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By A. H. PH. D Wilson

Compared to the rocks encountered in, say, metalliferous mining or tunnel drivage in igneous and metamorphic rocks, the strata associated with the coal measures are relatively soft. If excavations are made at depths greater than one or two hundred metres, zones of fractured or yielded rock develop around the openings. The stress which can be taken by yielded rock at the boundary of an opening is low, but because of the friction present between the fragments or particles, the sustainable stress rises rapidly with distance from the opening until sufficient confinement has been developed to prevent failure of the rock in the first place. Thereafter the laws of elasticity will apply and increasing distance will reduce the stress field until conditions consistent with the cover load are reached. For rocks which maintain their elastic condition right to the edge of an opening, finite element analysis and similar techniques have proved successful in determining the distribution of stress around the opening. This approach, however, is not successful when a yield zone is introduced between the elastic zone and the opening. Nevertheless, some estimate is required if help is to be given in the siting of roadways and in the determination of adequate pillar sizes. This matter is of particular importance in the planning of new mines especially if retreat faces are involved, as extensive roadway drivage must be planned and executed before experience of the effects of face extraction is gained. In 1977, the author published [I] a method based on 'stress balance'. The total aggregate downward load over a large area will remain that of the cover load even after part of the coal seam has been removed. Any rise in stress in ribsides and pillars must be offset by an equivalent stress reduction across roadways and other areas of extraction, and vice versa. Knowledge of one can lead to an estimate of the other, provided the general form of the stress distribution can be postulated. It is the object of this paper to revue the method in more detail and to give further examples of its successful application.

Jan 1, 1981

By Zach Agioutantis, Christopher Mark

"Thirty years ago, the Analysis of Longwall Pillar Stability (ALPS) inaugurated a new era in coal pillar design. ALPS was the first empirical pillar design technique to consider the abutment loads that arise from full extraction, and the first to be calibrated using an extensive database of mining case histories. ALPS was followed by the Analysis of Retreat Mining Stability (ARMPS) and the Analysis of Multiple Seam Stability (AMSS). These methods incorporated other innovations, including the Coal Mine Roof Rating (CMRR), the Mark-Bieniawski pillar strength formula, and the pressure arch loading model. They also built upon ever larger case history databases and employed more sophisticated statistical methods.Today, these empirical methods are used in nearly every underground coal mine in the US. However, the piecemeal manner in which these methods have evolved resulted in some weaknesses. For example, in certain situations, it may not be obvious which program is the best to use. Other times the results from the different programs are not entirely consistent with each other. The programs have also not been updated for several years, and some changes were necessary to keep pace with new developments in mining practice.The Analysis of Coal Pillar Stability (ACPS) now integrates all three of the older software packages into a single pillar design framework. ACPS also incorporates the latest research findings in the field of pillar design, including an expanded multiple seam case history data base and a new method to evaluate room and pillar panels containing multiple rows of pillars left in place during pillar recovery. ACPS also includes updated guidance and warnings for users and features upgraded help files and graphics.INTRODUCTIONPillar design is one of the fundamental elements of mining engineering. Pillars are necessary to control the great weight of the overburden during all phases of underground mining. Without stable pillars, ground control is impossible."

Jan 1, 2018

By Stanislaw Prusek

In this paper, results of underground measurements conducted in gateroad 3, seam 615, at the Bobrek-Centrum Colliery are presented. The Bobrek-Centrum Colliery is owned by Kompania Weglowa, the largest hard coal producer in the European Union. Yearly production at the colliery, including the longwall system, is about 2 million tonnes. Gateroad 3 was used simultaneously for two retreating longwall faces, No. 1 and No. 2. This working was also employed for the conveyor belt for longwall No. 1 and served as a transport road for longwall No. 2. Approximately 100 meters of gateroad 3 was maintained behind the face of longwall No.1 and then, with the progress of longwall No. 2, it was liquidated. In this paper, we present the results of underground measurements of vertical and horizontal convergence in gateroad 3. We also present the results of numerical modeling for deformation of the working. Finally, we analyse the accuracy of the numerical calculations by comparing them to the actual deformation of gateroad 3.

