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By Antoni Kidybinski

Coal mining both with longwall and room-and-pillar method often encounters severe operational difficulties due to had roof conditions. Rooffalls at entry cross-cuts or along the rib line in longwalls as welt as dynamic and excessive loads on supports due to overhanging of strong roof layers in a goab - are well known phenomena to the mine operators. Also, a number of practical examples may be quoted of dramatic improvement of working conditions after changing of coal winning technology or supports type. It is therefore a question to be answered how far we are able to recognize roof strata properties before mining starts and adjust both coal extraction technology and supports of entries to rock stability foreseen. Furthermore, engineering principles and criteria for decision making should be established. In recent years several in situ rock strength and discontinuities detection testing techniques have been developed both indirect (geophysical) and direct thus making possible closer roof-strata examination. In this paper critical roof span criterion is discussed as a basis for selection of proper mining system in terms of desirable stability of mine opening .

Jan 1, 1982

By Styles P.

To determine whether 130 felt earth tremors around Edwinstowe, Nottinghamshire U.K, which also experienced severe surface fissuring, were caused by coal extraction, a surface seismometer array was established around Thoresby Colliery. Over the next year, 785 microseismic events were detected. The spatio-temporal seismicity patterns are clearly associated with the commencement, continuing extraction and closure of faces. Of particular note are events which locate at the surface and appear to be related to the active fissuring. Events occur within days of commencement of production and cease when production finishes, with good correlation between face advance and hypocentral position. Naturally occurring microseismic events have also been detected up to 1 km ahead of active longwall faces in the Midlands using triaxial geophone packages grouted into the seam together with a surface seismometer in the top of the borehole. The quality of these data were very high and guided waves can clearly be seen with the dispersive characteristics associated with seam waves. In one experiment more than 2000 events were detected in only two days of monitoring even in a relatively noisy surface environment. This paper demonstrates how very accurate locations ( 5 10 metres) can be generated using three-component digital data from only one borehole. The surface seismometer and the borehole P-wave onset can give extra precision for the height of the event relative to the seam. The event distributions give a dynamic, three-dimensional image of the developing patterns of fracturing above, below and ahead of the longwall face with important implications for roof control, subsidence prediction and gas migration.

Jan 1, 1992

By M. K. Larson

The U.S. Bureau of Mines and Cyprus Shoshone Coal Corp. conducted a study of deformation mechanisms around a longwall gate-road system in an underground coal mine. Of particular interest was roof deformation over the headgate entry. The strata above and below the coal seam are very weak, carbonaceous mudstones that have planes of weakness oriented along the bedding. Visual observations of previous gate roads over several years suggested that deformation was time dependent and resulted in creep along joints or crack growth. Instruments were installed at a test site to measure deformation around the entry, strew changes in the pillar, and loads transferred to support systems. Multipoint extensometers, instrumented bolts, instrumented trusses, biaxial stressmeters, borehole pressure cells, and single-point extensometers were installed. Monitoring continued until the longwall face passed the test site by 38.7 m (127 ft). Results show that fracture zones arched over the entry from both ribs, most bolts and trusses were loaded past their yield point, and the largest change in secondary principal stress was almost horizontal in the pillar. Displacement measurements in the roof showed classic primary creep.

Jan 1, 1995

By D. A. Payne

Large excavations, such as Iongwall panels, result in extensive vertical stress redistribution in the surrounding strata. The large abutment stresses developed may produce damage to pre-existing or planned excavations in the same seam or in seams above and below the workings. Knowledge of the magnitude and location of these stresses is therefore important in the design of mine openings; pillar sizes for panel and pillar layouts, roof supports in longwall gateroads and workings over or above pre-existing or planned extraction in adjacent seams. In an attempt to reduce costs, the Cape Breton Development Corporation (CBDC), a Federal Crown Corporation responsible for operating two retreat longwall coal mines, examined the potential for either interpanel barrier pillar width reduction or entire pillar elimination by the adoption of dual life gateroads for the longwall panels. In order to assess the potential for reduced interpanel barrier widths or total elimination, an investigation of the redistribution of vertical stresses around longwall panels in the Sydney Coalfield was established. The study was conducted jointly by the Cape Breton Coal Research Laboratory (CBCRL), of CANMET (a division of Natural Resources Canada) and the Denver Research Center (DRC) of the United States Bureau of Mines. The program included monitoring of vertical stress changes around longwall panels and gateroad behaviour in two seams. USBM-style hydraulic borehole pressure cells connected to chart recorders for continuous monitoring were deployed at four sites, two at Lingan Colliery and two at Phalen Colliery. This report describes the investigations conducted at Phalen Colliery. Contoured plots of stress redistribution around two sites are presented.

