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By J. Nielen van der Merwe

The main objective of the research described in the paper was to determine the causes of falls of roof in South African coal mines. The south African Colliery Manager's Association and the members of the South African National Institute of Rock Engineering (SANIRE) who are employed by coal mines lent full support to the work. Fifteen major roof falls were investigated in detail and their causes established. This data was supplemented by mapping a large number of roof falls on four coal mines in less detail. The total number of recorded roof falls was 182. The data bank simulated the data bank of roof falls that resulted in fatalities in terms of the total number of falls recorded and the thickness distribution of the collapses. It was found that the causes of the falls differed for different thickness ranges of roof falls, The thin falls, classified as "Skin falls", which accounted for approximately 70% of all fatalities, were predominantly caused by ineffective joint support and excessive bolt spacing. Ineffective joint support was found to be a dominant cause of roof falls for all thickness ranges, while the influence of excessive bolt spacing diminished with increasing fall thickness. However, it remained a significant cause of roof falls even for the major falls. Burnt coal, dykes, bad mining practice, weathering and inferior materials were found to be minor causes, but nonetheless contributed to the falls. Horizontal stress was a minor cause, with increasing contribution for thicker falls. It was also found that the indicators of horizontal stress coincided with areas of reduced roof rock strength. In general, roof falls occurred in areas where the roof rock was found to be less competent in terms of rock mass rating systems. It was concluded that the Coal Mine Roof Rating (CMRR) system developed by NIOSH in the USA held promise for application in South Africa and should be investigated further.

Jan 1, 2001

By Nikolaos Polysos

Besides considering the given geomechanical conditions, the geological/geotechnical logging of drill cores and heading faces during the driving of parallel entries and the resulting rock classification form the basis of the assessment of the proneness to caving of the roof strata in the actual mining phase. Both types of separation planes, parallel to the stratification as well as intersecting the bedding, will be defined and assessed within the immediate roof strata depending on the lithological structure. These data may hold the potential to plan the winning and design its operational organization in a way that - to a major extent - future damage of the workings by increased caving or cavities can be predicted in advance. Following the general description of the separation plane structure, the combined influence of the petrographic and structural geological roof properties on the operating cycle of the planned working face will be presented and illustrated.

Jan 1, 2003

By David P. Conover

The effects of stress distribution on pillar stability were evaluated through rock mechanics studies conducted in three underground room-and-pillar coal mines. Results of the three field instrumentation programs are presented to illustrate the use of borehole pressure cells to determine vertical stress distribution in coal pillars at various stages of mine development. Vertical stresses and stress changes are determined using methods developed at the Bureau of Mines. Measured stress distributions are compared against theoretical distributions and possible explanations for observed behavior are discussed. Results indicate that stresses and stress changes calculated from pressure cell data are inconsistent with theoretical values; however, the data are useful for qualitative evaluation of trends over time. Calculated values are found to be sensitive to coal properties and initial setting pressures of the cells.

Jan 1, 1989

By Gustavo Herrera Rico

As part of a general effort to increase safety, efficiency, and profitability of its mining operations, MICARE personnel with the support of NorWest Mine Services, Inc. determined that the key to improving development mining was the implementation of roof bolting as the means of primary roof support for both main entry and longwall gate road development. Until mid-1994, MICARE had relied exclusively upon primary roof support systems composed of heavy steel beams and wooden posts. Support systems were designed, implemented, and evaluated that were successful in controlling the mine roof under extreme geological conditions. This paper describes the geological challenges, support selection criteria, and presents the instrumentation and 'mine measurements made to assess the bolting performance during the development and retreat mining process of longwall mining.

Jan 1, 1997

By J. D. Cole

In order to reduce the number of longwall face moves and to increase the rate of gateroad construction, super longwall panels, 1000 feet wide and 12000 to 14000 feet long, are planned for Wolf Creek Collieries No. 4 mine. The development of the longwall panels at the mine has evolved from 5-entry, to 4-entry, to 3-entry systems. A three-entry system including pillar size and entry supporting method for the super longwall panels was designed for long-term stability. The design procedures included field observation and studies, laboratory properties tests, in-situ stress measurement, and computer analysis. Three pillar design methods consisting of PILLAR, ALPS, and the Wilson Method, and one entry supporting method, ROOFBOLT, were adopted in this study. The procedures, methods, and results are discussed in detail.

Jan 1, 1990

By Kevin MacPherson

After 34km of roof bolted drivage the first fall of ground occurred at the Phalen Colliery in the maingate of the 8th East longwall panel. The fall occurred over the stage loader as the wall approached an area of poor roof geology. This area had developed a severe cutter since initial drivage and the longwall had yet to mine through 500m of even worse conditions. This paper describes the cause of the poor roof conditions, the use of polyurethane resin injection and cable bolting to re-stabilize the roof outby the fall, and the measurement of roof movement and performance subsequent to the continuation of mining under the rehabilitated roof.

