Company in Utah was planning longwall mine development adjacent to and downdip of older mine workings known to be flooded. Barrier pillar geometries and widths were proposed by mining company personnel. Hydraulic conductivity testing suggested that the general characteristics of the local coal seam, dikes, and faults are only weakly conductive and that leakage through the barriers, if any, would be minor. The mechanical performance of the proposed barrier pillar design was evaluated according to (1) published empirical design methods and (2) numerical stress modeling. Comparison of the proposed design, with the range of barrier widths derived from the empirical methods, served as a first-order check on the adequacy of the proposed design. Numerical modeling was used to evaluate conditions site-specific to the Skyline #3 Mine and barrier geometry. Models were developed to quantify the relationship between barrier width and the abutment stresses onto the future workings. The models were also used to estimate the stress distribution within the barrier pillars. The hydraulic performance of the proposed barrier pillar design was evaluated according to (1) published empirical design methods for hydraulic impoundments, (2) empirical method estimates of seepage through coal barrier pillars, (3) numerical hydrogeologic flow modeling, and (4) numerical strain modeling. Using the techniques described above, and based on relevant industry experience, underground observations, knowledge of local geologic conditions, hydrogeologic measurements, and analytical results, the level of geomechanical risk associated with the barrier design as proposed was considered low and the level of hydraulic risk was considered moderate. The largest uncertainty that remained was the possibility that unfavorable geologic structures, such as faults and dikes, would act as conduits for leakage of impounded water into the new workings. Although the study indicated only minor steady-state leakage would occur through a small number of known structures, the presence and full leakage potential of threatening structures in the barrier could not be reliably known until the barrier was mined. One longwall panel has been completed adjacent to the barrier with no evidence of water flow even in fractured zones. Thus, as predicted by the modeling and analysis, it appears that abutment stresses imparted on the barrier during mining did not substantially alter the natural hydrogeologic characteristics, or leakage potential, of the barrier. The longwall coal mine water-barrier pillar design and performance are considered a success.

Jan 1, 2007

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 Thomas Barczak

A cooperative study was conducted with Emerald Coal Resources, L.P., an affiliate of Foundation Coal Corporation, and the National Institute for Occupational Safety and Health (NIOSH) to evaluate the effectiveness of (partially) pre-driven longwall recovery rooms supported with pumpable roof supports. This paper evaluates the load transfer mechanics associated with the advancement of the longwall face into the pre-driven recovery room and the widening of the room to recover the longwall shields. The results show that the yielding of the panel fender produced uncontrollable convergence that caused yielding of the shields and stiff pumpable roof supports when the face was approximately 10 ft from the recovery room. The shields continued to yield until the face advanced into the recovery room. Convergence in the pre-driven recovery room typically reached 6-8 in once the pumpable supports yielded and shed load through a declining residual load capacity prior to being cut out by the longwall shearer. Roof deformations occurred beyond the 32-ft horizon with total deformation ranging from 2 to 4 in, indicating that standing support was necessary to help control the span as the longwall face advanced into the pre-driven recovery room. Stability improved as the pumpable supports were cut out, thereby reducing the span between the shields and the outby pillars. In the end, the shields were successfully recovered under stable ground conditions. However, a disconcerting discovery was the load shedding of the outby pillars as the recovery room was mined into. It is postulated that this may be due to the progression of the rear ?abutment? moving toward the recovery room as the panel fender and standing support in the recovery room yield and shed load. This behavior was not expected with a relatively narrow room that is fully supported with standing support. If this mechanism is indeed occurring, the width of the room and performance of the standing support becomes even more critical. Premature failure of the support can lead to excessive convergence on the shield line that cannot be controlled even with modern shields, resulting in roof instabilities that can lead to catastrophic results.

Jan 1, 2007

By Stephen Iverson

The National Institute for Occupational Safety and Health (NIOSH) is currently performing research to help mine operators minimize the amount of loose or damaged rock surrounding a blasted opening. Improperly designed blast rounds can result in excessive wall rock damage that may reduce the rock mass competency and increase ground support requirements. Unnecessary blast damage can cause many safety and operational concerns including an increased potential for injury from rock falls and other hazards resulting from sub-optimal blast designs. This paper focuses on the ground control and safety implications of blast damage in underground mines. A reduction in rock quality due to blasting can increase the amount of ground support required. Case study data collected by NIOSH in underground metal mines during standard drill and blast production rounds included blast vibration monitoring, detailed three-dimensional laser scanning of the as-built excavation, and geotechnical mapping of development headings.

