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By Ross Seedaman

The specification of roof support needs to consider rock strength, imposed ground stresses, and more importantly the bedding and joint structure of the immediate roof rocks. A database structure has been developed that allows these considerations to be incorporated into modern mine planning software so that better predictions of mining rates can be made. Rock strength is estimated from core and/or geophysical logs (especially the sonic velocity log) and ground stresses are estimated from depth of cover and knowledge of the regional stress field. The database is unique in the way it addresses bedding structure. Only 10 roof lithologies are allowed and these are allocated a numerical value that reflects an increase in the average spacing of bedding partings. These are referred to as 'Geotechnical Ferm Units' (GFU). The locations of all bedding discontinuities logged in the core are entered into the database and referenced to the GFUs. It has been found that, within each GFU, bedding spacing follows a negative exponential distribution and so it is a simple matter to predict the probability of the occurrence of any roof beam thickness. Output from the database can include plans of roof strength, the onset of overstressing, and the statistics of the distribution of beam thickness in the immediate roof.

Jan 1, 2003

By Vladimir I. Klishin

Complex method of ruling of rock pressure is considered in conditions of rapid roof subsidence. The purpose of this method is elaboration of emergency devices into hydraulic props. Also method of unloading of solid roofs is worked out. An analysis of means of protecting hydraulic props of powered supports 6om dynamic loads made it possible to divide them into four classes according to principle of operation: emergency, operating hm an increase of pressure; emergency damping; emergency, operating according to the "plastic element" principle; inertial emergency, operating hm dynarmc loads. It was established that the overwhelming 'majority of designs of the first three classes of emergency devices (EDs) are based on !mditional approaches, which exhausted their possibiiities of high-speed response. Delay on their opening is caused mainly by inertia of the moving locking parts. In our opinion, the most prospective are EDs using the inertia of the moving locking parts for depressurizing the piston cavity of the hydraulic prop with a minimum delay (inertial EDs). The essence of the designs of EDs having an inertial principle of action is that into the hydraulic prop containing movable and immovable elements is additionally introduced a spring-operated contracting support, which is installed on the movable element. Under dynamic loading of such a hydraulic prop hm the roof, the support moves relative to the movable element, performing the role of an inertial mass, which leads to instantaneous depresswization of the piston cavity of the hydraulic prop. Technological measures for unloading of solid roofs consists in creating of long oriented cracks in rock massif. Description of method for creating cracks, demanding equipment and means for cone01 the progress of splits are included. Also technological schemes for realization of the method of saving underground mines and rock mass are included. The results of experimental realization of this method on the Kuzbass and Poland coal mines are also included.

Jan 1, 1997

By Hamid Maleki

Results of geologic investigations, in-mine instrumentation, and numerical modeling are presented for two longwall mines in which pillars with large width-to-height ratios (8 to 9) are used. These investigations revealed that wide pillars unload when subjected to sufficient stress and that pillar unloading is influenced by geologic conditions. Field measurements in mine 1 clearly indicated load transfer from the longwall area toward the gate roads, resulting in pillar failure near the margin of a fluvial channel. Loads were shown to transfer to a harrier pillar (width height ratio equal to 9), contributing to progressive failure of this pillar. Unloading of the barrier pillar was associated with significant load transfer to the two-seam submains to the north of the harrier, contributing to roof falls, floor heave, and rib spalling within the submains. Stress analysis results addressed pillar peak strength and unloading behavior that was in agreement with the measurements and underground observations. Underground observations and stress analyses were completed before and after longwall mining near a six-entry submain in mine 2. This submain was intended to he used as a temporary bleeder and tailgate for a small longwall block consisting of one to two panels, depending on the development schedule in other areas of the trine and observed ground conditions. Numerical modeling using both elastic and inelastic pillar behavior for submain pillars with a width-to-height ratio of 8 was completed before mining began. Results indicated that pillar unloading was likely. During retreat of the first longwall panel, pillars unloaded. as evidenced by progressive rib spalling along the cleats, This behavior was mostly confined to the immediate face area, an observation that was in agreement with the modeling results,

Jan 1, 2000

By J. L. Condon

The Bureau of Mines, through in-house and contract research, monitored mountain bump-prone areas of the Olga #2 Mine, near Welch, WV, using microseismic techniques for 15 months during 1985 and 1986. During this period, two ground failures occurred that were considered "mountain bumps" owing to excessive stress buildups. One failure damaged the roof and 4 pillars around an intersection; the second failure, which released much greater energy, resulted in significant damage to over 100 coal pillars. Analysis of the microseismic data collected provided significant insights into the magnitude and location of each failure. The actual failure zone was clearly delineated prior to the mountain bump occurrences through analysis of source locations of each microseismicevent. Trends in cumulative rate plots indicated the onset of structural instability. Although the data could not be analyzed automatically, the significance of the data interpretation cannot be overemphasized. Microseismic monitoring of bump-prone areas can be successful in providing a means to actually observe the structural stability occurring in underground coal mines. This paper discusses data collection techniques, analysis methods, and implications of the monitoring technique, with special emphasis on actual data from two large mountain bumps in West Virginia.

