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By D. S. Choi

This paper presents a theoretical analysis of the effect of shape and size on strength of geologic materials. The results indicate that the shape effect is caused by the development of confining pressure at the interfaces. The linearization of the obtained formula is of the same form determined by experiments. The size effect is attributed to the fact that the geologic materials contain discontinuities in the form of fractures, bedding planes, and cleats. The maximum reduction factor for the size effect is the one-sixth power of the volume of a cubic test specimen. The results of this work are particularly useful in determining the strength of coal pillars in underground mines.

Jan 1, 1989

By Andrew Starkey

Ground anchorage systems in the form of rock bolts are used extensively throughout the world as supporting devices for civil engineering structures such as bridges and tunnels. It is widely recognized that non-destructive testing methods for ground anchorages need to be developed as a high priority [1 ], with only a small proportion of anchorages currently being monitored in service [2]. The condition monitoring of anchorages is a relatively new area of research, with the objective being a wholly automated or semi-automated condition monitoring system capable of repeatable and accurate diagnosis of faults and anchorage post tension levels. The GRANIT tm system developed by the University of Aberdeen is capable of measuring the integrity of ground anchorages and has won two major UK Awards - the National John Logic Baird Award for Innovation in 1997 and a Design Council Millennium Product Award in 1999. The normal procedure with the GRANIT tm system [3,4] is to train a neural network on dynamic response data that has been sampled from an anchorage over a range of post-tension levels. Further data is needed in order to test the neural network. The paper presents neural networks that have been trained on data taken from a number of anchorages on different sites, and the results of the diagnosis of these neural networks on new test data is presented. The results show that the cross-diagnosis of anchorages is possible, where a neural network trained on one anchorage is capable of successfully diagnosing the post-tension level for an anchorage of similar construction, and the high level of accuracy that can be attained with the GRANIT tm technique is shown. The paper also introduces a mathematical model of the GRANIT tm process.

Jan 1, 2001

By B. G. D. Smart

An understanding of the nature of yield zones surrounding coal mine excavations is fundamental to the design of mine support systems. Various methods have been designed for investigation of underground rock masses and an extensometric technique, the Magnetic Rod Extensometer, has been developed to overcome inherent design limitations in existing equipment of this type. This paper refers to the prototype form of the extensometer and details changes designed to overcome a variety of drawbacks indicated by routine use of the equipment. The principal change has been one of manual to computerised data acquisition and presentation in the laboratory phase of the system. This has dramatically reduced the time taken to process underground measurements and produce numerical and graphical results. The revised Magnetic Rod Extensometer represents an integrated instrumentation system that combines speed and accuracy in all areas of data handling.

Jan 1, 1984

By V. D. Jalevsky

A new methodology of an experimental-analytical approach has been developed. It is used in building an information geomechanic monitoring system for the modeling of geomechanic processes during coal seam working. The approach synthesizes experimental (in-situ) measurements of rock pressure manifestation in separate points, methods of numerical modeling of processes on rock boundaries with coal, and a seam analytical calculation of rock mass in a spatial strained state. One of the realizations of this approach is the developed theory and calculation method of adjoining rock surface shift (overhang) formation during underground working of sheet deposits and of rock mass geomechanic state determination. There are description algorithms of cavity configurations with broken lengths of faces during underground working of useful mineral sheet deposits. These algorithms have been used in computer modeling of formations of adjoining rock surface overhangs and abutment pressure upon rock mass in the area of coal faces with irrectilinear longwalls. Peculiarities of surfaces shifts and stress allocation are established for different face schemes. The use of theory and methods alloww working out techniques of automatic control to receive processing algorithms of RPM measurement results and calculate geomechanic situations within panels in the process of planning and development as well as during mining, i.e., to carry out long-term prediction, in-line control, and short-term prediction of rock pressure manifestations.

Jan 1, 1996

By Collin Stewart

Bowie Resources maintains one of the most pro-active ground control programs in the U.S. mining industry today. Its guiding philosophy is that everyone in the organization, from the miners at the face all the way through the top management, are involved in ground control. Some of the unique elements of the Bowie ground control program include: ? Roof bolt operators log the stratigraphy of test holes in nearly every cut. These ?lithgraphs? are used by section foremen to determine the need for additional roof support, and they are entered into the geologic data base for use in geologic modeling and mapping. ? Underground core drilling is conducted to obtain data for CMRR and stability maps. ? Weekly ?Quality? meetings, involving mine management and technical staff, are held to ensure that ground control information is integrated into production operations. The paper will describe the program in detail, discussing the involvement of roof bolters, section foremen, surveyors, technical staff, and production managers. It will also show how the program has contributed to the mine planning process. Case histories of successful ground control interventions will be discussed.

