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By Michael Alber

Coal resource extraction reaches higher depth of overburden and rockbursts as well as associated seismic events appear to become an additional problem to he addressed by ground control engineers. For example, German coal mines approach a depth of overburden of some 1400 m (4200 ft). Certain strata at those depths and associated high stresses may react upon resource extraction and subsequent deformation by violent release of strain energy, termed rodcbursts. Rock mechanics tests on numerous rock types have been conducted and three types of brittle rock behavior were classified. This paper describes different types of rock behavior in uniaxial compression and discusses a possible classification of rock with respect to their likelihood of violent response upon loading.

Jan 1, 2004

By Buddhima Indraratna

The paper highlights the conceptual developments in rock mechanics applied to quantify the effect of bolts in the stabilization of joints and yielding soft rocks.

Jan 1, 1992

By Gift Makusha

The Witbank Coalfield in South Africa contains up to five economically viable coal seams. These seams have been mined continuously for over 100 years and the reserve in now becoming depleted. The normal mining method has been bord and pillar with examples of stooping. The situation now occurs where the formed pillars, left to maintain stability, have a higher quality than the coal that is left in the unmined reserves. Where the reserves are less than 70m deep and continuous over several kilometers, opencast mining is being successfully applied. After several years of investigation, Anglo Coal has identified blocks of coal that warrant investigation with regard to pillar extraction by underground methods The proposed method is pillar extraction using Mobile Roof Supports (MRS) It is proposed to mine three panels at a depth of 120m, by keeping the panels narrow and sub-critical. The sub-critical layout does not allow caving to surface and more stress is thrown onto the barrier pillar rather than the face pillars. The sub-critical span is anticipated to be 100m. The paper describes the full geotechnical investigation, including numerical modelling, instrumentation and drawing on the local South African experience of shortwall mining. The project will be unique in that pillars were not designed for pillar extraction and have low factors of safety (l.4 - 1.8) and width to height ratios of less than 4. The mining height will be approximately 4-4 5m, and this represents "new" operating heights for Mining Roof Supports. The required equipment was ordered in late 2002 and pillar extraction is anticipated to begin in June 2003.

Jan 1, 2003

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 Kikro Matsui

Longwall mining 1s more productive in comparison to other methods but requires particular conditions for its effective use This paper relates to longwall roof instability at Ikeshima Colliery In order to make clear the cause of the longwall roof instability, scientific investigation in terms of geomechanical factors and longwall support system was planned and carried out

Jan 1, 1997

By Kent D. Mctyer

Mining commenced at Tasman Mine in late 2006. The current method of mining is bord and pillar using continuous minerbolters and shuttle cars for first workings and secondary extraction using mobile roof supports. The two stage process was chosen to accommodate irregularly shaped coal deposits, allowing adjustments to be made to extraction ratios for better management of subsidence and to maximize the efficiency of the operation. Following the completion of the first three full/partial extraction panels, the mining method was changed due to variable caving. Another partial extraction method, referred to as the Duncan Method was adopted; it involves stripping the developed square pillars on four-sides on retreat to leave a load-bearing remnant coal pillar. The system of partial extraction has been successful in meeting safety, productivity, and subsidence targets.

Jan 1, 2011

By Marek J. Mrugala

Coal strength based on scaled uniaxial compressive strength from the laboratory and back calculations from in-mine observations of pillar stability indicate that material strength scaling rules are open to debate. Analyses presented indicate that the application of scaling factors lower than 0.5 provide a better correlation with field observations of pillar stability.

Jan 1, 1989

By S. M. Matthews

The magnitude and orientation of the in situ stressfield has been determined as a key factor in controlling the stability of openings for both coal mine development and extraction. Monitoring of roadway and pillar behaviour in conjunction with numerical analysis has identified specific stress control measures that have been successfully applied to improve productivity in underground coal mining operations. The influence of the major horizontal stressfield on the stability of openings has been identified at a wide range of sites including mines in Australia, Britain, the USA, New Zealand and Japan. Stress mapping, stress measurement and stress monitoring techniques have been applied to define the in situ stress regime and, together with the correlation of measurements of rock deformation and rock reinforcement behaviour, has enabled appropriate stress control measures to be developed. Stress control measures for development headings have required the selection of roadway orientation, roadway sequencing, reinforcement density and pillar size. The monitoring of behaviour at a range of trial sites has confirmed significant improvements in development stability. Stress control measures for longwall extraction have required rational selection of longwall orientation and extraction direction. Where a mine layout design incorporating the optimum longwall layout design has not been possible, the use of sacrificial roadways has been successfully applied to protect key roadways from stress concentration effects. Comprehensive in situ monitoring of behaviour has been carried out to confirm the mechanisms involved.