Jan 1, 2014

By Behdeen Oraee, Amirhossein Bangian, Kazem Oraee

"Determination of the optimum design of chain pillars has a significant effect on the economics and safety of longwall operations. Most pillar design formulae are based on empirical methods and supplemented by local experience. They therefore lack versatility of application under different geotechnical conditions.In this paper, in order to illustrate the shortcomings of the above, a typical real coal pillar in Tabas coal mine has first been studied and the conventional formulae have been used to determine the optimum dimensions. The results show that a wide difference exists between the predicted and the real field data. The Oraee-Hosseini formula has then been applied to this mine in order to determine the optimum design. The results from this formula demonstrate a close fit between the theoretical VALUES (and those produced by laboratory tests and in practice. It is further demonstrated that the wide discrepancy between the results obtained from the two formulae is attributed to the dissimilarities between geotechnical conditions of Tabas and the original regions whose data were used to devise the empirical formulae.It is finally concluded that the application of numerical simulation methods and experimental equations together with engineering judgment used by the mining design engineer, will provide the most accurate design characteristics.INTRODUCTIONLongwall is an underground mining method which has a high rate of production capacity and mechanization ability. This mining method, which is appropriately applied for soft rock fiatlying bedded ore bodies, gradually became a widespread mining method amongst European coal mines at the beginning of the 19'h century. It was then applied in some of the U.S. coal mines with the development of self advancing support mechanisms in early 1960's (Oraee, 2001).Underground coal production around the world using Longwall mining method doubled during 1990s. The main reasons for this improvement were: the high level of mechanization, increased 10 safety, adequate design of panels, and well designed ventilation systems (Peng, 2006)."

Jan 1, 2010

By Tobias U. Papst

The German Ruhrgebiet is slowly transitioning into a post-mining state. It has been subjected to industrial-grade hard coal mining operations for more than 150 years. Due to a political decision, German hard coal mining will come to an end in 2018. By then the surface will have subsided up to 20 meters. After all mining activities in the area have ended, there will be no need to maintain the current mine water level. Accordingly, the company operating the last remaining coal mines has decided to let the water level rise to an as yet undetermined level. Alongside other issues that may be caused by rising groundwater levels, the protection of the area?s drinking water supply is of utmost importance. This notion is also represented by European and German law. The primary intent of the applicable laws is to protect the health of the citizenship. This means protecting ground water that may be used by humans from being contaminated by the mine water. This paper outlines the consequences of European and German legislature on mining aftercare using the example of one of Europe?s largest agglomerations.

Jan 1, 2014

By Andre Zingano

The coal seam strength and the failure criteria that represent the behavior of the coal seam are very important for mine section and pillar design. Additionally, mine section and pillar design take into account roof and floor layers, which form the roof-pillar-floor system. Therefore, underground ground control design begins, most importantly, with rock mass characterization. Another issue is the distribution or behavior of the coal seam strength for the whole mine area. Sometimes, the coal seam strength is assumed to be the same for an entire area, but this is not true because the coal seam is heterogenic and anisotropic, and also the coal strength is affected by faults and spacing between fractures. The objective of this paper is to demonstrate the use of the methodology for geomechanics characterization to determine the strength for the Bonito coal seam, and discuss the coal seam strength distribution around the entire mine area. The intact coal strength was determined by laboratory tests that were completed during the last 17 years, including triaxial and uniaxial strength tests. It was noted that there is a strong correlation between all laboratory strength tests with 90% confidence interval. The explanation for this coal seam strength variation is the presence of the discontinuities and their variation in quantity and quality at each location in the mine area, because the coal seam rock classification and its behavior will be affected by the quantity and quality of fractures.