Jan 1, 2007

By Dave Minney

The South African mines health and safety regulations require that mine management establish a quality management system for rock support material. Quality control of resin capsules used as a support medium in South Africa is specified under SA National Standards No 1534. Experience has shown that this specification, while addressing the chemical and physical aspects of the products, but does not address the required performance criteria. In order to address this shortcoming, the industry has put a lot of emphasis on short encapsulation pull tests, performed insitu, in the underground workings of the mine. This is also fraught with many shortcomings, particularly as regards site availability, cost and repeatability of results. To try and address these issues, the Department of Mechanical and Aeronautical Engineering at Pretoria University were approached to develop equipment and machinery that could produce reliable, repeatable results in a laboratory environment which reasonably simulates installation practice underground. The test method is based on the ASTM F432-95 Strength Index test, but modified to use push test rather than pull tests. The paper describes development of the test method and the preliminary results and findings of the work.

Jan 1, 2007

By André C. Zingano

The coal pillars in room-and-pillar mining method are subjected to vertical stress due to overburden rock mass and horizontal stress relieve due to entries excavation around them. Some coal mining companies are applying rock bolts and steel straps to improve the pillar strength, consequently to reducing the pillar size and increasing coal recovery. This kind of approach deserves some discussion, whether it is pillar reinforcement or just rib support. The efficiency of pillar reinforcement (or rib support) is directly related to the initial load capacity of coal pillar, the type of excavation, and the width to high ratio of the pillar. The objective of this paper is to understand the mechanism of pillar reinforcement (or rib support) using rock bolts and steel straps on pillar ribs. Numerical simulation taking into account the relations listed above. The results showed that the increase of coal pillar strength is very short, and there is no influence on improvement of load capacity of pillar. However, the rib support avoids block falling, rib failure, and displacement, reducing the pillar failure propagation into the pillar center. Keywords: coal, pillar reinforcement, room and pillar.

Jan 1, 2006

By Christopher Mark

The first International Conference on Ground Control in Mining opened with the topic of pillar design. Two classic papers were presented, one by Bieniwski and the other by Wilson. Unfortunately, the two methods were so radically different from each other that it was nearly impossible to reconcile them. Adding to the confusion were the many other pillar strength formulas (such as the Salamon-Munro, the Holland-Gaddy, and the Obert-Duvall, just to name a few) that were also available. Little wonder that discussions of pillar design in those days often ended with anguished cries of ?but which formula is the right one?? The past 25 years have seen substantial progress in the science of coal pillar design. Indeed, one testament to the improvement is the relative scarcity of papers on the topic at recent Conferences. Two factors have been largely responsible for the progress that has been made. The first has been the collection of large data bases of actual case histories of pillar performance in a variety of settings, from shallow room-and-pillar mines through deep cover longwalls. These have made possible the development of empirical design procedures that are closely linked to real world experience. The second important factor is the development of sophisticated computer models that can accurately simulate pillar behavior and roof/pillar/floor interactions. Together, these two lines of research have led to a new understanding of pillar mechanics that identifies three modes of pillar failure: ? Sudden, massive collapse, accompanied by airblast, for slender pillars (width/height<4) ? Squeezing, or slow, non-violent failure, for most room and pillar applications (4<w/h<10) ? Entry failure or bumps for deep cover and longwall applications (w/h>10) It is particularly satisfying that the insights gained from numerical models broadly support those obtained from the empirical studies. While far less controversial than in the past, pillar design problems continue to arise. One recent example is pillar design for highwall mining. NIOSH has just released a software package, called ARMPS-HWM, which employs a number of modern pillar design concepts. Since highwall mining web pillars are long and slender, the greatest danger is that of a sudden collapse. ARMPS-HWM suggests two possible prevention strategies, one which concentrates on the SF of the webs, and the other which creates a ?pressure arch? using properly sized barrier pillars. The paper will close with a discussion of some current needs in coal pillar design, including: ? Updating older empirical methods, such as ALPS, where changes in technology (new types of roof support, more demanding ventilation requirements, faster retreat rates) may have made some of the original case histories obsolete. ? Methods for determining site-specific coal strengths, focusing on bedding plane strength and other factors that may effect confinement, as input for both empirical and numerical design. ? Improved methods for evaluating coal pillar performance for environmental issues, such as surface subsidence and hydrologic impacts, which consider such factors as depth, w/h ratio, water immersion/drainage, and time dependent seam strength.

Jan 1, 2006

By Stephen C. Tadolini

The International Ground Control Conference in Mining has always provided an open forum for the publication, presentation, discussion, and often heated debate on roof bolting systems mostly with attention to how, when, and why they work. During the last 24 Conferences at least 170 papers have been presented by International Experts on these ?simple? devices that range in length from 30-inches to 20 ft and from 5/8 to 2 inches in diameter. Roof bolts are primarily mechanically anchored, glued, cemented, or driven. Bolts are placed vertically, angled, or tied together with special fixtures (trusses and slings). They have been termed active, passive, stick-slip, and several other descriptive acronyms or mining slang expressions. This retrospective paper will present the changes in bolt types and usage, point out some of the biggest myths and hail the most significant advances (according to the authors? opinions of course).