Jan 1, 1999

By Yong-Ming Jiang

In cooperation with Cyprus Twentymile Coal Co., researchers from the National Institute for Occupational Safety and Health (NIOSI-I), Spokane Research Laboratory, conducted a study at the Foidel Creek Mine, an underground coal mine near Oak Creek, CO, to evaluate the stability of underground working conditions as a longwall advanced through a series of backfilled cross-panel entries. Instruments installed in the cross-panel entry pillars, backfill, longwall panel, and adjacent headgate entry were continuously monitored by a computerized data acquisition system as the longwall approached and advanced through the backfilled section of the panel. Data collected from these instruments were analyzed to determine the magnitude and direction of the secondary principal stress changes ahead of the longwall face, deformation and closure of the headgate entry, and stability of the cross-panel entry pillars and backfill. A three-dimensional, boundary element model was calibrated to accurately predict mining-induced stress changes and the distance they occurred ahead of the longwall face. As the longwall advanced through the hackfillcd entries, most of the mining-induced load was supported by the cross-panel entry pillars. The backlill provided stability for the root and Moor of the in-panel entries and also confined the entry pillars significantly improving their load-carrying capacity. Comprehensive information derived from backfill material property tests, instrument data, underground observations, and numeric modeling results document the safety and stability of the 8-Right minethrough and should provide beneficial case study information fur other longwall backfilling applications in the future.

Jan 1, 1998

By Laxminarayan Holla

There exist many formulae for designing coal pillars. However, when applied to a given set of mining parameters, they lead to different pillar sizes and factors of safety. From the subsidence point of view, the pillars designed as protection pillars not only have to be stable, but also have to limit subsidence over them to a level acceptable to the structure being protected. The subsidence over a pillar of given width to height ratio depends upon the mining depth. A pillar may be stable, but the subsidence over it may be quite large and unacceptable for the structure. In many cases even though pillars may yield, they support the undermined strata and reduce surface subsidence. In addition, the yielding pillars eliminate peaks and troughs in the surface subsidence profile and reduce ground strains. In a given situation, whether a pillar yields or not depends primarily upon its width in relation to mining depth.

Jan 1, 1992

By John W. Fredland

Conducting mining operations under a body of Water can create hazardous conditions for miners. The potential for a sudden inrush must be considered and abnormal ground control conditions can be encountered. This paper synopsizes the factors which mine operators should take into account when evaluating whether an inrush may occur. It addresses the applicable Federal regulations and discusses some items which coal operators should consider in applying for a permit, Through collecting and analyzing the pertinent information, the likelihood of encountering problems can be judged. Through proper mine planning, the potential for creating hazardous conditions can be mitigated.

Jan 1, 1982

By M. Afsari Nejad

A Transversely Elasto-Plastic Model was configured in FLAC (Fact Lagrangian Analysis of Continua) as a more realistic simulation of stratified and inclined strata behaviour examining surface subsidence above the inclined longwall panels (I ). The failure criterion employed was a modified Mohr-Coulomb failure criterion that assumed a variable cohesion, which changed with respect to the orientation of the plane of anisotropy . Using the proposed numerical modelling technique several longwall panels with different configurations were simulated. The results of both surface subsidence and horizontal displacements were validated against UK data using the Subsidence Engineer's Handbook (S.E.H) subsidence prediction method and the Influence Function Method (2). It was found from the research that the incite properties as well as laboratory to field reduction factors arc depth dependent. A set of relationships for calculation of anicotropic stiffness and strength parameters have been derived. The proposed modelling approach has been successfully applied to surface subsidence prediction for 3 collieries in the UK allowing the results of the simulation to he validated directly against measured data.

Jan 1, 1998

By Pier Paolo Manca

The investigated area lies close to a large town and covers an area of about lkm2. The mine was worked at depths ranging from 5 to 100 meters but mining operations actually ceased some 50 years ago. There are seven coal seams, each one to two meters thick, overlain by sedimentary rocks in a strongly faulted area. Different kinds of subsidence have occurred, crown holes, shallow depressions and hummocky ground. The results of rock mechanical characterisation (RMR method) in the area have been used to define three different types of numerical model (FLAC 2D). A large numbers of simulations has been performed to solve three different problems at three different scales: to determine the, mechanical properties of the roof rock and overburden involved in the subsidence process, to assess the crown hole subsidence potential and to define the properties of the beam foundations.

Jan 1, 1999

By S. Jayanthu

This paper presents analysis of strata behaviour with special reference to convergence and stress variation during an experimental trial of extraction of pillars with cable bolts as major support system in 6.5 - 8.0 m thick coal seam. Critical span for roof falls was estimated through empirical models and also evaluated by numerical models. In situ strata behaviour studies during 1992-96 for about 70 local/major falls revealed inadequacy of the available literature for interpretation and prediction of the roof falls. Convergence acceleration and variation of induced stress based on continuous monitoring data showed distinct anomalies and potential for better understanding of strata mechanics during pillar extraction.