Jan 1, 2014

By Mario G. Karfakis

Mining related subsidence is a major concern over abandoned shallow coal mines. Many of the subsidence prone areas are presently used or will be used in the future for residential housing development. Concern over mine subsidence is inhibiting the development of surface land uses on previously mined area. This concern prompted public officials to realize the need to develop guidelines which can be used to assess and reduce the risk of mine subsidence damage to building and other improvements. Subsidence and the associated ground movements must be defined before one can relate them to building damage. Surface movements resulting from underground coal mining occurs in two phases termed as: active subsidence and residual subsidence. Active subsidence refers to all surface movements occur in^ simultaneously with the mining operations, while residual subsidence is-that part of the surface deformation which occurs following the cessation of mining. Residual subsidence is the major concern over abandoned mine lands. The duration of residual subsidence is of particular importance from the standpoint of damage at the surface to structures built after the mine has been abandoned. Time spans during which surface subsidence may occur, vary markedly with the mining method being used. In total extraction methods, such as longwalls, subsidence is induced rapidly, beginning almost immediately after mining. Subsidence concurrent with coal extraction is desirable for a safe longwall mining operation. Usually subsidence is complete within a relatively short time. The remaining settlement occurs as residual subsidence due to the gradual compaction of the subsided ground. The relative proportion of the residual subsidence movements is dependent upon the presence of remnant pillars and roadside packs. rate of face advance and depth of workings. In partial extraction methods, such as with room-and-pillar systems. major occurrences of surface subsidence may be delayed for decades until the support pillars and/or the roof have substantially deteriorated and collapsed. The actual time involved depends on a number of factors such as the strength of coal, roof, and floor, extent of fracturing. presence of water, depth of workings, pillar size and percentage extraction. Consequently residual subsidence is more likely to occur over abandoned room-and-pillar mines. The following sections define subsidence over abandoned mines, review the proposed mechanisms, identify the controlling factors and examine the prediction methods.

Jan 1, 1987

By Sean C. Farrell

In underground mining and tunneling, machine design is predominantly dictated by mine conditions and individual customer desires. In partnership with Australian companies WDS Limited and Cram Fluid Power, the engineering department of J.H. Fletcher & Company was tasked to design and manufacture roof bolting modules to be mounted off both sides of a roadheader, providing in-cycle bolting. The objective was to produce a boom and bolt module that would allow an operator to bolt from a remote location, eliminating the need for miner to work in unsupported ground. The bolt module needed to operate in front of the cutting boom, but retract to a parked position out of the way during the cutting cycle. Due to Australian electrical approval requirements, it was desirable to design the module with minimal use of electrical controls. The need for multiple, complex, drill steel and bolt handling steps led to the need for a custom hydraulics package to automate several of the movements. The result of the design is dual drill and bolt modules mounted to a multistage boom with over 24 feet (7.31 m) of extension. Each unit rotates and tilts to meet the bolt pattern requirements in conjunction with the stow and load requirements. The bolt module builds on previous designs incorporating a modular assembly with each sub-component designed to be replaceable and adjustable independently from other machine components. Each major component is painted a different color to help the operator make the connection visually from the module to the operator?s control valves. Hydraulic package valves were designed to automatically control several of the sub-components during the drill cycle, reducing the number of steps a drill operator would, otherwise, have to do manually. This design removes the operator from working in unsupported roof and away from other inherent hazards, putting them at a safe distance on the operator?s platform. The cutting and bolting equipment has been combined into one machine, which is advantageous in single entry drifts because there is no need to place change equipment. The hydraulic package valves eliminate the need for any electronics, thus avoiding any electrical regulations with respect to the controls. The addition of these modules to the roadheading machine increases miner safety and streamlines production while meeting the ground control support requirements.