Jan 1, 2006

By Naj Aziz

A series of laboratory experiments were conducted on a variety of bolt types to examine the load transfer capacities of different profiled bolts in short encapsulation push testing. A 75 mm section of 150 mm long bolt specimen was anchored in a 75 mm long stainless steel tube using full resin encapsulation. Six types of different profiled bolts and two non - profiled bolts were tested. Bolts with higher profile were in general found to have greater shearing resistance and higher stiffness than low profile bolts. Widely spaced profiles allowed greater displacement at peak shear strength, and bolts with no profiles produced very link load transfer capability. Rough surfaced plain bolts showed a significant load transfer capability in comparison to a factory supplied smooth surface bolt which supports the belief that rusted bolts have higher lnad transfer capability that un-rusted bolt surfaces.

Jan 1, 2003

By Kenneth Rigsby

Black Mountain Resources' Highlands Mine No. I is a full extraction room and pillar operation located in Harlan County, Kentucky. Black Mountain extracts the 0.9 to 1.1 meter (36- to 42-inch) thick Owl Scam that is situated 60 to 70 meters (200 to 235 ft) above the longwall panels of the abandoned Arch No. 37 Mine, in the 3.4 meter (I I ft) thick Harlan Scam. In the areas where full extraction has taken place, the overburden above the Owl Seam ranges from 60 to 430 meters (200 to 1,400 ft). A short room and pillar panel was developed above an extracted longwall panel to test the selected pillar size, to evaluate support systems, and to verify the shape of the subsidence profile over the recovery end of the depleted longwall panel. It is over the recovery end that Highlands Mine No. I would access the Owl Seam reserves that exist above the No. 37 Mine. Black Mountain selected pillars sizes to be employed above the longwalls based on the ability to fully extract a pillar employing extended cuts. After a mechanical testing program was completed and mine behavior and top disturbance were observed in the test panel, John T. Boyd Company (BOYD) verified pillar stability and designed support systems. BOYD previously designed No. 37 Mine's gate pillars and studied subsidence and gob formation over its longwall panels; this effort aided the analysis of overburden movement and pillar integrity at Highlands Mine No. 1.

Jan 1, 2003

By Daniel W. H. Su

Current longwall pillar design relies heavily upon past experience and many assumptions about pillar and entry response to longwall abutment pressures. Caving characteristics of the immediate and main roof strata are known to play an important role in the overall pressure redistribution, which determines the maximum abutment pressure, abutment pressure at the T-junction, and the width of influence zone. This paper presents the results of an extensive geomechanical study in a longwall panel at a West Virginia coal mine. The response of eight headgate pillars and the adjacent entries was monitored as the longwall face approached and passed by the instrumented locations. Overburden response to longwall mining was also monitored, and height of the highly fractured zone above the gob was determined by postmining drilling. Good agreement was observed between the measured stress changes and those computed using the known height of the highly fractured zone. The results provide a detailed picture of the response of overburden strata and chain pillars to longwall mining.

Jan 1, 1987

By Rafal Misa

In order to prevent the formation of mining damage or to reduce its reach, it is advisable to look for technical solutions which would enable effective protection of construction sites from the impacts of underground mining. So far, other authors have mainly focused on developing methods of securing construction sites in order to minimize the damage resulting from underground mining; the issue of geotechnical methods for protecting buildings against mining damage has not been thoroughly covered in those articles. It should be noted that there are relatively few publications directly related to this issue, nor are there practical examples of geotechnical protection. In this paper, a geotechnical solution available for securing building structures is discussed. Trenches presented in this research reduce the size and range of mining deformation. The results of numerical modelling for sample protective solutions, aimed at reducing the impact of mining activity on the terrain surface under various conditions, are presented. The calculations were performed with the aid of the Abaqus 6.10-2 software, based on the Finite Element Method.