Jan 1, 1995

By R. D. Lama

The concept of rigid or yielding bolts is discussed based upon support requirements for excavations of equivalent geomechanical behaviour. The concept of equivalent geomechanical behaviour is introduced for comparison purpose where the rock mass properties are varied for a given depth (stress) so that the deformation of the roof without any support remains constant = 300mm. A simple experimental theoretical analysis for a uniform stress field acting on an axi-symmetric excavation is conducted to establish roof bolt requirements under different conditions such as width of a given excavation, time of setting of support etc. Results are presented in graphical forms.

Jan 1, 1992

By Zhou Ying

Based on the mining condition and roof lithology of the 2-3 coal seam at Gengcun Mine, the simulated materials modeling method was used to study the movement process and characteristics of overburden strata movement and to analyze the movement laws of overburden strata during top coal caving mining. Based on the modeling results, variation of the peak abutment pressure and its effect on the top coal's crushing and caving in the mining process was discussed.

Jan 1, 2001

By Frank E. Chase

Sacrificial entries, roof slotting, and other tactics have been used to combat high horizontal stresses during roadway development in U.S. coal mines. In Australia, the "pillar extraction on the advance" mining method has been successfully employed as a horizontal stress control technique. The concept of advancing and relieving mine workings through pillar extraction developed from observations that some mines experience difficult ground conditions when advancing and retreating the first panel in a new reserve. However, conditions dramatically improve in subsequent panels. Apparently, the creation of a gob area stress relieves the adjacent panel workings. After much debate and close scrutiny, the first U.S. roof control plan to advance and relieve was approved in 1998, for the Sargent Hollow Mine in Wise County, VA. The major concern was that this mining practice places section personnel inby of the gob. The tentative approval permitted barrier pillar slabbing and removal of the adjacent development pillar during panel development. The intent of the plan was to provide a "stress shadow" for the advancing faces and outby workings to reduce the excessive number of roof falls. Sargent Hollow successfully extracted their first panel by pillaring on advance and retreat mining using posts. Mobile roof supports were employed to extract pillars while advancing the second panel. This paper summarizes Sargent Hollow Mine's experiences with the advance and relieve mining method.

Jan 1, 1999

By Stephen Tadolini

Standing crib supports have been applied in underground mining programs to resist large roof movements and sustain high¬loads. The strength and deformation capability of these systems has been documented under both laboratory and field conditions. The parameter that has not been examined and is not well understood is the effect that a crib or other types of standing support has on the primary and secondary bolting systems. A crib support system with low system stiffness may allow large amounts of closure and deformation and cause the bolting systems to yield or even fail. Conversely, cribs or standing support that are too stiff may experience brittle or buckling failures, negating the advantage of the intrinsic supports previously installed. Utilizing a combination of field measurements and 3-dimensional finite element modeling techniques, the relationship between system stiffness and the subsequent performance of the installed bolting system is evaluated. Additionally, a simple method for calculating the combined system stiffness for standing supports, when using materials with different strengths such as steel, concrete, wood, etc., is presented.

Jan 1, 2003

By Christopher Mark

Current longwall pillar design methods are based on many assumptions about pillar and entry response to longwall abutment loads. Knowledge of the magnitude and time-of-arrival of abutment loads is essential for Improving design procedures. The abutment loads are in turn related to the development of yield zones and horizontal confining stresses in pillars. Entry stability, which is the goal of any longwall pillar design, is affected by in situ stresses as well as the interaction between the roof, pillars, and floor. The paper describes a study performed in a longwall panel at a West Virginia coal mine. The behavior of two headgate pillars and the adjacent entries was- monitored as the longwall face approached and moved past. The results provided a detailed picture of the response of long- wall pillars and entries to abutment loadings.