Jan 1, 2014

By S. J. Jung

The goal of this research is to demonstrate how optimization methodology can be coupled with the finite element method for greater stability of underground mine openings. As a result of increasing mining depth and intersection of geologic discontinuities, mining environments are becoming more complex. Current design procedures are rapidly becoming inadequate. In this research, the finite element method has been coupled with optimization criteria to design more stable and safe underground openings

Jan 1, 1993

By Gift Makusha

Pre-feasibility work at Goedehoop Colliery indicated that stooping of the undersized pillars could be undertaken safely and economically. In light of the findings a decision was taken in may 2003 to undertake the project in the 600 area where three panels (628, 630 and 632) were identified for pilot test sites. The findings of the pre-feasibility studies were presented in a paper to the 22nd International Conference on Ground Control in Mining held at Morgantown in 2003 (1). This paper is a progress report for the project 12 months down the line. Work commenced to prepare the sites for mining including, acquisition of equipment, equipping the panels, selecting and training people to undertake the project. Three monitoring sites were set up distributed evenly along panel 628. Each site comprised of a surface extensometer station in the overburden strata adjacent to a pillar underground containing hard inclusion vibrating wire extensometer cells. This combination would be used to record pillar loading ahead of stooping as well as the depth of caving and strata deformation in the goal zone behind the stooping face. In July 2003, support work began to secure the roof and rib sides. The support was required to be at least three-pillar center distances ahead of the mining front to ensure that the effective stooping zone is secured at all times. The NEVID method of pillar extraction developed at Sasol and described in detail by Ismet and Moolman (2) in a Coaltech 20-20 report was adopted for the pilot project. As part of training the selected stooping team was send for a week to Sasol's Bosmanskrans Colliery to familiarize with the NEVID method of stooping. Further training sessions were carried out on site especially on the use of Mobile Roof supports. The first cut was taken on 20 August 2003 by implementing a form of chequerbord mining taking every second row of pillars. Three rows were mined this way which period was considered training to get workers familiar with the new equipment and mining method. On the fourth row the full design span equivalent to mining 4 pillars in a row was implemented taking out all the pillars in successive rows. Production figures gradually increased from 300 to more than 1,000 tons run of mine coal per shift. Panel 628 was mined out at the end of December 2003 and the second of three panels (630) is being mined. Substantial data from the monitoring set up has been recorded since the start of the project. Monitoring will continue to the end of the pilot project and the data will be analysed and fed back into the design tools to improve the design in future undertakings. The pilot project has been a success so far and other old workings are being evaluated for stooping.

Jan 1, 2004

By Thomas E. Marshall

During the past few years, the Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) examined and characterized conditions at a majority of the underground stone mines in the United States. Observations at these mines revealed a limited degree of roof monitoring beyond visual inspection and sounding. When monitors are used, they typically require the miner to measure movement at the roof. If conditions are unstable, the miner may be in a hazardous situation while recording data. Based on this scenario, researchers surmised that a simple, inexpensive monitor with the capability of recording data at a distance from the mine roof would be a safer way to gain the information. Additionally, more widespread use of monitors could potentially lead to better understanding of roof movement in general. A monitor to meet this need was designed, tested, and subsequently improved as experience was gained in its' use at a number of underground stone mines. The Roof Monitoring Safety System (RMSS) can provide an initial indication of movement in roof beams. By understanding and measuring roof movement in underground mines, the potential for injuries and fatalities to mine workers From falls of ground can be reduced. Also, officials at a mine with a history of data are better prepared to make a decision on remedial actions in the event of ground falls. This paper will outline the evolution of the RMSS and how it can be used in a comprehensive pro-active ground control safety program. Also included is a cast history describing how the RMSS was used in an evaluation of the effectiveness of a mechanical impact scaling machine at an operating limestone mine.

Jan 1, 2000

By Ali Naeimipour

Availability of accurate geological information about the rock mass surrounding a tunnel or mine working is the key to enhance and optimize the required ground support measures. Various approaches have been used to obtain data on ground conditions, including the possibility of characterizing the ground while drilling for roof bolts, by the Measurement While Drilling (MWD) system, via optical, sonic and acoustic televiewers, and using various types of gamma probes. While there are many different systems for characterizing the ground in longer, larger-diameter holes drilled downward, the options for using probes and logging systems in short, small-diameter holes drilled in horizontal or an upward direction are very limited. This paper will present the results of recent field tests using optical televiewer (OPTV) to analyze the joints and voids in boreholes drilled in different directions. The studies show that OPTV is a promising tool; however, getting a good quality image from the upward drill holes still need to be studied. Meanwhile, estimated in-situ strength of the intact rock is needed to complement the jointing pattern for rock mass classification. For this purpose, preliminary steps for the development of a borehole scratcher to estimate rock strength will be discussed in this paper.