Jan 1, 2006

By Francis Kendorski

The mining engineering design professional has limited practical and reliable tools for planning successful room-and-pillar stone mines using readily-available and collectible information. Three techniques are in common use today: the hard rock CANMET method of Hedley and Grant, the hard rock method of Stacey and Page, and the oil shale method of Hardy and Agapito. Other methods have been proposed, such as the USBM method of Obert and Duvall, the CSIR/Penn State method of Bieniawski, and the soft rock confined core method of Abel, Wilson, and Ashwin. However, the latter have practical shortcomings when applied to room-and-pillar stone mines such as developed for construction aggregate production. Ideally, the use of multiple techniques resulting in the same acceptable and reliable answer is the goal. Recently, in several underground stone mines areas of undersized pillars were examined. These undersized pillars apparently resulted from non-adherence to a mine plan. These undersized pillars now exhibit strong evidence of incipient failure such as slabbing, opening of through-going fractures, and hour-glassing. This situation allows examination of pillar designs at a ?safety factor? of essentially one. This rare opportunity allowed the examination of the suitability and adjustment of the first three design methods discussed, resulting in greater overall confidence in the methodologies.

Jan 1, 2007

By Kourosh Shahriar

The occurrence of rock falls in underground coal mines entails detrimental effects as fatal or non-fatal injuries on workers, stoppages in mining operations and breakdown of equipment. In this paper, a risk assessment approach on the basis of a decision analysis trend is employed in order to assess the possibility of and manage roof falls. Risk is then assessed by determination of likelihood of occurrence and the cost of consequences (outcomes). In this regard, collected real roof fall data from Tabas and Kerman coal regions comprising of several underground coal mines are used. It is concluded that the annual accidents due to the roof falls occurrence in the all investigated mines are so high that it is economically feasible to improve the support systems and to implement a suitable educational program as well as an accurate supervision and other elements of safety management.

Jan 1, 2009

By Christopher Mark

Multiple seam interactions are a major ground control hazard in many U.S. underground coal mines. The two most common types are: ? Undermining, where stress concentrations caused by previous full extraction in an overlying seam is the primary concern, and; ? Overmining, where previous full extraction in an underlying seam can result in stress concentrations and rock damage from subsidence. The National Institute for Occupational Safety and Health (NIOSH) has completed a major study aimed at helping to identify the location and likely severity of these interactions. In the course of field visits to mines throughout the U.S., more than 300 multiple seam case histories were assembled into the largest data base of multiple seam case histories ever collected. These data were analyzed with the multivariate statistical technique of logistic regression. The study also employed LaM2D to estimate the multiple seam stresses, ALPS and ARMPS to determine pillar stability factors, and the CMRR to measure roof quality. The study resulted in the development of a computer program, called Analysis of Multiple Seam Stability (AMSS), which can help mine planners to evaluate each potential interaction and take steps to reduce the risk of ground control failure. AMSS first evaluates pillar design by calculating the single seam Stability Factor (SFSS) using ALPS or ARMPS. It also automatically generates a LaM2D analysis that provides the additional multiple seam stress so that the final, multiple seam SFMS can be determined. The second part of the AMSS procedure builds upon the statistical findings that overmining is much more difficult than undermining, isolated remnant pillars cause more problems than gob-solid boundaries, and weaker roof significantly increases the risk of multiple seam interactions. AMSS quantifies these effects and predicts the outcome in terms of three levels of risk: GREEN (where a major multiple seam interaction is considered unlikely), YELLOW (where adding a pattern of cable bolts or other equivalent supplemental support could greatly reduce the probability of a major interaction.), or RED (a major interaction should be considered likely, and it may be desirable to avoid the area entirely).

Jan 1, 2007

By Panfeng Gou

In order to solve the problems of supporting the roof of the gateroads affected by a fault, the movement characteristics of roof strata must be fully understood. In this paper, a plane stress physical simulation model was constructed to study roof strata displacement and deformation of gateroads driven in the hanging wall of the fault after the coals in the footwall were extracted. The results indicated that, due to the effect of fault, vertical cracks, orienting parallel to the dip of the fault, occurred in the horizontal direction of the model within the areas affected by the fault. The roof strata are discontinuous. On the other hand, non-uniform subsidence of roof strata occurred due to the bedding plane separation in the vertical direction. Roof strata, especially the lower roof strata, dip toward the fault. Failure of the surrounding strata is not symmetrical with the roof and floor close to the fault being deformed severely. In the selection of type of supports for the gateroads, the U-shaped steel supports are superior to roof bolting.