Jan 1, 1992

By T. J. McMahon

This Bureau of Mines study describes the development of a large-scale, three-dimensional, finite-element model of a deep-vein Coeur d?Alene District mine. The three-dimensional model was prepared from combined digitized mine maps and generated levels that may also be analyzed as independent two-dimensional models. A comparison of the stress changes and displacements was made following a simulated vein excavation in both the three-dimensional and two-dimensional models. A one-cut excavation was simulated with the two-dimensional model and a multiple-cut excavation was simulated with the three-dimensional model. The compared results from the two analyses are discussed in terms of the inherent differences between the models.

Jan 1, 1989

By David Hill

Over the last decade, Beltana No.1 Mine has generally been Australia?s most productive longwall operation, with a ROM output of 7.2 Mt in YEJ2007. Characteristically favorable roof conditions and low depths of cover resulted in typically high rates of development drivage and longwall retreat. As a consequence of these high rates of development and extraction, there was a continuing focus on recovering and relocating the longwall face as efficiently as possible to minimize associated production delays and operational costs. This paper presents a comprehensive case study of the conventional longwall recoveries (?take-offs?) at the mine, from Longwall 1 to 14. By examining the take-off monitoring data collected over the life of the mine, the key geotechnical issues are demonstrated, including their relationship to the ground behavior experienced and the issues overcome.

Jan 1, 2011

This paper presents a two-dimensional numerical procedure which incorporates concepts related to the mechanics of strata deformation' as well as' empirical indices associated with subsidence engineering. Regional deformation data were employed in order to establish overburden deformation zones, while material properties were kept constant. Scaling of final displacements was based on empirically predicted values for subsidence and strain for the Appalachian coalfield. The model was validated with a number of regional case studies and predicted subsidence and strain curves were compared to those obtained using the semi-empirical influence function formulation developed by VPI&SU.

Jan 1, 1988

By Francis Kendorski

One of the most dangerous events in underground coal mining is unexpectedly encountering water inrushes from undetected abandoned mines in the same seam. The surest and most confident method is probe drilling either from the mine or from the surface. However, drilling is expensive and may even miss the suspected mine voids entirely by drilling through pillars. Many operators rely upon one or more of several remote sensing techniques for detecting mine voids. However, mine "voids" often are not air- or water-filled open cavities, but are collapsed, rubble-filled chimneyed columns in the strata. Geophysical techniques, such as seismic reflection and refraction, electrical resistivity, magnetics, ground-penetrating radar, and others, often assume a continuous or fractured rock mass that has varying properties which provide the signatures that allow discrimination of one strata from another, or of strata from voids. However, a rubble-filled cavity has rock block-to-rock block contact throughout its volume and can still respond as a continuous rock mass with the rock blocks allowing signal transmission or mass detection, rather than a void space. Hydraulically, a rubble-filled cavity is essentially as transmissive to water as an open mine void. Thus, the problem of detecting a mine void with confidence is significantly compounded.

Jan 1, 2004

By Al Campoli

The majority of the 100 million roof bolts installed each year in the United States are 5/8 inch deformed rebar fully grouted into a one inch borehole. Commonly referred to as B-Series or No. 5 bar, this system is proven effective and cost efficient in coal mines with average to above average roof conditions. One of the critical variables affecting resin bolt performance is the size of the resin annulus, calculated as the difference between the hole and bolt diameters. The optimum annulus has been proven to be '/4 inch, which provides for effective mixing of the two polyester resin components and incorporation of the plastic film packaging within the cured resin. Glove fingering occurs when the plastic film is forced against the borehole wall potentially interfering with the mechanical interlock of the resin and the rock mass. Glove fingering is not a problem with the optimum '/n inch annulus but has been know to occur with the B-Series 3/8 inch annulus. Laboratory testing correlated the degree of Glove fingering with resin formulation and the bolt head to shaft axes alignment. Glove fingering occurred over 45 % of the grouted length of standard B¬Series bolts. The newly formulated Eclipse B-Series resin when combined with a 1/8 inch offset between the bolt head and shaft axes reduced the glove fingering 70%. The Eclipse system allows the 3/8 inch B-Series annulus to achieve the positive mixing characteristics associated with the / inch optimal annulus systems.