Jan 1, 2014

By Gabriel Walton, Sankhaneel Sinha

"Rib failures are a major hazard in underground mining. When pillars are subjected to loading, extensile cracks are generated parallel to the excavation boundary, which can manifest as rib falls. A common mitigation measure is to install supports, especially bolts, normal to the rib surface. The type and spatial distribution of these supports along the pillar is often based on experience rather than comprehensive analysis. Since conducting field experiments is often impractical and expensive, a convenient alternative is to perform 3D numerical modeling. In this study, FLAC3D has been used to capture the interaction of rib bolts and the failure processes governing the stability of pillars. An associated goal is to determine whether, at all, a continuum software is capable of modeling the effect of supports. The modeling involved two important steps: selection of a proper constitutive model, and, explicit simulation of rock bolts. Based on precursory work in the last decade, an S-shaped strength criterion was chosen that can describe the tensile fracture mechanism of rock at low confinement and shear failure mechanism of rock at high confinement. The built-in pile structural element in FLAC3D is a simplified representation of the complete bolt system. Hence, the different components of a rock bolt (i.e., steel, grout, steel-grout interface, and grout-rock interface) have been explicitly modeled within the rock mass and then tested for its ability to replicate the ground-support interaction behavior. INTRODUCTION Rib failure-related hazards and injuries have plagued the mining industry for decades (Mohamed et al., 2016; Iannacchione and Prosser, 1998). In highly stressed environments, excavation parallel cracks are the primary initiators of skin instability. In absence of any support, this failure can indirectly destabilize the excavation by increasing the exposed span of intersections and roadways. Over the last two decades, there have been numerous injuries (fatal and non-fatal) due to rib failures in underground coal/non-coal mines."

Jan 1, 2017

By Christopher Mark

Colombia is 11th largest coal producing nation in the world, and the world?s fifth largest coal exporter. But while most of Colombia?s coal is mined at large modern surface mines located near the Caribbean coast, there are also underground mines that produce high-grade metallurgical coal. These mines employ traditional hand-loading techniques in a variety of geologic environments, and they have some of the highest accident rates of any Colombian industry. The National Institute for Occupational Safety and Health (NIOSH) has been collaborating with POSITIVA, the primary provider of worker?s compensation insurance to the Colombian underground coal industry, to reduce the economic and social cost of rock falls in these mines. The specific goals of the collaboration are to 1) document practices and conditions in Colombian underground coal mines, 2) identify relevant international best practices, and 3) provide recommendations for developing a national campaign to reduce fatalities and injuries associated with roof falls. NIOSH conducted a survey of seven underground coal mines in four separate coalfields. Five of the mines were characterized by steep dips and weak roof and floor rock. The coal was loaded by hand at all seven mines, though two employed partially mechanized longwall techniques. Wood posts were by far the most common ground support. Roof bolts were not observed in use at any of the mines, and they are unlikely to replace timbers any time soon. While long-term entries generally appeared to be adequately supported, the survey found that less attention was paid to the temporary support that is installed right after the coal is removed. This temporary support is critical for protecting the workers from rock falls, however, because with hand-loading mining methods most of the activity takes place between the permanent supports and face. The study concluded that improving the temporary support in the working faces would be the most effective way to reduce the number of rock fall injuries. The key principle should be that every worker is protected by planned roof support at all times. Ground Support Management Plans, consisting of specific rules about when, where, and what roof support is to be installed, should be the focus of the campaign. Each Plan should be tailored to a specific mine based on the geologic conditions and mining methods in use. The Plan provides miners with clear, simple rules that can be easily understood, while management can easily check to see if it is being followed. Ground Support Management Plans will also provide the campaign with a concrete focus for the necessary cultural change.