Jan 1, 1986

By Shosei Serata

Serious floor heave of up to 2.4 m in a 2.4-m high mine entry was eliminated by applying the stress control method of mining, as a last resort, at the No. 5 coal mine of Jim Walter Resources, Inc., in the Black Warrior coal basin near Birmingham. Alabama. Underground observation of the first 3-room entry created using the stress control method is discussed here. The behavior of the test entry, which eliminated the heave problem, is analyzed in relation to similar studies conducted in a salt mine under similar ground conditions by utilizing finite element analysis.

Jan 1, 1984

By A. Grenoble

The mining of seams in close proximity can greatly accentuate interaction problems. At distances of less than 110 feet vertically interaction can occur for both over and under mining. Research into ground control problems resulting from undermining a previously mined seam has been carried out using finite element and body-loaded photoelastic modeling methods in conjunction with numerous case studies. Analysis of field data was used to validate and expand the model results. Research findings demonstrate situations under which the worst ground control conditions can be expected to occur for a specified geologic environment, depth, innerburden spacing and mining geometry. Special emphasis has been placed on the effects of pillar load transfer on the lower seam in terms of pillar and floor stability and roof control. Roof and floor condition indexes are used to summarize structural and lithologic conditions in the affected seam. The effects of joint orientation and continuity are incorporated in the analysis and the research results have been compiled into a software package for application by the field engineer.

Jan 1, 1984

By Brian E. Travis

Recent Developments in the utilization of polymer grid structures for Longwall Shield Recovery operations are discussed. Specifically: various Longwall screen designs and the numerous methods employed during installation. Also discussed is the role of polymer grid structures in head and tailgate mf and rib control and their role in controlling soft bottom often associated in shield recovery operations.

Jan 1, 1993

By Susan Robertson

Many injuries are caused each year by rock falls in coal mines. Most of these injuries are not caused by major roof collapses, but from falls of smaller rocks from the immediate top or roof skin. Various surface controls are used in mines to control this surface rock. One that has been found to be very effective is roof screening. Depending on the size of the screen, roof coverage approaching 100% can be achieved. Many mines are reluctant to use screen for primary skin control because of additional costs of time and materials, but others are having great success at both controlling costs and surface rock. Data is presented from two mines that show a dramatic reduction in roof skin injuries when screening is used. Much of this success is due to innovations in roof bolting machines. Four case studies of roof screen experience are presented along with associated costs of materials, impact on bolting advance rates, and potential ergonomic risks. The effects of roof screening on skin control and safety are also included. Finally, this paper will provide information about features of different roof bolting machines that affect production and safety.

Jan 1, 2002

By Deno Pappas

During the past few years, the Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) has been evaluating developments in sealant support systems from a ground control perspective. The proper selection and use of a sealant material can significantly enhance ground control which may result in a decrease in worker related injuries due to ground falls. The purpose of this paper is threefold: to briefly review the current state¬of-the-art technology in shotcrete and membrane developments, to evaluate the preliminary findings of a long-terra underground study of various types of sealant materials, and to examine installation practices that are critical for an effective sealant material. The underground study utilizes NIOSH's Lake Lynn Laboratory Experimental Mine to evaluate the long-term performance of several types of shotcrete and membrane materials. Sealant performance to date have been evaluated on a regular basis over a two year period. Although the study is still ongoing, critical mining practices were identified that may seriously effect the bond of the sealant materials to the mine roof and rib; most notably, the importance of scaling and thorough cleaning of the rib prior to application. Also, results from an extensive series of Schmidt Hammer tests found that the shoterete increased in strength by 70% during the humid summer months.

Jan 1, 2002

By P. A. Gray

Artificial ground support has developed into a relined science over the past 20 years. From reactive support systems such as timber props and steel supports, to active systems such as roof bolts and cable anchors, ground support technology has now developed efficient systems that work well in many instances. However this paper demonstrates that there is still a long way to go to develop optimum ground support systems. Some conventional rock bolts and cable anchors emphasise their tensile capacity only, without considering other factors such as their shear strength or shear modulus, or if the support member can transfer all of its support capacity to the surrounding rock. Examples are given of the use of ground anchors with high tensile and shear capacity in underground coal mining as well as examples of slope failures in open pits where the cable bolts have remained intact because they have been unable to transfer their full capacity to the surrounding weak rock mass. For underground coal mines, speed of installation is also of primary concern, but it must also have high tensile and shear capacity both in terms of strength and stiffness. Ground support systems of the future must therefore address these problems. An innovative new rock bait is discussed which goes some way towards solving these problems. This new rock bolt is screwed into the rock and thus has a direct mechanical interlock with the rock. Initial field results show that it has an extremely high anchor capacity and can be fully installed in approximately 30 seconds. It has considerable capacity to improve the productivity of the coal mining industry.