Jan 1, 2014

By E. M. Williams

The U.S. Bureau of Mines is investigating the use of in-seam seismic methods to monitor stress-induced changes in coal near the working face of longwall mines. Seismic surveys performed through a longwall panel and pillars near the face can provide valuable information on geologic anomalies, high stress regions, tailgate support performance and other conditions that could endanger miners or interrupt production. This paper describes the details of the equipment and data analysis techniques used, and the significant results obtained and their implications for ground control. Using seismic tomography, it is possible to conduct measurements on the pillar and reconstruct an image representing the internal velocity distribution. Successive measurements over several days identify changes in velocity attributable to changes in stress. Several field measurements were conducted at a western U.S. longwall mine to observe stress induced changes within the panel and pillars during mining. A permissible data acquisition system was designed to meet mine safety requirements for operation in gassy air. Certification from the Mine Safety and Health Administration (MSHA) was obtained, and this approved system was used to monitor stress changes in several pillars as the longwall face passed during first panel mining. Tomographic surveys were completed across the longwall panel to map the apparent abutment stresses ahead of mining. Velocity distributions show an increase in vertical stress across the panel from the headgate to the tailgate. Additionally. a yield zone along the face near the tailgate and the forward abutment approximately 100 feet outby the face in the tailgate side of the panel were detected. These results agree well with data obtained from other instrumentation emplaced in the panel near the tailgate. Using velocities for both compressional and shear waves, the Poisson's ratio was calculated for the panel.

Jan 1, 1994

By C. C. White

Much benefit can be achieved by a mining company and a manufacturer if both parties can communicate effectively with each other and assist in product development; mutually beneficial ideas and products will develop. This communication can best be achieved if both parties have the opportunity of having on their respective staffs, specialists whose function is to improve the use of products used in mine development. Although a great wealth of roof control knowledge has long existed among mine operators, until recently there was not an abundance of joint development of roof control material by manufacturers and mine operators. This was perhaps due, in past lean years, to the preoccupation of mine management with other overwhelming problems that arise in mining. Manage¬ment had little time for the lengthy process of product development. Until recently, only a few of the largest mining companies had fulltime personnel devoted to roof control. A bolt manufacturer who is invited to work with an operator's roof control people in developing new material is lucky indeed in that he learns what is wanted and he acquires the benefit of the operator's knowledge and experience. He has only to come up with a satisfactory product at an acceptable price - which is not always easy: The development of new products is usually costly and time consuming, and many new products fall by the wayside. In the case of point anchors, however, this product is here to stay. The advantage of point anchors over mechanical anchors is now a proven fact. This advantage is due to less bleed off and higher ultimate load capacity of the resin point anchors. Point anchored bolts have proven to be superior to fully grouted bolts in many areas. We at Birmingham Bolt Company are fortunate in that we have been involved in joint efforts to develop point anchors with several coal companies from whom excellent advice and direction was received. It is appropriate to mention before continuing that the use of point anchors began in the 1960's in Europe and became widespread only recently in the United States. They fall in two categories, point anchors and combination point anchors. These categories have been defined by the Bureau of Mines as follows: (1) point anchors having 24" of grouted length and (2) combination anchors having 24" or more of grouted length. In the present nomenclature, all are anchored with resin or some sort of cementing compound between the rebar or tensioned element and the wall of the hole. The latest development includes an expansion shell as well as rebar, the purpose of the expansion shell being to facilitate installation without the delay necessary in use of types that require a waiting periond for setting of the resin before tightening. (See figures #1 and #2.