Jan 1, 2014

By Stephen C. Tadolini, Peter A. Gray, Robert Hawker, Andrew Sykes

"Self-drilling rock bolts (SDRBs) that use pumpable cement grout to anchor bolts have been used successfully in the tunnelling and civil engineering industries for many years. Pumpable cement grout offers a considerable advantage in that the SDRB can be fully encapsulated right down to the bearing plate, including filling cracks and cavities in the borehole. However, cement grout cannot provide the immediate rock support required for the coal industry. This paper describes a new SDRB system that uses pumpable resin to anchor the SDRB in a borehole, which not only provides all the advantages of a pumpable grout anchoring system, but also provides immediate roof and rib support as required in the coal industry. This paper further outlines the latest developments in SDRB technology including (1) extendable self-drilling rock bolts, which can almost double in length during the installation process and are particularly suited to low seam heights and rib support and (2) pre-tensioned SDRBs, where a pre-tensioned load can be applied during the installation process. These new SDRB systems offer both considerable advantages in rock support and improvements in mining productivity.INTRODUCTIONThe mining industry is continually striving to develop more efficient methods to support the roof and walls of roadways and tunnels, and rock bolting has proven to be one of the most significant improvements in ground support over the last 50 years. Rock bolting is now almost universally used in the underground mining industry worldwide, and there are many different rock bolts available.Rock bolts are traditionally made from steel or fiberglass and are anchored into a borehole with a friction-anchoring device, such as an expansion shell mechanism; cement grout; or chemical resin. Friction-anchored bolts are widely used in the Australian metal mining industry, predominantly as short-term support. They are used in conjunction with resin-anchored bolts where there is a requirement for longer life. Resin-anchored bolts are used in the coal mining industry.Friction-anchored bolts provide immediate rock support, but their load transfer capacity is poor compared to resin or cement anchored bolts. In addition, friction-anchored bolts are more susceptible to corrosion than resin or cement anchored bolts."

Jan 1, 2010

By Ismet Canbulat, John Naylor, Jason Emery, Peter Craig, Andy Sykes

"Longwall mining is by far the most common method of underground coal extraction in Australia. The industry trends and expectations are placing increasing emphasis on the reliability and productivity of these operations. The longwalls are becoming wider and longer while retreat rates are continuously increasing due to significant improvements achieved in longwall equipment reliability and automation. This increased longwall productivity is placing significant emphasis on the reliability of longwall panel development.Although there have been significant improvements in the reliability of continuous miners and hydraulic drill rigs, the traditional resin encapsulated bolt installation is still the principal method used in all major coal-producing countries around the world. Anglo American realised an opportunity existed to introduce an alternative roof bolt installation technique called “spin to stall” in Australia. Spin to stall was first introduced in Anglo American Coal in South Africa where it has been successfully used for over a decade, though implementation of South African spin to stall resin in Australia has proven to be near impossible due to a significant difference in geotechnical conditions, mining method, and, subsequently, roof bolting equipment. Therefore, a new spin to stall development project was initiated between Anglo American Coal and Jennmar Australia (SPIN2STALL®). This paper summarises the journey of this project, the results, and the successful implementation of spin to stall achieved at Grasstree Mine.INTRODUCTIONThe Anglo American Coal business unit operates Grasstree and Moranbah North underground longwall mines in the Bowen Basin of Central Queensland, with a third longwall mine, Grosvenor, currently being developed, and two more (Aquila and Moranbah South) in the project pipeline. The vision for Anglo American is to achieve “Zero Harm” through the effective management of safety at all operations, while for the coal business unit, at the same time to achieve industry-leading operational performance. Currently, both Grasstree and Moranbah North longwalls are operating at industry best practice rates. This is placing increased pressure on the development process to allow such high productivity levels to be sustainable."