Jan 1, 2002

By Bruce K. Hebblewhit

Australia is well endowed with extensive reserves of thick underground coal scams, particularly in the range of 4.5m to 9m thicknesses. (For the purposes of this paper, thick scams are defined as being greater than 4.5m thick.) At the present time, the thickest, high production, high productivity underground mining operation in Australia operates at a maximum of approximately 4.8m using single pass longwall methods. In a number of instances, conventional' height operations are mining thick seam coal, effectively sterilising considerable reserves - either because of financial or technical risk concerns, or in some cases poorer quality coal at some horizons in the scam, UNSW has been involved in thick scam mining research over many years, most recently through an ACARP -funded project, jointly with the CMTE in Australia. A key part of that project involved the assessment of the relative geotechnical risks associated with the various methods under consideration. The methods were: single pass longwall; multi-slice descending longwall; soutirage and related caving methods (including the Chinese Longwall Top Coal Caving method (LTCC)): and hydraulic mining. A formal risk assessment process was adopted to review the risks, relative to a conventional 4m high longwall operation. This paper presents the findings of that risk review and discusses the geotechnical issues (and some possible solutions for risk management) which need to be dealt with in order to bring these methods to fruition in the context of the Australian coal industry environment.

Jan 1, 2001

By Kikuo Matsui

In a difficult economic environment, the coal mines in Japan have been making extensive efforts in order to survive by rationalizing the mining operations. Introduction of a new longwall mining system is one of these efforts to establish the rational production structure. In the high production longwall mining system, gateroads should be developed rapidly and maintained effectively during their lifetime. Strata bolting support system has been employed in the gateroads to raise their stability levels and reduce their maintenance cost. However, excessive gatemad closure caused by longwall mining is still occurring, impeding the passage of equipment, personnel transportation, and ventilation air in the longwall panels. This paper describes the development system of gateroads and proposes a new mining method to control the excessive roadway closure problem.

Jan 1, 1995

By J. Nielen van der Merwe

The effects of horizontal stress in coal mine roofs in South Africa did not receive the attention it deserved for several years, but now there is a danger that it may be over emphasized. A number of phenomena that are ascribed to elevated levels of horizontal stress, are shown to he explained by fundamental considerations. The stress concentration caused by the creation of an opening in rock and the beam effects in a stratified roof are briefly reviewed It is shown that guttering can be present without high horizontal stress, depending on the nature of the roof material. Three examples from practice are presented where horizontal stress was initially erroneously believed to be responsible for roof collapses A comprehensive program of roof displacement monitoring in South Africa yielded results that are compatible with gravity loading on roof beams without the assistance of horizontal stress. It is also shown that one of the diagrams that are used to explain guttering as a result of horizontal stress is misleading The over emphasis on horizontal stress may obscure the real causes of roof instability, resulting in ineffective remedial action to be taken by mine operators.

Jan 1, 2000

By J. A. Nemcik

Although floor failure at longwall faces has been traditionally associated with a soft floor, in many csses floor buckling has been observed just ahead of the longwall supports, despite the floor being a competent rock The floor failure is associated with the presence of weak bedding planes in the floor and the movement of strata towards the goaf opening. This paper presents the principles of the floor failure based on numerical modelling and field observations and outlines the practical approach to estimate the risks involved with floor failure.

Jan 1, 1995

By Steve Signer

The San Juan Mine and the National Institute for Occupational Safety and Health conducted a study to measure how development mining effected bolt loads. Twelve fully grouted, instrumented roof bolts were installed at two three-way intersections as part of the standard bolting pattern. Newly developed miniature data acquisition systems (MIDAS) were used to measure bolt load changes during initial face advance and cross-cut breakthrough. The effects of cut placement and depth on roof bolt loads were studied This test showed how bolt loads increased at five positions along the bolt length during initial mining. Both entry advance and cross-cut breakthrough produced a similar percentage of increase in bolt loads. Geologic differences between the test sites were probably responsible for the differences in amounts of bolt loading. The test site with more top coal and a higher rock quality designation (RQD) had lower bolt loads.

Jan 1, 2004

By Shu Cheng

Abstract: Based on the consolidation theory of soil dewatering and land subsidence prediction methods in soil mechanics, preliminary analysis of surface subsidence and movement due to groundwater withdrawal was performed using the stochastic theory and the principles of effective stress and the method for subsidence prediction was derived, Surface subsidence due to coal mining was then superimposed to obtain the total subsidence. Keywords: land subsidence, rock and soil consolidation, stochastic theory, effective stress

Jan 1, 2002

By Frank Kendorski

Several startup or longtime underground stone mines have experienced unanticipated roof control difficulties due to the variable geologic character of the roof strata. Careful mapping of roof conditions, and identification of controlling geologic and mine geometric factors using sedimentological descriptors, lead to an understanding of the mechanisms causing roof problems. Types of bedding plane features can be categorized into geometric, internal structures, bedding-plane markings, deformed bedding, solution structures, and accretionary structures. Factors such as encountering open water-filled master joints; roof strata delamination due to internal water pressures; local karst features with mud, sand, and water hazards; and sudden loss of limestone roof due to paleo-channel erosion are some recent problems described.

Jan 1, 2002