Search Documents

By Eric R. Bauer

Skin failures of roof and rib in underground coal mines continue to be a significant safety hazard for mine workers. Skin failures do not usually involve failure of the support systems but result from rock or coal spalling from between the support elements. For instance in 1997, over 800 miners were injured by roof and rib falls, of which 98 pet were the result of skin failures. Also, nearly 80 pct of the roof and rib failure injuries occurred at or near the working faces in development sections. The face area is a zone where the potential for skin failure accidents and injuries and roof and rib failures, in general, is high because of mining activity, ground readjustment due to changing stress conditions, and the higher exposure of mine workers. In addition, failures occur where the roof and rib are unsupported. This paper features a review of the roof and rib accident statistics resulting from skin failure, highlighting the incidences by average days lost, in-mine location, state, MSHA District, and worker activity at the time of injury. A detailed analysis of recent fatalities caused by skin failure is included. Also discussed are the causes of roof and rib skin failures, current methods and support materials for skin surface control, as well as a historical literature review of skin failures and control methods.

Jan 1, 1999

By S. K. Sharma

The present study attempts to critically review the pillar-design as required by Coal Mines Regulations of India vis-a-vis latest rockmechanics formulae, Equivalent material modelling was the approach adopted for testing the safety and stability of the pillar sizes predicted by various formulae. Two parameters, considered here for calculating pillar sizes was the stresses acting over the pillars and the strength of the pillars. Stresses acting on the pillars were calculated by the tributary area method. Based on the studies of Coates and Hustrulid, the stress obtained was reduced by 40 percent. For calculating the strength of the pillars, three formulae, namely Holland and Gaddy, Salamon and Munro and Central Mining Research Station, India, were used. The pillar widths were then determined by equating the product of recommended factor of safety and pillar stress with the pillar strength.

Jan 1, 1992

By Larry Stolarczyk

The mining industry would benefit greatly by imaging geologic conditions well in advance of mining. In layered deposits such as coal, trona, quartz, and potash, natural waveguides form and enable electromagnetic seam waves to travel great distances. In the early 1980s, the Radio Imaging Method (RIM) was developed to capitalize on electromagnetic seam wave propagation in the assessments of seam conditions in longwall panels. The initial applications of RIM tomography and comparisons to in-mine mapping of geologic conditions proved that faults, paleochannels, and rapidly thinning coal could be detected and imaged with seam waves. This paper describes the application of electromagnetic seam waves in mapping the margins of a paleochannel crossing a longwall panel and the interesting possibility of adopting the newly developed Full Wave Inversion Code (FWIC) )Newman 1995) to significantly improve imaging resolution. When tomography images are combined with ground control science, a significant reduction in cost, roof fall potential, and waste rock in underground mining may be realized.

Jan 1, 1999

By Thomas L. Vandergrift

Multiple-scam mining is becoming more common in the United States, especially in the East, where primary reserves arc being depleted, forcing mining companies to develop secondary seams, Although mine engineers and planners are aware of the basic effects of seam interaction and generally follow guidelines for columnizing entries and avoiding load abutments, mine design earn greatly benefit bum more quantitative estimates of scam interaction effects. This paper presents case studies from two eastern mines subject to seam interaction effects where pre-mining stresses were quantified: the Mingo Logan Coal Company's Mountaineer Alma A Mine, and Peabody Group's Lightfoot No. 2 Mine. In each of these mines. mining is being carried out within 60 to 90 ft of overlying longwall and pillar panels under maximum cover of about 1,100 ft As part of overall mine planning and design efforts al these mines. NSA Engineering performed numerical modeling analyses using the non-linear boundary-element code MULSIM/NL Input parameters were calibrated by modeling areas where ground conditions resulting from seam interaction were well documented. By incorporating actual cover loading based on topography and seam dip, and the integrity of remnant, gate road, arid harrier pillars in the overlying mines, the pre-mining vertical stress in the underlying scants was estimated based on the pre-mining stress, plans for mains, gate road, bleeder, and pillar panels were developed. Although mining experience in these areas has beers somewhat limited to date, she designs developed as a result of these analyses have so far proven successful.

Jan 1, 2000

By Z. T. Bieniawski

A major study was performed on determination of long term strength of coal strata directed to the design of coal pillars to ensure their stability beyond the life-time of a mine. Based on field investigations, numerical modeling and reliable data from 12 case histories, three aspects of time effect in the strength of coal strata were studied; (1) strength reduction based on the concept of volumetric strain estimate from in-situ tests, (2) strength reduction in terms of three failure criteria applicable to coal strata: original Hoek-Brown criterion, modified Hoek-Brown criterion, and Generalized criterion, and (3) long-term strength estimation as a function of coal strata quality RMR based on case studies. From these investigations, an integrated approach to long-term pillar stability is proposed, based on numerical modeling and empirical pillar strength formulation. The values of factors of safety for coal pillars are analyzed in terms of probability of failure and practical meaning for mine design purposes.