Jan 1, 2015

By Gamal Rashed

This research is divided into two parts: the first part of this research focuses on understanding coal mine bump mechanisms, and the second part focuses on mitigating the violent failure associated with a bump. Many researchers believe that the coal bumps happen because of sudden loss of constraint between pillar/ roof and pillar/floor or the hammering effect due to the breaking of thick, massive strata above the coal seam, creating a sudden impact load on the pillars and causing them to bump. However, very little work has been done to explore and understand the consequences of these proposed mechanisms for coal bumps. In the first part of this research, computer simulation of coal specimen behavior have been used to explore two issues: 1) How would an instantaneous loss of constraint or a small sudden impact load cause violent failure? 2) How does the strength of the coal specimen control coal mine bumps? Eliminating or reducing the violent failure is the main objective of the second part of this research. The coal specimen needs to be softened either in the rib zone or the core zone and then its mode of failure studied to see whether it is violent or not. Instantaneous loss of constraint and the hammering effect are associated with sudden release of elastic energy, increase in the kinetic energy, and very rapid transient vertical stresses of the coal specimen. These are three of the major proposed mechanisms for bumps. The potential for violent failure increases with increasing the unconfined compressive strength, UCS, of the coal specimen. Failure of the rib zone is not the main factor for the violent failure of a coal specimen. The energy stored in the core zone is the key factor for the violent failure.

Jan 1, 2013

By James Pile

The San Juan Coal Mine, located near Farmington, New Mexico, supplies the San Juan Generating Station with more than 6 million tons of coal annually. To replace dwindling surface mine production, San Juan installed a longwall in 2002. The roof rock in much of the underground reserve consists of interbedded coals, carbonaceous shales, and mudstones. Roof support consists of fully-grouted roof bolts, both tensioned and non-tensioned, supplemented by modified cable trusses. Short-encapsulation pull tests conducted by San Juan's geotechnical engineering department showed that some of the immediate roof layers provided variable bolt anchorage. To ensure that the roof bolts were obtaining the necessary anchorage, additional tests were conducted in a variety of locations. The results have been used to determine the optimum support design for different areas of the mine. Another series of tests evaluated the performance of several bolt and hole diameter combinations. The experience at San Juan Mine shows that short-encapsulation pull tests can provide the information necessary to improve ground control in variable roof conditions. It also illustrates how a pro¬active geotechnical engineering program can be an essential component of a state of the art underground coal mine.

Jan 1, 2003

By J. Nielen van der Merwe

During the process of pillar extraction at the Welgedacht Colliery, an unexpected pillar collapse resulted in the entrapment of equipment. The area was visited soon after the event to evaluate the stability of the area for the purposes of extracting the equipment. It was found that the area had stabilised sufficiently to allow work without undue risk to the safety of workers. The equipment was reclaimed successfully. A subsequent investigation into the reasons for the collapse was conducted. It entailed detailed underground and surface subsidence observations and numerical modeling. It was found that none of the perceived negative factors that were present at the time could result in pillar loads that were sufficient to cause the collapse. It was concluded that the local pillar strengths were less than the nationally used value and that they were possibly further reduced by aging.

Jan 1, 1999

By R. Fritschen

In order to investigate locations, source parameters, and source mechanisms of mining induced seismic events, a local network of seismic stations was installed at a coal mine in the eastern part of the Ruhr District (Germany). The recorded events were located with the help of an anisotropic velocity model, which accounts for the fine layering of the carboniferous strata. Most of the events took place about 50 m above the working Luise 76-10. Their locations correlated with the advancing working face, and the number of events increased when pillar edges were undercut. The source mechanism, i.e., the orientation of the fracture plane and the direction of displacement on this plane, could be determined for several seismic events. The majority of them was characterized by a fracture plane parallel to the working face and an almost vertical dislocation on this plane towards the goaf a mechanism resembling the breaking down of the hanging wall. The fracture plane had a dimension of about 100 m. Probably the fracturing of a sandstone layer, located approximately 40 m above seam Luise, was responsible for the observed seismicity

Jan 1, 1999