Jan 1, 1992

By André Vervoort

In coal room and pillar, and pillar extraction panels, the sidewall conditions were investigated based on underground measurements and numerical simulations. Three-dimensional linear elastic models provided a useful and quick prediction of the amount of sidewall spalling. However, more complex constitutive models were required to simulate more accurately the extent of fracturing and its effect on the stress re-distribution. The input parameters of a Mohr-Coulomb model were determined by back-analysis of underground measurements, and the effect of the depth of the workings and the time dependent occcurrence of coal pillar fracturing were analyzed.

Jan 1, 1992

By Larry Stolarczyk

A clear vision of the future coal mining industry was developed into a hierarchy of technology needs by the National Mining Association (NMA) and the Chief Executive Officers of the mining industry. Although improvements in safety and environmental factors were the main drivers in the technology roadmapping workshop sponsored by the Department of Energy, the agile mining machine of the future must efficiently extract coal from deeper, thinner, and more geologically complex coal beds. One of the challenges will be to develop sensors and software that enable clean coal cutting in a changing geologic environment. The roadmap identifies imaging ahead of raining and horizon control within the top ten needed technologies. For these technologies to be adopted by the mining industry, they must be proven to be effective while driving down mining cost and risk. This paper describes how three-dimensional imaging ahead of mining and horizon sensors enable improved ground control and safety in the mining of underground coal.

Jan 1, 2003

By Yi Luo

The severity of disturbances caused by longwall mining subsidence to various residential structures can be re¬duced. The success of such mitigation efforts depends on accurate subsidence prediction, correct assessments of the subsidence influences, and careful design and implementa¬tion of the proper mitigation measures. This paper presents the systematic approach involved in the subsidence mitiga¬tion efforts with a case demonstration.

Jan 1, 2003

Coal mass strength properties must be estimated to obtain realistic estimates of coal pillar strength. The limitations of existing techniques for approximating these properties are discussed. The coal mass is anisotropic. The Hoek and Brown approach to rock mass property estimation is modified to allow for this. This approach requires the Rock Mass Rating (RMR) for the strata. In the normal case where bedding is sub-horizontal, these major planes of weakness have little effect on the strength of pillars. As a result of this the RMR must be measured parallel to bedding. This means that a significantly higher rating than that usually obtained for coal is observed. The use of mass strength property estimates in extant empirical relationships is outlined. At high width to height ratios, an acceleration in the rate of increase in pillar strength with increasing width to height ratio has been observed. The empirical relationship developed by Salamon (the so called squat pillar formula) takes account of this. This relationship was found to have significant limitations with respect to its universal application. Modifications arc made to Bieniawski's empirical formula to extend its use to large width to height ratio (squat) pillars. This new squat pillar formula has none of the disadvantages associated with Salamon's relationship. Strain softening was incorporated into non-linear finite difference numerical modelling to estimate pillar strengths. The technique outlined above is used to obtain peak coal mass properties. Modelled pillar strength was found to be sensitive to both peak and post peak mass strength properties. Post peak properties were found to be difficult to obtain and required some back analysis. Modelling results were used to estimate both the strength of squat pillars and parameters for use in a now squat pillar empirical strength relationship.

Jan 1, 1992

By J. R. Stoltz

To date, there has been limited formal research dealing with the use of Mobile Roof Supports (MRS) for pillar recovery. This paper, which is a portion of a larger project, presents convergence data from 4 different mine case studies. MULSIM was used to create a numerical model. The resulting information from this model was then compared to the actual measured values. MULSIM was also employed to analyze the industry-accepted pillar/barrier layout for mining underground reserves with the Joy Highwall Mining System. This layout was examined for both a low cover and a high cover extreme,

Jan 1, 1999