Jan 1, 2006

By Murali M. Gadde

Perimeter mining is a special form of very productive partial extraction method practiced at some mines in the United States. In the Illinois Basin mines where all major coal seams have weak underclay in the immediate floor, experience shows that any long-term ground control issues will arise due to floor failures rather than instability of the coal pillar itself. While there is a significant amount of work done on floor stability associated with regular pillars, no work has been done so far on the stability of perimeter pillars. This paper reports the results of a comprehensive study that was conducted for the first time on floor stability during perimeter mining in the Illinois Basin mines. A special plate template, which mimicked a common perimeter pillar configuration was created to conduct in-situ bearing capacity tests at five mines in three states of the Basin. Additionally, because of the complex shape of pillars created in perimeter mining that defies any closed-form solution, floor bearing capacity was studied using three-dimensional nonlinear numerical modeling. Based on these in-situ and numerical studies, some guidelines are provided for perimeter pillar stability analysis.

Jan 1, 2009

By Shu Yan

The high resolution resistivity method (HRRM) is developed specifically for detecting underground cavities and tunnels. HRRM has a special feature in that it can recognize and cancel static shift, which makes it different &om the high density resistivity method (HDRM). In searching for various means for detecting underground cavities, we found that the detecting depth of the transient electromagnetic field method (EM) depends mainly on the measurement duration, not the loop area. In addition, TEM can also be used to locate the high-resistivity bodies. In order to use them for local working site conditions, we combine HRRM with TEM to become a combined electromagnetic method. Since the late 1980&apos;s this method has been used to locate abandoned mines and air passages and burning centers of mine fires with great success. This paper cites case studies for which this technique was successfully used.

Jan 1, 2005

By Thomas Barczak

Roof support systems are designed for roof control to prevent unplanned falls. It sounds logical and has been the conventional thinking in support design since supports were first installed. In order to determine which support system should be used or which would be the most effective, the degree of control provided by the support system must be known. This question embodies the concept of a ground response curve, which is a measure of support control by assessment of the convergence in the mine entry as a function of the support capacity. In this National Institute for Occupational Safety and Health (NIOSH) study to optimize standing roof support design, ground response curves were developed for longwall tailgate conditions from numerical models of Pittsburgh Coal Seam geology. The models were calibrated against tailgate convergence measurements that were made in two Pittsburgh Coal Seam mines as the depth of cover varied during the panel extraction. Ground response curves were developed for four loading conditions: (1) development, (2) side abutment, (3) front abutment near the longwall face, and (4) full extraction inby the face. In general, the tailgate convergence and required support capacity increase through each of these loading stages. It was concluded that prior to failure of the rock mass, the support system regardless of its capacity has relatively little impact on tailgate convergence. Recognizing that the support cannot prevent much of the (pre-failure) convergence that is occurring is an important part of support design. Supports that exhibit high loading stiffness but cannot sustain that loading through a displacement compatible with the ground response curve will not provide adequate roof control because they can fail prematurely. The required capacity depends on the loading plus yielding characteristics of the support and the amount of rock failure that has occurred. The loading plus yielding character establishes where the support loading intersects the ground response curve. Design criterion for support capacity based on the identifying the onset of strain-soften rock response leading to ?damaged roof? is proposed as a foundation for assessing support design requirements. The capacity without accounting for the loading and yielding character of one support should not be used to assess the capacity requirement of an alternative support design, particularly when the loading and yielding characteristics are significantly different. A sensitivity study was made to evaluate the impact of mining height and overburden depth. As expected, the results show that convergence increases with increase in mining height and increase in overburden depth. Some specific support examples are also analyzed. Although this is a first step and the two-dimensional numerical models have limitations, these initial studies provide valuable insight into the control provided by the roof support system and will ultimately lead to optimizing support design based on specific mine site conditions.

Jan 1, 2008

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 Hakan Gurgenli

Many coal seams have weak shale immediate roofs that cause ground control problems. Therefore, it is important to know the properties of these shales so that preventive measures can be developed in advance. However, it is sometimes hard to understand this rock type due to its unpredictable behavior. A good example of this situation is found in two shale coal mine roofs located in the Illinois coal basin (Herrin No. 6 seam). Although, these mines are located very close to each other, one of them has many roof falls, whereas the other has no measurable ground control problems. The aim of this paper is to explain why and how one shale roof behaves worse than the other. Laboratory tests, including point load strength index, slake durability index, moisture activity index, weatherability index, water content determination, swelling strain and x-ray diffraction tests were performed to analyze the shale properties. According to these analyses, some useful correlations were found between the engineering properties, moisture-sensitivity indices, and weathering properties. Finally, mine and support design considerations based on the test results were discussed and recommendations to prevent ground control problems were given.

Jan 1, 2006