Jan 1, 1998

By R. J. Morrell

The Bureau of Mines developed a ground support system for use with an underground coal auger. The system consists of inflatable airbags that are inserted in the holes as they are drilled, and then gradually deflated to provide controlled roof caving. This paper presents results of laboratory and field tests of the system that show it to be technically feasible.

Jan 1, 1988

By John J. Bowders

The stability of surface slopes undergoing subsidence due to longwall mining was investigated to examine the effect the subsidence had on the stability of the slopes. Stability analyses were performed on all slopes along both longitudinal and transeverse sections of the longwall panel before and after subsidence. Both finite- and infinite-slope analyses were performed with both drained and undrained cases. The results indicated that the changes in the stability of slopes due to subsidence were insignificant for the cases analyzed. Although significant lateral ground movements were recorded, these were not attributed to slope failure. Explanations for these movements are presented in the paper.

Jan 1, 1988

By Chris Adams

The application of pre-tensioned tendon roof support assisted Harworth Colliery in their revival, from impending closure to lowest cost deep-mined producer in the UK, over just six months last year. The paper describes this success story, how it came about, the reasons "why", and the method "how"

Jan 1, 2003

By André C. Zingano

The domino effect in a violent pillar collapse occurs due to the redistribution of stress in the remaining pillars near the collapse, and this stress transfer occurs in the immediate and main roofs. The remaining pillars do not have enough load capacity to support this overstress and also fail, and the domino effect propagates further to the entire panel. The domino effect does not occur only due to low load capacity of the pillars, but also the roof rock mass quality, because the stress is transferred by the roof. In high competent roof without discontinuity (fault), like massive sandstone seam, the deformation of the roof is the cause of pillar collapse extension and propagation. Depending on how close it is approaching the collapse limit; the immediate and main roofs behave differently. When it is close to the collapse limit roof convergence and pillars compression occur. Conversely when it is far from the collapse limit divergence and pillars decompression occur. This is called the arc-effect. This paper studies this behavior of the roof (immediate and main roofs) using numerical modeling to simulate the roof deformation and stress distribution in the roof and pillars. This simulation represents the progression of pillar collapse and shows the zone of convergence and divergence of the roof, and also the effect of this behavior on pillar confinement. Two-dimensional plain strain model was applied to simulate the arc-effect. The simulation results were compared to those of the instrumentation that was installed before the collapse. Keywords: pillar collapse, room and pillar, numerical modeling

Jan 1, 2005

By H. N. Maleki

The results of five years of geotechnical investigations are presented to develop productive and stable longwall layouts for the Foidel Creek Mine. The program was initiated during the pre-mining stage, and has continuously provided the data required for mine design. From several stress determinations at the mine, the relationship between geologic structure and the stress field/depth was established. The results were used for orienting the entries for improved stability. Cave conditions and load transfer to panel boundaries were evaluated from closely monitored full extraction mining and computer analysis. Back-analyses of ground movements, as compared with the actual measurements, indicated that cave conditions were favorable in spite of the presence of competent, thick-bedded sandstones in the mine roof. Gate pillar design using a yield-rigid concept was developed through computer analyses and underground instrumentation, resulting in 35 and 75 ft wide pillars. The influence of a slickensided bedding plane at the roof contact on pillar yield zones, core confinement, and pillar behavior was evaluated.

Jan 1, 1988

By G. K. Rogers

Portals, which are the surface entrances to underground excavations, are frequently overlooked and often difficult areas In terms of ground control. Failures commonly occur In the typically high angle approach cuts and Initial subsurface portion of the portal. In order to permit rigorous design of all types of portal structures, failures are Identified and subsequently classified. Based on the failure classification and suggested analysis methodology, optimum ground and support loading can be determined.

Jan 1, 1988

By Tetsuro Esaki

This paper describes the present condition of mining-induced damages in Japan and the characteristics of them. The change of mining operation and environmental condition has caused various significant impacts on the ground environment: subsidence, cave-in, springing out of ground water etc. The characteristics and the influences of these phenomena are discussed by investigation of the actual conditions and by fundamental studies. Some counter measures are also introduced.

Jan 1, 1989

By M. Terry Hoch

This paper presents information on low-coal ATRS systems that are now in use. A study was conducted consisting of an in-mine evaluation of ATRS- equipped roof bolters capable of bolting in seams below 42 inches. Machine manufacturers, MSHA personnel, and bolter operators were asked to assist in reviewing the operation of various low- coal ATRS systems. Features of each system are outlined and operational problems and limitations of each are analyzed. Specific applications, such as ATRS-equipped bolters designed for use with auger-type miners, are also examined.

Jan 1, 1988