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By F. E. Chase

Mobile roof supports are shield-type support units mounted on crawler tracks. Mobile roof support (MRS) units are used during retreat mining, and they eliminate the setting of roadway, turn, and crosscut breaker posts which are required during pillar recovery operations. Mobiles are a more effective ground support than timbers, and their usage enhance the safety of section personnel and reduces material handling injuries. MRS usage is rapidly increasing, and approximately 40 U.S. coal mines have successfully employed this relatively new technology. This paper is in response to increasing requests from operators, State and Federal Regulatory Agencies, and others on the practical aspects of MRS usage in underground coal mines. During this investigation, nearly half of the U.S. mines which have utilized mobiles were visited. This report depicts the more common pillar extraction methods which operators have found successful. The Christmas tree and outside lift methods are illustrated and discussed. Roof control plans that do not require breaker posts or allow pillar extraction with fewer than four mobiles are also examined. In addition, operators' experiences with setting pressures, loads, and rates of loading during pillar extraction are addressed. Mining and support strategies to more effectively control hillseams, weak roof, and gob overrides which have entrapped equipment are also discussed.

Jan 1, 1996

By Don Kay

A two year study into the effects of coal mining induced subsidence on cliff lines was undertaken by the Department of Mineral Resources with the support of a grant under the National Energy Research, Development and Demonstration Program. A new monitoring technique, using an Electronic Distance Meter and acrylic reflectors attached to the cliffs, provided safety for the surveyors and extensive and accurate three dimensional displacement data on the movement of the cliffs as they were being undermined. The study report included many graphs showing the measured vertical and horizontal displacements, differential subsidence, tilts and strains and included analysis of these movements with various parameters, such as, seam thickness, depth of cover and distances between the reflectors, longwall face and the edge of the chain pillar. Some of these graphs are presented in this paper and could be used, with caution, to predict subsidence movements around other cliff lines, after allowing for differences in geology and mining layout. A mechanism is suggested as an explanation for the observed high horizontal movements around the cliff lines. Sixteen percent of the overall lengths of cliffs that were undermined during the monitoring study fell and the contributions of various factors that appeared to influence the cliff falls are discussed. It is concluded that no single factor dominated as the major cause of the cliff falls and, often, many factors combined to cause cliff face instability.

Jan 1, 1992

By Victor V. Nazimko

Long term stability of the pillars that have been remained after R&P extraction causes postponed subsidence. To prevent unforeseen subsidence and to diminish postponed subsidence 25-50% new method for its control has been developed by the author. It supposes to use drilling and blasting technology and other original approaches. New method has been fulfilled through the holes being drilled from the surface or from the active underground openings. Explosives had been charged into the holes and were blasted. This had increased bulking factor dramatically and prevented natural caving of the strata. Physical scale modeling corroborated efficiency of the new approach. The bulking factor has grown from 0.0623 to 0.1185 and surface subsidence has been diminished from 1.1 m to 0.2 m or 5.5 times.

Jan 1, 1996

By M. M. Zarnan

This paper presents the results of s study involving characterization of ground subsidence of abandoned lead-zinc mines in northeastern Oklahoma. Two coordinated efforts were undertaken: (1) collection and analysis of available data such as driller's logs, geologic cross-sections, structure contour naps and stratigraphic sections, and (2) development of a numerical (finite element) technique for prediction of collapse and mine subsidence. An in-depth analysis of a major collapse in the region was conducted. The factors which contributed to this collapse were identified.

Jan 1, 1989

By Lewis Martin

Spokane Research Center personnel are currently conducting research on cable bolts to assess their material and support properties, to provide design criteria for roof control systems under various types of underground mining conditions, and to provide background data for computer models of cable bolt supports. Laboratory studies of cable bolts made of conventional steel cables with Garford bulbs and collars resulted in recommendations on characteristics such as embedment length, hole diameter, cable bolt diameter, and configuration of cables. A completed series of laboratory tests has determined the minimum embedment length of resin required for resin-grouted, cable bolt ground support.