Jan 1, 1994

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 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 Alan D. Smith

[A standardized checklist in a questionnaire format, was established to investigate selected factors associated with coal nips roof falls, as derived from an intensive review of the literature. A plot study was undertaken to aspect and explore possible statistical relationships that may exist with these parameters. the Purposes of toe study were to: 1. Study in a systematic and empirical fashion selected parameters that may be capable of differentiating characteristics of roof falls; 2. Obtain some predictive indices cased on selected physical and management related variables so that predictions of future problems associated with mine roof falls can be completed, prior to its actual occurrence. Accomplishment of these goals should result in valuable guides to the mining engineer in scheduling exploration work and 4 determining economic feasibility of certain ground control and .lining pro and practices. The parameters associated with mine roof failure that where included in the questionnaire are: working section, county, spar., orientation degrees), pillar dimensions width, length), seam thickness, roof fall dimensions (length, width, surface area, volume), shape of fall (arch, dome, laminar), thickness of thinnest immediate layer, lithology of the first four i.e.-diate roof layers, thickness of h e first four immediate roof layers, type of initial support systems before failure, type of resupport system after failure, length and spacing of bolts in use before failure, length and spacing of bolts planned after allure, presence of .cater, time failure after It coal excavation, distance t3rest ace, physical operation during, time of coo failure, location of the roof fall entry failure occurred ence of sloughing of ribs before failure, and presence of floor heave condition before failure. The typical roof fall had an average roof spar of 20.3 feet, -06 feet depth o coal seam, occurred 136 feet from nearest face, coal seam thickness of inches, p--ar :width off 45 feet surface area D' roof fall 1151 ft2, mainly lamin- shaped, previously rat fore fall between good to very good, little floor heave presence, usually cracks before the fall about 40 percent of reported cases, shales in most of the immediate layers, and usually located in an entry or intersection. in addition, multiple linear regression techniques were employed in the testing process of 18 major research hypotheses in a search for relationships among the various parameters studied of which were found to be significant, once corrected for multiple comparisons.]

Jan 1, 1984

By Xun Luo

In this paper we report on the use of microseismic monitoring techniques to investigate the fracturing process associated with longwall mining. The study was carried out at three mines in Australia and concentrated on some major concerns of the mines, such as failure patterns and growth rate associated with mining progress, their implications to water ingress and gas emissions, caving processes in the goaf and precursors of roof falls. The results have shown that a zone of shear fracturing has occurred further ahead of the longwall face than predicted using conventional longwall geomechanics theory. Failure patterns in the roof and floor vary from mine to mine and this appears to be dominated by geological structures. The reaction of faults, the formation of fractures which allow gas emissions and precursors roof falls have also observed. Results from this work have opened up new understanding of longwall caving processes and proved that microseismic monitoring is a very useful technique for studies of longwall geomechanics and hazard controls.

Jan 1, 1998

By Y. Luo

An extensive subsidence research program conducted by the authors has greatly improved the accuracy and the capabilities of subsidence prediction under complicated mining and surface conditions. The techniques developed for assessing and mitigating subsidence influences on various surface structures have been applied in numerous cases with a very good re- cord of success.

Jan 1, 1997

This paper describes the experiences with the application of the Coal Mining Roof Rating (CMRR) System to South African strata conditions. Roof classification studies using the CMRR system were undertaken as part of a SIMRAC investigation into falls of ground in South Mean collieries. Colliery roof rating exercises were undertaking in a range of geotechnical conditions from shallow level coal seams under flat topography to workings with cover depths of greater than 300m A comparison is made between the CMRR and other Rock Mass Rating systems used to classify different roof geotechnical conditions. The need for geological discontinuity orientation, blasting and rockmass stress adjustments to the CMRR is discussed. The effects of acid mine water on strata weatherability and the need to incorporate immersion tests using acid mine water are also detailed. In addition limited correlation is given between CMRR values and other geotechnical factors such as joint trace length, joint shear strength, joint orientation, weatherability and roof damage due to horizontal stress. Taking cognizance of these observations, a concept of stress induced roof damage triggering is discussed.

Jan 1, 2001

By Ross W. Seedsman

The in situ behaviour of claystone floors associated with the Wallarah, Great Northern and Fassifem Seams has been studied using a comprehensive suite of stress and displacement monitors. The instrumentation was supplemented by in situ and laboratory testing of the claystone materials. The instrumentation demonstrated that the presence of claystone floors is associated with floor heave, extra rib spall, and settlement of the pillar into the floor. Horizontal floor extensometers showed that the depth of extrusion of clay was limited to 1-2 in. A pore pressure transducer installed in a claystone floor demonstrated the possible existence of short-term undrained behaviour of the claystone. Long term swelling of destressed claystone was also documented. Settlement of pillars into claystone floors can be analysed using simple elastic theory. The assumption of plane strain is valid for most claystone thickness/pillar width aspect ratios. Floor heave can be analysed using bearing capacity concepts. However, it is considered that heave is the result of local bearing capacity failure in the rib spall zone. The advantage of this model is that the bearing capacity equation can be applied without resort to unrealistically high factors of safety.