Jan 1, 1982

By William B. Beerbower

When full length resin grouted bolts were introduced into U. S. coal mines some 11 years ago, it was assumed that safety would be the primary benefit. There is ample testimony to substantiate the safety factor as an important reason for using resin roof bolts. More recently, it has been determined that many coal operators not only concur with the safety consideration but also benefit economically from the safer working conditions resulting from a reduction in roof falls. IMPROVED SAFETY Some typical roof fall reduction experiences with resin bolting follow: ? Western Kentucky Mine - Resin bolting reduced roof falls by 80%. ? Southern West Virginia Mine - Experienced 141 roof falls between 1971 and 1972. In 1973 with resin bolting, roof falls were reduced 85%. ? Western Pennsylvania Mine - In 1973 had 59 roof falls with mechanical bolts. During the same period of 1974 with resin, only 19 falls were experienced - a reduction of 66%. The overall experience with resin bolted roof support has been an average 50% reduction in roof falls. Long term, this can translate into improved worker morale. As a matter of fact, a general mine foreman at the same Southern West Virginia mine mentioned above commented that improved miner morale was noticeable following the introduction of resin bolting. HOW GREATER SAFETY CAN BE COMBINED WITH INCREASED PRODUCTIVITY IN COAL MINING OPERATIONS Experience has also shown that a reduction of roof falls with resin bolting has resulted in increased production for many coal operators. Some typical examples with mines using resin bolting follow: ? Western Kentucky Mine - Resin bolting contributed to increasing production from 7000 to 11,000 tons per day, due to a 80% reduction in roof falls. This was accomplished in part by a reduction of 50% in timbering requirements and the reassignment of three roof fall cleanup crews to coal production. ? Pennsylvania Mine - Following resin bolting introduction the section showed a 50% increase in production. ? Central West Virginia Mine - Continuous miner production averaged 6 cuts per unit shift prior to resin bolting. Reduction of cleanup time, resulting from improved roof support with resin bolting, increased production to an average of 9 cuts per unit shift. PROPERTIES OF RESIN BOLTING SYSTEMS Several of the important properties of resin bolting systems which contribute to its improved performance are listed below: ? Full length anchored resin bolts and many resin anchored, point anchored bolts provide effective resistance to changing rock strata stresses throughout their length, thereby improving stabilization of rock strata over long periods of time. ? Individual layers of rock are anchored into a single high strength beam providing excellent resistance to vertical and lateral strata movement, the most common cause of roof failure. ? Moisture cannot corrode the fully grouted bolt. ? Has twice the pull strength of mechanical bolts (See Figure 1)

Jan 1, 1982

By Tim Miller, Michael M. Murphy, John L. Ellenberger, Gabriel S. Esterhuizen

"A limestone mine in Ohio has had instability problems that have led to massive roof falls extending to the surface. This study focuses on the role that weak, moisture-sensitive floor has in the instability issues. Previous NIOSH research related to this subject did not include analysis for weak floor or weak bands and recommended that when such issues arise they should be investigated further using a more advanced analysis. Therefore, to further investigate the observed instability occurring on a large scale at the Ohio mine, FLAC3D numerical models were employed to demonstrate the effect that a weak floor has on roof and pillar stability. This case study will provide important information to limestone mine operators regarding the impact of weak floor causing the potential for roof collapse, pillar failure, and subsequent subsidence of the ground surface.IntroductionMassive roof falls extending to the surface at a limestone mine in Ohio have been studied collaboratively by the National Institute for Occupational Safety and Health (NIOSH) and the mine owners. This paper provides a summary of the sequence of events leading to observed instability and presents a numerical modeling analysis that investigates the role a weak floor can have on roof and pillar stability.This study builds upon previous NIOSH research in underground limestone mines undertaken by way of a survey that analyzed performance of pillars in 34 different stone mines in the Eastern and Midwestern United States between 2005 and 2009 (Esterhuizen et al., 2011). This survey led to the development of underground stone mine pillar design guidelines based upon a “stability factor” for the pillar system. The factor is compared to operational experience to determine ranges of values that are typical of stable pillar systems. A software package titled Stone Mine Pillar Design (S-Pillar) was developed for easy application of these guidelines (Esterhuizen and Murphy, 2011).Weak floor in the present case study departs significantly from cases included in the previous NIOSH research, in that none of the 34 study mines had weak floor issues. Moreover, this case study is the first in which a weak floor has been monitored routinely as roof falls are occurring. A detailed report on the observations and measurements taken at the mine has been published (Murphy et al., 2015). The previous observations led to the conclusion that although there were weak bands in the pillar, the weak floor was the significant defect that led to the instability issues. The pillars were initially mined in 2004, approximately 10 years before the instability issues began to occur. The objective of this study is to use numerical models to analyze a variety of floor strengths and the impact of moisture content to find the critical point at which the floor becomes unstable, leading to long-term instability issues."