Jan 1, 1996

By M. K. Larson

The U.S. Bureau of Mines and Cyprus Shoshone Coal Corp. conducted a study of deformation mechanisms in strata around a longwall gate road system at two sites in an underground coal mine near Ham, WY. Of particular interest was timedependent (on the order of several months) roof deformation. Strata above and below the coal seam are very weak, carbonaceous mudstones that have cohesionless planes of weakness oriented along bedding, which dips approximately 8° to 16° at N 49° E. An extensive array of instrumented roof bolts, roof extensometers, and biaxial stressmeters were installed during development mining at these two sites under variable depth of cover [I83 to 335 m (600 to 1,100 ft)], seam dip (10° to 16°), and top coal thickness [estimated to be 0.30 to 0.61 m (1 to 2 ft) on the downdip side of the entry]. Bolt load and roof deformation histories during the entry development periods were compared. Bolt loads at site 2 were less than those at site 1, which is consistent with the amount of overburden. Deformation magnitudes at site 2 were similar to those at site 1, but deformation rates were approximately 44% greater and nearly constant over time, whereas the deformation rates at site 1 decreased exponentially over time. Also, most deformation occurred above the bolt horizon at site two, but extensometer measurements at site 1 showed that a significant amount of deformation occurred within the bolt horizon. The most significant factor that influenced bolt tension and timedependent strata deformation was strength and Young's modulus of the strata. Finite-difference models support this conclusion.

Jan 1, 1996

By Mikhail P. Nesterov

The paper presents the results of the research that can forecast the development of subsidences and deformations of the earth surface with time in the permanent marginal areas of the movement troughs arising when mining sylvinite beds in the Upper Kama deposit of potash ores where the rock formation support is made by means of interchamber (rib) pillars of various yieldability.

Jan 1, 1997

By Peter Altounyan

The rate of decay of the energy from a hammer blow to a coal mine roof is related to the size and nature of the contacts between the rock being struck and its surroundings The use of this principle as a basis for testing roof condition in South African coal mines has been investigated under a SIMRAC funded project, with the objective of reducing the number of roof falls. ( I ) A prototype instrument having a simple numeric display related to the energy decay rate (the Acoustic Energy Meter) was constructed and used to carry out surveys at mines covering the range of coal mine roof geology and condition. The equipment was simple to use and provided excellent discrimination between visually intact roof and less stable conditions, once the normal/abnormal threshold had been established for a given site. Areas of potential roof failure were identified close to roof slips and igneous dykes, and confirmed in areas of visible deterioration. Directional stress effects on roof condition were also identified at some sites, with consistent differences in readings obtained depending on the direction of drivage. Repeat surveys also indicated areas where roof conditions were deteriorating with time. Based on the success of the prototype, a hand held version of the instrument. has been developed to a specification from Anglo Coal. This incorporates a simple 'traffic light' green / amber/ red display, making the device suitable for routine use by mine operatives This version is now undergoing trials at an Anglo Coal mine. It is concluded that the Acoustic Energy Meter shows great potential as a test instrument providing output which relates to the stability of the immediate roof.

Jan 1, 2000

By Xu Lin Sheng

During the mining course, when the roof above coal seams is constituted by thick and hard sandstone (conglomerate), sudden roof movements often take place with great force and de¬struction which is the biggest danger to the safety and production. Such sever movements of the roof in mining directly relate to the rock mechanical properties which are the main fac¬tors that influences the regularity of hard roof movement and its destruction form. A significient work and important technical way for applying rock mechanics into mining en¬gineering is studying the rock mechanics of the hard roof and altering some of the rock me¬chanic parameters which induce the improvement of the rock properties in order to efficiently control the movement of the hard roof.

Jan 1, 1989

By Yajie Wang

High horizontal stress and its adverse effects on longwall gate entries have been witnessed in many US coal mines for the past years. It can cause cutter roofs and leads to roof falls either in the headgate or tailgate entries depending on the relative orientation of the panel to the maximum principal horizontal stress. Underground observations have shown that if the maximum principal horizontal stress is perpendicular to the panel entries, the roof is in the worst condition and if the maximum principal horizontal stress is parallel to the entries, the entry is in the best condition. However, little has been done about the high horizontal stress effects on panel entries subject to different orientations of high horizontal stress. In order to design a longwall panel and control the roof effectively, the redistribution and concentration of high horizontal stress field must be fully understood. Using 3-D finite element analysis, this paper presents the high horizontal stress redistribution and concentration for a retreat longwall face with different orientations of high horizontal stress. Based on the results, possible roof failures for different orientations of high horizontal stress are analyzed. A panel design criterion is recommended.