Jan 1, 1987

By W. J. Guo

Overburden movement as a result of longwall mining can be classified into, in ascending order, caving zone, fractured zone, and bending zone. Operation experience and laboratory research have demonstrated that there are bed separations in the bending zone due to differential movement and rock mechanics properties between adjacent strata. In china several coal mines have injected flyash into bed separations to prevent strata movement and control surface subsidence. This method is very effective because it can reduce surface subsidence by 35-55%. Combining the field survey results and laboratory simulation research, the author has investigated bed separation and its mechanisms and effect of backfilling the bed separation and its mechanisms. Finally backfilling application in three mines are introduced, including grout flow chart, grouting pressure, and radius.

Jan 1, 1994

By John D. VandeKraats

Yielding rock bolt systems have been developed to provide ground support while accommodating shifting, creeping, or swelling ground movements. The Dywidag Systems, Inc. (DSI) Slip Nut System is one such system. It has been tested in the laboratory and installed and monitored in the interbedded halite and anhydrite at the Waste Isolation Pilot Plant (WIPP) since 1994. The Slip Nut System is installed on a point anchored threaded bar or on a threaded bar coupled to a cable bolt. At a pre-engineered load the nut acts as a die and cuts through the thread on which it rests, thereby reducing the load and coming to rest on the next thread where the process repeats. Two types of laboratory tests were performed using slip nuts on DSI's threaded bars and their Tensionable Cable bolts. Straight pull tests exceeding 0.3m (1 ft.) of displacement were performed on the systems. Also, offset tests were performed on these bolt systems in a test frame designed to simulate WIPP geologic conditions. A room scale test area consisting of 186 threaded bars began in 1994. Select bolts were instrumented to track bolt load. All bolts were tracked through time for operation. Through three complete years since installation over twelve slips, corresponding to approximately 0.15m (6 in.) of ground movement, have been observed in some areas. Since installation strata offset has exceeded 0.08m (3 in.) at a supported clay layer in the roof. No bolt failures have been observed. Slip loads observed underground generally approximate those observed in the lab. To date, the slip nut systems tested and installed have performed as expected.

Jan 1, 1998

By S. P. Singh

Rockburst is a sudden manifestation of the release of elastic strain energy stored in the rock. Therefore the tendency of the rock to burst should depend upon its ability to accumulate elastic strain energy. The use of Burst Proneness Index as a simple and reliable indicator to recognize the burst prone situations in the mines has been advocated in this study. The Burst Proneness Index exhibited a strong dependence upon the brittleness, compressional and shear wave velocities and the point load strength index. It was also observed that the Burst Proneness Index increases with the increase in the strain energy per unit volume, c ressive strength and the modulus of rigidity of the rocks. In addition, important causes and the measures to alleviate rockbursts have been outlined in this paper.

Jan 1, 1986

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 David C. Johnson

Coal mining in Illinois has resulted in many structures being exposed to subsidence damage. Damage mitigation techniques (DMT) applied in the foundation design/construction have proven successful in minimizing damage in several test cases. To determine the practicality of these techniques, the Costs of their application are investigated. The AMT costs are compared to foundation and total house costs for various classes and sizes of houses. An economical and effective subsidence resistant foundation construction is proposed.

Jan 1, 1993

By Klaus Opolony

The world's most popular method of longwall mining requires multiple entry systems for the panels. In contrast to this mine layout the roadways in German coal mining are used for advanced mining from the rock mechanic point of view or are even re-used by a second panel. This procedure of roadway use is presented within this paper using a 3D visualisation software tool. In the previous conferences in Morgantown several papas had addressed the basics of rock mechanic background, planning tools and variants. This paper gives a practical case study for reuse of a roadway in a German coal mine. The heading and the use of various alternatives are compared under the aspect of efficiency and costs. Within the comparison the specific requirements and benchmarks of German coal mines are taken into consideration. The costs for a multiple entry system and first of all the development rates that can be achieved with common road heading systems are compared with international layout alternatives. A prospect includes a discussion of pro and con for various development methods. Devices are given due to this prospect if and how multiple entry systems are applicable to German coal mines. The paper gives an initial point for international comparison and is a base for further discussion.