Jan 1, 2015

By Tom Denton

The bolt design criteria, since it was first published by Dr. John Stankus (Stankus, 1997), has been recognized as an important tool in ground control design. This concept is based on the Optimum Beam Building Effect (OBE), which is defined as the roof beam that has no separation above or within the bolted range and having the shortest bolt possible. This paper will present an application of this concept at the American Coal Company. Galatia mine. This mine previously used a 6' and 7' length bolt. However, since seam thickness has gotten smaller in the Southern area of the nine, a two-piece bolt is required. A two-piece bolt increases ground control costs and delays production. Therefore, a one-piece, shorter bolt, if feasible, can reduce costs and expedite development. In this analysis, Computer modeling is first conducted to determine bolt length and installed load. Second, a special bolt is developed, that can achieve the proper installed load as determined by the computer modeling. Third, underground bolt pull and load cell tests were conducted to evaluate bolt performance. Finally, an underground "bench" test was conducted. The test results have shown that entry convergence in the short bolt (5) test area is less than the 6' bolt area. Overall roof condition in the test area was as good as the 6' bolt area. Based on this analysis, the mine has converted to a shorter bolt in the low seam area and it is working well.

Jan 1, 2000

By Yi Luo

The potential influences of mining subsidence on structural stability, integrity and functionality of guyed tower structures have not been systematically studied before. This paper presents a ease study where a guyed steel tower of a telecommunication company was mined under by a longwall operation. Prior to mining, every aspect of the subsidence influences to this structure was studied. The pre-mining assessment indicated that the structure could experience vibration problem in the second half of the dynamic subsidence process. Bawd on the study, a simple mitigation measure was recommended and implemented and the structure was successfully protected.

Jan 1, 2003

By T. P. Mucho

Strata behavior and support performance were evaluated in a longwall tailgate test area at the Eagle Nest Mine near Van, WV. The mine operates in the Eagle coalbed which ranges in height from 4 to 6 ft (1.2 to 1.8 m) on the mine property. Rugged topography results in rapid changes in overburden which ranges from 200 to 1200 ft (61 to 366 m). The immediate roof at this mine transitions from sandstone to shale. In some areas the sandstone appears to be massive while in many locations it is highly laminated, fossilized, and interspersed with coal streaks. Horizontal stress levels appear to be sufficient to create instabilities in the roof in some locations, especially where the roof is thinly laminated. Traditionally, hardwood cribs have been used at the mine to provide secondary support for longwall tailgates; Strata Products' Hercules cribs are installed on 8 ft (2.4 m) centers staggered left and right in the tailgate entry. In a 250 ft (75 m) tailgate test area, however, cable bolts were installed in lieu of cribs. The bolts were 12 ft (3.6 m) long tensionable cables anchored with 5 ft (1.5 m) of resin. Four cables were installed across the entry and rows were spaced on 6 ft (1.8 m) centers. Primary roof support was maintained in the vicinity of the test area using 42 in (1.1 m), grade 60, No. 6 (19 mm diameter) resin bolts installed on 4 ft (1.2 m) centers through T-2 channels. The cable test area was located under a stream valley in an area of relatively shallow overburden. Strata instabilities consistently had been associated with the stream valley in both the Eagle and the overlying Powellton coalbeds. In an effort to further expose the cables to the most difficult stress conditions available at the time of the test, the specific site chosen was directly beneath a barrier pillar remaining in the upper Powellton mine (approximately 150 ft (45 m) of interburden separates the two coalbeds).

Jan 1, 1996

By Gennaro G. Marino

Documented case histories of surface ground movements from mine subsidence were utilized to help determine when the initial collapse of the overburden occurred above longwall panels. By studying this relationship between initial movement at the surface and the amount of face advance, the least-complex conditions exist for empirically assessing the span capacity of the overburden. The objective of this work was to obtain a better understanding of the overburden response to longwall mining. This knowledge will give a better insight on the long-term effects that different overburden types have on the stability and the subsidence potential of room and pillar mines, as well as, the influence of the overburden on subsidence over longwall panels. Theory on the overburden span capacity was developed and correlated to field observations. Data were used on the overburden lithology, depth, width, and face advance of the panel, and the subsidence conditions that were noted on the surface. The subsidence measured on the surface was used to determine when complete failure in the overburden rock had occurred. According to spans predicted by theory and the subsidence measured in the field, the competent lower units failed at a face advance less than when significant subsidence was observed on the surface. In each case studied, there were competent lower beds; however, the full overburden weight was not sustained to the point of subsidence. The best correlation with the surface movement was found with the most shallow, competent bed. Elastic plate solutions with various support conditions were found to best monitor the surface subsidence of these upper roof units.