Jan 1, 1996

By John C. Stankus

Jennmar Corporation. in an effort to improve mine roof control, has conducted extensive study and research comparing tensioned point anchor resin systems versus the use of non-tensioned fully grouted rehar. Preliminary indications are showing that in thinly laminated, weak strata, tensioned point anchor resin systems are providing more effective roof control than the non-tensioned fully grouted rehar. In this paper two case studies are presented in which tensioned point anchor resin systems were utilized to stabilize and control the mine roof in two different mines. Both mines are alike in many ways, but yet very different in other aspects. However, the mines do share one main common factor. and that being the non-effectiveness of the non-tensioned fully grouted rebar in controlling the thinly lamininated strata in each respective mine.

Jan 1, 1990

By Peter Cain

In response to the appearance of excessive convergence of bolted gateroads in the front abutment zone of retreating coal faces at Phalen Colliery, staff from the CANMET Cape Breton Coal Research Laboratory in Sydney, N.S. initiated a programme of roof profile measurements in the front abutment zone of longwall headgates. The results of these measurements allowed a characteristic deformation curve to be determined for each headgate, and the early identification of areas showing an unstable deformation trend. These areas were then reinforced with supplementary support. The monitoring programme was adopted and enlarged by the rock mechanics staff of the Cape Breton Development Corporation, and has been continuing for a period of approximately two years. A sufficient quantity of data now exists to be able to make an assessment of the effects of a number of mining parameters on the performance of the gateroad in the front abutment zone. The following paper describes the method of measurement, illustrated by typical results, and goes on to relate variations in rate and patterns of movement to geology, interaction, method of support and pre-mining stresses.

Jan 1, 1996

Knowledge of initial ground pressures or in-situ stresses and mechanical properties and geological conditions is of great importance in drivage and maintenance of a mine roadway. Field measurements of the in-situ stresses and the geological and mechanical properties have been carried out. However, these methods are really impractical in using in underground mines. Alternatively, back analysis method using measured displacements during drivage of roadway can be used. The paper presents discussion on some problems in applying inverse formulation of back analysis to a roadway driven in an in- homogeneous rock mass. The back analysis is formulated by 2-0 finite element method. The inverse procedure is carried out using a microcomputer, but for the multi-rock layer, a larger computer or a main-frame computer is used due to an iterative procedure.

Jan 1, 1990

By Stephen Tadolini

The annulus thickness, for bolts anchored with resin, effects the systems final performance. Recent developments in the manufacturing process of these support systems, coupled with improved resin characteristics, have resulted in design changes to minimize hole drilling requirements and reduce resin usage. The amount of resin to properly anchor a 7/8-SRD (Special Rolled Dimensions) bolt in a 25.4 mm (1-in) diameter hole has been reduced to about one-third of the resin required to anchor a No. 7 rebar bolt (22 mm (7/8-in) diameter) traditionally installed in a 35 mm (1-3/8-in) diameter borehole. More importantly, laboratory and field tests, indicate that higher bond strengths and anchorage capacities can be realized with reduced annulus systems. These factors have a direct bearing on reinforcement efficiency and subsequent roof support performance.

Jan 1, 1998

By L. R. Stace

Arguably, one of the most significant technological breakthroughs that had been experienced during the period when British Coal was the major coalmine operating company in the UK, the use of rockbolts as principal support in underground roadways brought with it the need to re-assess the technical knowledge, management controls and legislative frameworks associated with the support of mine workings. Following a major roof fall in a roadway supported principally using rockbolts in which fatalities occurred, the process of the design of rockbolted support and the systems and procedures that had been put in place to control its use in the UK were further reviewed. The paper discusses the outcome of these reviews, and how the incident initiated a major new interest in strata control research in the UK against the background of the fragmentation of the ownership of the mines following the sale of the former British Coal assets at the end of 1994, together with the proposed introduction of new strata control legislation in 1996.