Jan 1, 2003

By Kanchan Mondal, Anthony (Sam) Spearing, Gopi Bylapudi, Joseph C. Hirschi

"The U.S. coal mining industry uses about 100 million rock anchors per year. ASTM F432 specifies the requirements for rock bolts on mines but does not address corrosion. Corrosion has been found to be an issue in Australian coal mines (Hebblewhite et al., 2003 and Villaescusa et al., 2007), where the problem has been researched and stress corrosion has been found to be a significant cause of rock falls. Corrosion is also a major concern for underground civil construction in the U.S. but has not been considered an issue or adequately researched in coal mines. Underground conditions are conducive to corrosion mainly because of water quality and humid conditions. There is a perception, however, that when bolts are fully grouted, adequate corrosion protection is offered to the steel. Research has shown that this is not necessarily the case, due to the formation of micro-cracks as the resin sets and with subsequent rock movement shearing the resin column. This paper outlines a method to determine the corrosion potential of bolts used in long-term excavations and suggests ways to mitigate such effects based on research conducted in the lab and on three mines.INTRODUCTIONResearch on rock anchor corrosion in mines has not been extensive enough in the past, and the effects of several factors in the mine atmosphere and waters are not clearly understood. One probable reason for this lack of research may be attributed to the time required for gathering meaningful data, which makes the study of corrosion quite challenging.Almost all the rock bolts used in coal mines are made from steel, hence the corrosion of iron needs to be understood. The corrosion of iron is an electrochemical phenomenon with the overall equation"

Jan 1, 2010

By Robert W. Bruhn

A 125 foot high steel lattice frame tower supporting a 500 kV EHV transmission line and located over the gateroads of a longwall mining operation was subjected to ground movements from the mining of adjacent longwall panels. Under an agreement with the owner of the powerline, the mine operator was required to minimize mining impacts on the transmission line. In view of this requirement, the mine operator prepared a forecast of the expected subsidence ground movements prior to the onset of mining. These forecasts, in conjunction with finite element analyses of the tower superstructure under the imposed ground movements and wind loadings. suggested that the structural capacity of the tower would probably be diminished little, if at all, by mining. Ground movements determined by survey during mining of the two longwall panels were found to be in close agreement with predictions. Strains measured in selected tower members during mining generally agreed with values calculated by the finite element analyses. The study confirmed a methodology to predict subsidence and to assess its effects on steel lattice transmission towers.

Jan 1, 1990

By André Zingano

This paper analyses possible causes for floor heaving in room-and-pillar coal mining. The coal mines are in the southern part of Brazil, where the Barro Branco Coal Seam varies in depth from 20m to 400m, and a very thick volcanic formation overlies part of the sedimentary basin. The erosion of part of the volcanic formation causes this large range of overburden thickness, which formed large V-type valleys. The mine in study is located near the slope of one of these valleys. The mine inclined shaft is at the lower overburden thickness area of the valley and the mining is developing toward the thicker overburden area. The floor heave occurred in the crosscuts, which are parallel to the slope valley direction. Numerical modeling was applied to study the stress distribution around the openings; and fieldwork on the floor heaved entries was carried out. The geometry of mining (pillar and openings width) also was analyzed. The results showed that the very weak rock and the high horizontal stress were the causes of floor heave. As the mining gets near the valley slope, the stress distribution around the crosscuts changes considerably. In addition, the stress distribution changes as the mining goes far from the crosscut. Keywords: floor failure, room and pillar, finite element model

Jan 1, 2002

By X. L. Yao

This paper analyses the effects of mining extraction geometry (width/depth ratio) and dip of seam on subsidence parameters based on a large Database, containing 120 longwall mining cases all within the UK coalfields. The subsidence parameters evaluated in this paper include the following: the maximum subsidence value with different goaf-treatments, the position of maximum subsidence, the position of half-subsidence, subsidence value over the rib-side, positions of maximum tensile strains, positions of inflection points and the magnitude of maximum tensile strains on both rise and dip sides. Additionally, the maximum compressive strain and two limit angles are also studied in this paper. Based on a comprehensive Database analysis, several equations are derived relating these parameters to the width/depth ratio and the dip of seam. The results should assist engineers to design new structures, or take measures to protect existing structures from strain effects. The findings of the research assist in extension of the SEH to inclined seams with greater certainty.

Jan 1, 1990

By Mingju Liu

In this research, we measured fractal characteristics of over ten typical outburst areas on different scales and related them to their outburst proneness, in attempts to provide new methods for outburst prediction. It was shown that both faults and folds follow the power law on coal basin scale, mine scale or mining district scale, whichever it is. Fractal dimensions are appropriate indices to describe geological structures quantitatively. Preliminary relationships have been established between outburst proneness and fractal dimension. Comparisons have been made among fractal characteristics and degrees of outburst proneness on all three scales. The results indicate that multi-scale assessment of outburst proneness can be made based on fractal measurements independently or supplemented by other assessment methods.