Jan 1, 1988

By Dion Pastars

Resin based grouts are the main form of rock bolt anchorage in the underground coal industry in Australia and New Zealand. To be effective, the system requires the mixing of the catalyst and mastic components of the resin, as well as shredding of the laminate cartridge that contains the resin. An unknown measure is the load transfer characteristics of a bolt where the resin is well mixed but remains encased in the cartridge (gloved). Laboratory and in situ field investigations have been undertaken to quantify the performance loss due to mixed gloved bolts. This work showed repeatable results, indicating serious performance loss of the gloved and mixed system, with load transfer approximately 10- 15% of a non-gloved system (MacGregor, 2004). The in situ testing has demonstrated the relationship between the adhesion qualities of the resin and the mechanical interlock generated by radial confinement with progressively increasing tensile load. Effective load transfer is defined by the ability of the system to sustain shear stress on the bolt hole wall.

Jan 1, 2005

Effective strata control, utilising fully encapsulated roof bolts, is dependent on the installed quality of the reinforcement elements. One mechanism by which fully encapsulated roof bolts may become less than fully effective is by gloving (glove fingering) and un-mixing of the resin. Following some small roof failures containing gloved bolts, investigations have been undertaken in some New Zealand coal mines to determine the extent of, and mechanisms involved in, the gloving of fully encapsulated roof bolts for a range of roof types and installation methods. Gloving, in this context, refers to the resin cartridge partially or completely enveloping a length of the bolt, typically with unmixed resin filler and catalyst/hardener remaining within the cartridge. Results have shown that gloving and un-mixing is a systematic and widespread phenomena, occurring across a range of resin and/or bolt manufacturers, and across a variety of roof types. Gloving was found in bolts installed using: hand held pneumatic and continuous miner mounted hydraulic methods, under run of mine (ROM) conditions by mine workers, and under controlled "best practice" conditions. The roofs bolts investigated are all 1800mm (6ft) long, and have shown that, on average, 485mm (-19") of the bolt length is gloved and/or unmixed. This equates to a 27 percent reduction in effective bolt length. The effected length of bolt is sufficient to impact on the load transfer/reinforcement capabilities of the bolt system, and hence impacts on the design assumptions and subsequent stability assessments. Furthermore, there is a direct relationship between the mechanism for the gloving of bolts and an increased occurrence of resin loss and reduction in encapsulations length in soft (less than 10 MPa) and/or highly fractured rock. Investigations have concentrated on developing an understanding of the mechanisms involved. This has been undertaken using both field installations and recovered bolts, as well as test bench trials. The mechanisms involved have been confirmed as being: the development of a pressure front as the bolt encounters the resin and is spun up the hole, which, in turn, leads to over-pressurisation and radial expansion of the resin cartridge, resulting in an increase in diameter of the plastic resin cartridge, which allows the bolt to be spun up into the cartridge without making sufficient contact to shred the cartridge or the hardener tube, resulting in an unmixed portion of resin.

Jan 1, 2003

By Alexander Kattner

Surface fractures in urbanized areas show the risk potential of near-surface mining. This kind of mining subsidence damage occurs mainly in areas where hard coal was mined near the surface with very little overlying rock strata. In North Rhine-Westphalia (NRW), the mining authorities assess areas that are at risk for caveins by means of envelopes constructed according to Hollmann and Nürenberg (1972). Drafting abandoned underground structures depends on the assessment of historical documents such as mine plans, mine titles, and operation plans of prior mines in this area. A research and development project establishing a comprehensive Geo Information System (GIS) for hazard prevention was undertaken in cooperation with the mining authorities of NRW. It aims to display the areas endangered by abandoned, shallow underground mines for the purpose of hazard prevention. A tool was programmed using ArcGIS as a result of this project. It is used to digitize underground workings from preserved mine plans, calculate outcrop lines of coal seams and display hazardous areas. In the future, a database of shallow mine workings will be established. The tool will be used for a systematical mapping of shallow workings and assessment of endangered areas.

Jan 1, 2011