Jan 1, 1992

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 Brian G. D. Smart

A scheme is presented whereby the complexities of design and operation of a hydraulically powered coalmine support are modelled using the finite element method. This enables predictions to be made of the interaction of the support with its environment in a variety of operating conditions. The model is a two dimensional representation. The modelling scheme was applied in two case studies. In the first, the performance of a four leg support was uprated by increasing the front leg force. The finite element model enabled changes in overall support performance to be verified. In the second case study, the change in lateral force developed by a two leg support was assessed with changes in support operating height and floor foundation stiffness. The modelling scheme is capable of further development with the possibility of extending the model into three dimensions and the addition of control elements that would enable an actual mining sequence to be represented.

Jan 1, 1992

By Paul Hagan

A diverse range of rockbolt designs and resin anchors are available for use in underground mines. Despite their wide spread use, the factors that affect the performance a rockbolt is not fully understood. The laboratory-scale rockbolt test facility at the UNSW Mining Research Centre is being used to gain a better understanding of the behaviour of rockbolts leading to an improvement in ground support. A test program investigated the nature of load transfer between a fully encapsulated rockbolt and the surrounding rock mass. It was found that with all other factors held constant, there were differences in the behaviour of load transfer depending on the method of loading. The typical pull-out arrangement as used in most field test work resulted in a different load transfer function to that observed when the load on the rockbolt was induced through bed separation.

Jan 1, 2004

By Stephen C. Tadolini

Cable supports offer several advantages over traditional secondary support methods. Cable supports enhance stress redistribution to pillars and gob areas, minimize or eliminate timbers and cribs which reduce ventilation capabilities, eradicate material handling injuries related to the placement of crib supports, and provide a cost-effective alternative to secondary support. The U.S. Bureau of Mines has designed and installed cable supports to improve the stability of tailgate entries in an underground western coal mine. Two entries closest to the Iongwall panel were solely supported with high-strength cables installed with cement grout. Basically, with modifications in the intersections, 360 ft of roadway nearest the longwall panel was supported with two rows of 22-ft-long cables installed on a 7-ft spacing. Approximately 250 ft of a second bleeder entry was supported with two rows of 16-ft¬long cables installed on a 7-ft spacing. Individual cable loads were monitored with hydraulic U-cells and pressure pads. The roof behavior was simultaneously monitored with differential magnetic sag stations and closure meters on 25-ft spacing during longwall panel retreat to evaluate the response of the immediate and main roofs. Cable supported gateroads appear to be a viable alternative in secondary support when compared to wood timbers and cribs. This paper describes the theory, application, and advantages of cable supports and presents the results of the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1993

By Jay Keng

During the construction of shaft on the fractured and weak rock strata, the stability of construction plays an important role. This study includes earth stress estimation, support method, and stability analysis. Stresses on the site of three shafts include structural residual stresses, earth pressures, and hydrostatic pressures were determined. The basic method of estimating the support strength is dependent upon the' size of the plastic ring after rock excavation. During the construction of the shaft, geotechnical measurements, physical properties of the rock, and the change after excavation have to be evaluated again. Geology and construction methods need further consideration.

Jan 1, 1988

By W. G. Pariseau

A new approach to the direct inclusion of the effects of joints and geologic structure in rock mechanics analyses is applied to an elastic analysis of stability of a Lucky Friday undercut and 4111 longwall stope (WL) using the finite element method (FW). The approach is base+ on the concept of equivalent properties of a heterogeneous test volume, but does not depend on the assumption that a test volume is a representative volume element. Comparison with conventional FW results show greater variability in fields of stress, strain, displacement, safety factor and energy densities. Stability requires a knowledge of rock mass strengths as well as elastic moduli.

Jan 1, 1989

By S. M. Hsiung

In late September 1984, a longwall panel in West Virginia lost 60 powered supports when the face had advanced for 250 ft. from the setup entry. An in-mine investigation showed that it was a first weighting failure resulting from inadequate Support capacity. This paper presents the results of finite element modeling, which have led to a better understanding of the first weighting failure mechanism. The dynamic impact of the first weighting failure on the powered supports is analyzed. The optimum capacity for the powered supports in order to control the roof at face area under the geological condition is discussed.

Jan 1, 1984

By Mao Bai

Mining in situations where there exists a potential hazard from inundation has always existed in coal mining, and often reserves were left unmined in favor of reserves with no such hazard. However, increased pressure on expanding the minable reserves and on producing low-sulfur coal, has made mining under water hazards necessary in many parts of the world. Recent joint efforts by the authors to develop case-history data bases of mining under water hazards has allowed the bringing together of Chinese and United States experience and guidelines for such mining. In China, mining under water hazards has been successfully accomplished for a number of years, allowing the development of a good data base from which to draw guidelines. In the United States, much more limited experience, and the longer delay in adopting longwall mining compared to the rest of the world, has forced reliance on a more limited data base and European experience. In the United States (and Canada), the data base is continuing to increase as more operations start-up. Developing a generally-applicable set of guidelines for international use, based upon extensive Chinese and other experience, appears feasible. The Chinese and United States experiences are detailed by case histories.

Jan 1, 1995