Jan 1, 1996

Historically, the selection of mining sequence for multiple seam conditions has been based on many factors other than ground control considerations and minimization of interaction effects. Today, numerous operations are finding themselves mining above existing or abandoned operations, some of which exhibit large scale caving, pressure arches and subsidence troughs. Development of design criteria for the upper seam that would alleviate damage due to lower seam interaction effects can contribute substantially to the safety and productivity of many mining operations. The primary objective of this re search has been to identify methods of improving ground control conditions in the upper seam when mining through either passive or active subsidence waves. These methods concentrate on improved ground control practices for the upper seam, rather than reducing the amount of subsidence in the upper seams by improved practices in the lower seam. Design factors such as pillar size, pillar and opening orientation relative to the subsidence trough, offsetting intersections, and different support methods have been studied for the upper seam in a variety of typical geologic environments. Analysis has demonstrated criteria for ameliorating negative interaction effects and facillitating design in the superincumbent seam.

Jan 1, 1986

By Rudiger Baltz

The sudden failure of bodies of rock is a serious problem commonly experienced in the international mining industry. These explosive releases of energy are referred-to as ?bumps?, ?outbursts? and ?rockbursts?. Rockbursts can occur at such a rapid speed that they present a direct threat to safety in large parts of the mine workings. The build-up of excess stress within the body of rock can be attributed to different causes. In shallow workings, for example, sudden and violent failure of the roof strata can be caused by leaving insufficient pillar width when practising room-and-pillar mining. At much greater working depths, as it is the case in the German coal industry, rockbursts are generally attributable to high levels of rock pressure. This can even result in coal outbursts when driving individual roadways. More than thirty years ago the German coal industry suffered from numerous rockburst incidents which caused many deaths and serious injuries. This led directly to the establishment of the Rockburst Prevention Centre, which was created at the instigation of the mining companies and the chief mines inspectorate. The consistent logging and analysis of all rockburst incidents has led to the development of a system that is still used as a basis for preventing underground rockbursts and minimising the risk of such phenomena occurring. Working closely with the mining authorities and mining companies the accredited experts of the Rockburst Prevention Centre have played a key role in helping to develop and implement a range of methods for the early detection and prevention of rockbursts.

Jan 1, 2008

By G. K. Rogers

The portal, which is the near-horizontal, surface point of entry to an underground excavation, can often be an exceedingly difficult area in terms of ground control. Surface and subsurface failures at portals, as discerned during a study involving over 300 case histories, are unfortunately, common. To aid in the engineering evaluation of portals, the Geomechanics Classification System was appended with discontinuity orientation assessment values and support/excavation guidelines. Also, a design model utilizing RMR predicted rock loads in conjunction with half-dome theory is proposed for the most common type of portal failure. Other pertinent comments and recommendations relating to portal stability are included.

Jan 1, 1989

By Anthony Iannacchione

Designing coal pillars to provide resistance against overburden loads has long been an aim of rock mechanics engineers. The need for accurate pillar strength models has become more urgent as greater overburdens are encountered and pillar sizes grow larger. Current pillar design models differ widely in their predictions of the trend in pillar strength with increasing pillar width-to-height ratio. The goal of this paper is to evaluate current pillar strength theories, using a comprehensive data base of stress measurements from coal pillars. The stress measurements indicate that coal pillars maintain relatively high stresses near the ribline, and that the stress gradient within the yield zone can be approximated as a straight line. Several problems are identified for further research, including calibration procedures for different types of stress cells, strain softening behavior in the yield zone, and measurement of stresses in the cores of very wide pillars.

Jan 1, 1992

By Joe F. T. Agapito

High stresses and adverse geology in deep coal mines in the state of Utah, USA, have caused numerous bumps. The larger bumps have been associated with seismic events with Richter magnitudes of 3.6 and greater, and in some cases have filled openings for lengths of 150 m. A better understanding of the mechanisms and stress levels involved in bumping is needed to help develop improved stress control design and bump mitigation methods. The geology of the area is notoriously bump-prone. The coal has poorly developed cleating and occurs in multiple seams that are often bounded by very strong roof and floor sandstone/siltstone beds. The overburden is formed by thick, competent strata with numerous sandstone channels This geology and deep cover are the major source of high stresses, causing bumps. This paper evaluates live common stress factors responsible for bump problems: depth, sandstone channels, arching, faults, and coal thickness. It uses ease study data from longwall panels with two-entry/yield pillar systems typical of deeper Utah mines. Results illustrate the importance of analyzing stress factor experience to allow a better understanding of the problem.

Jan 1, 2000