Jan 1, 2000

By Neil Styler

This paper presents an analysis of measurements of inter-burden de- formations above six longwall faces. An attempt is made to demonstrate some correlation between the movements at the various sites, and to examine their importance with respect to predicting caving height, disruption of overlying seams, and disruption of aquifers. This analysis demonstrates some significant differences between predicted surface subsidence, and inter-burden deformation. In addition it is shown that the caving height above a longwall face is equal to 8 to 12 times the extraction height, with a zone of fractured rock ex- tending to approximately 50 times the extraction height above the seam.

Jan 1, 1984

By Vasundhara

A chance detection of a seismic event of unexplained origin in the mining district of Appin in New South Wales. Australia, led to a detailed investigation of recording and analyzing seismic events occurring in the area. Multiple events were recorded each day on the seismometer stations. The larger events were strong enough to be recorded on a number of stations spread out in the district. Background investigation of seismicity stimulated due to engineering activities revealed that similar cases of mining triggered seismicity have been reported previously from many mining districts around the world. If the tectonic stresses are close to failure in a region then a small perturbation of the stress state may lead to instability in the region which may manifest itself as seismicity. The parameters that control the occurrence of such siesmicity include tectonic stress magnitudes and orientations. geological weaknesses or structural features and the amount of stress contribution due to mining. This paper reports on the seismic investigative study conducted in the Appin area and considers its association with the geological layout of the region and the mining activity.

Jan 1, 2001

By Richard C. Repsher

The U.S. Bureau of Mines has developed a method and device designed to detect lose rock material in underground mines. The technology is designed to be an aid to mine workers in detecting hazardous roof conditions in underground mines which can complement or replace the traditional roof sounding techniques where the miner relies on experience to determine whether rock conditions are sound. The leading cause of accidents and fatalities in underground mines is falls of lose rock pieces or rock slabs from the mine roof. The device is an electronic roof sounding device that acoustically determines the integrity of mine rock. In previous research the Bureau of Mines found that loose rock, when impacted, vibrates at a much lower frequency than intact rock material. A major problem in determining rock stability using this technique has been the repeatability of the matt signal. This difficulty baa been greatly reduced in the current design by measuring the power spectra contained in two separate frequency bands of the signal produced by striking the rock in question. The ratio of the energy contained in each band is computed. This process minimizes any striking force differences, producing accurate, repeatable results for solid rock as well as loose, drummy material. The prototype has been successfully- tested in a variety of underground environments including coal, uranium, molybdenum, silver and salt. The technology has been investigated by the U.S. Mine Health and Safety Administration and the Department of Energy for use in detecting detached tunnel lining areas in nuclear repositories. The paper will discuss the technique, applicable results, and future applications.

Jan 1, 1990

By Peter Altounyan

Goedehoop Colliery in the Witbank Coalfield of South Africa produces some 8 million tonnes of coal from fully mechanised room and pillar mining methods, using resin anchored bolts as roof support. Investigation of the quality of the installed bolts resulted in the finding that it was possible to pull out fully encapsulated roofbolts. This led to a drive to improve the quality of installation by making it easier to install a roofbolt correctly and more difficult to install incorrectly, whilst at the same time achieving high bond strength. The method of installation which was investigated, developed and implemented is known as 'spin to stall.' The roofbolt is pushed through the resin capsule to the top of the hole and then spun to mix the resin, until the increasing resistance to rotation exceeds the breakout limit on the bolt torque nut. The nut then runs up the bolt thread and is tightened to the roof as the drill stalls. This procedure is simple and avoids human factors such as non adherence to 'spin' and 'wait' times which contribute to poor installation with conventional methods. However spin to stall installation has been avoided in the past because of concerns over damage to the resin bond. This paper reports on the efforts made to investigate and understand the factors involved, and the progress made in overcoming the problem of resin damage so that the advantages of this installation method can be realised.

Jan 1, 2003

By S. F. Smith

Following a roof beam collapse on a major production district at the British Steel Corporation's Dragonby Iron Ore Mine, Scunthorpe, South Humberside, UK, rock mechanics investigations were undertaken in this room and pillar operation to assess the stability of workings throughout all the underground mine area. Over a four year period room convergence and roof sag were monitored in different mining areas and various junction configurations in order to assess and compare the relative movement of the strata resulting from mining activities, where varying rock bolt lengths and patterns have been utilised. The paper, after shortly describing geological aspects and mining methods, presents the research procedure adopted together with the instrumentation employed and discusses the results obtained during these investigations.

Jan 1, 1995