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By Jingyi Cheng

Many longwall panels in China are laid out along the dip direction such that the direction of longwall retreat mining is along the strike direction of coal seams. Downward mining longwall panels in steeply inclined seams refers to those that are laid out in a non-uniformly developed fold structure where the seam is not only steeply dipping, but also highly undulated along the strike direction, causing longwall mining extremely difficult because it has to deal with large dip angles that vary both in the mining and faceline directions. Obviously, the characteristics of mine pressure and overlying strata movement in downward mining longwall panels in steeply dipping seams have unique patterns that are very much different from those cases of mining the flat or gently inclined coal seams. The effects of those unique patterns presented a big challenge to production and safety. Almost all previous and current researches on ground control have been concentrated on issues related to the flat and gently inclined coal seams. Conversely, research on steeply inclined coal seams, especially those with large angle variation both along the seam dip and strike directions is almost none. Therefore, it is very important to initiate a study to better understand the characteristics of mine pressure and overburden strata movement for this special type of geological condition. The objective of this paper is to monitor and analyze the characteristics of mine pressure and overlying strata movement in downward mining longwall panels in steeply inclined seams, the angle of which varies greatly along both the seam dip and strike directions. The measured data has conclusively demonstrated that the characteristics of mine pressure and overlying strata movement in downward mining longwall panels in steeply inclined seams are different from those for the flat and gently inclined coal seams. The large downward angles of coal seam undulation along the strike (mining) direction can obviously affect the roof movement and reduce strata pressure. Study in this research showed that appropriately increasing the rate of mining (or face advance) is positive to roof control.

Jan 1, 2013

By Hani Mitri

A new monitoring device called SAFEBOLT (Stress And Force Evaluator in Bolts) has been recently developed. The device SAFEBOLT, essentially replaces the load cell, it permits the monitoring of bolt axial load from the time of its installation underground. The SAFEBOLT system consists of a portable readout unit, connecting cable and the SAFEBOLT device, which is installed in the bolt head. SAFEBOLT can be installed on any industry standard rockbolt. No special installation method is required; the conventional method of bolt installation by the stoper or roofbolter remains unchanged. The device is completely encapsulated in the bolt head, which makes it resistant to corrosion, rugged, and free of any electrical wires. Pilot tests at three underground Quebec mines were carried out and helped improve the initial design of the device connector. The paper reports on two of such pilot tests.

Jan 1, 2001

By Vasundhara

Underground coal mining in soft floor environments has often led to strata control problems ranging from small scale issues such as floor heave which disrupts traffic and ventilation, rib spalling and induced fractures in pillars, through to large scale instability and long term subsidence. Past investigations of the problems encountered in Newcastle Coalfields of New South Wales, Australia, have revealed that the soft floor unit comprises of claystone which is very sensitive to moisture. It is prone to swelling, reduction in stiffness and loss of strength in the presence of moisture. Long term stability of mine panels in such an environment is governed by the extent of deterioration of the floor strata. The paper reports the field investigations conducted at Cooranbong Colliery to assess the extent of softened floor zone in the roadway and under a pillar.

Jan 1, 2001

By Thomas L. Hutchinson

The mechanical design of a roof support is basically a matter of a statics and dynamics problem, assuming of course, that the imposed loads and mining conditions are known. Here in the Appalachian coal fields the general conditions are known well enough to allow the majority of the proposed new faces of longwall to be furnished with standard commercially available supports equipped with suitable variants and options. While not trying to negate the primary importance of thoroughly investigating a potential face and the selection of the correct equipment for that face, it still becomes clear that an equally important aspect of the design of an adequate ground control program has in the past too of ten been overlooked, having been overshadowed by the ?endless? selection process of the roof supports proper. The equally important aspect which this paper discusses is the overall design of a mining system, which design incorporates not only the size and capacity of the equipment but the following points as well, namely: Equipment adaptability to mining scheme, Equipment va. human constraints, Operation at the designed levels, and Coordination of operation, maintenance and support design. Unless the coordination of these factors is considered in the design and selection of the equipment, a lopsided and inefficient operation is possible. How can we afford to put an 8-10 million dollar investment up to chance as to whether it will yield optimum results or not?

Jan 1, 1981

By David A. Newman

In the 1950s, an underground limestone mine in the Camp Nelson Formation was developed from an outcrop exposure. The Camp Nelson Limestone met the requirements for DOT stone and aggregate. The mine ownership changed several times, which resulted in distinct mine designs in Level 1. Based upon the production requirements and an attempt to maximize limestone recovery from a single level, a variety of pillar dimensions and three mining heights (28 feet, 68 feet and 83 feet) were developed. This was sufficient prior to reaching the mineable extent of Level 1. Level 2 introduced previously unknown ground control challenges. These included: ??Pillar spalling and a significant back fall on the Level 1 - Level 2 decline; ??Laminated immediate back and the need to bolt the back on Level 2; ??Delamination of the back adjacent to the Level 1 - Level 2 decline resulting in bolt shearing, and floor heave in isolated areas of Level 2; ??Multiple level interaction, the need to design pillars based upon numerical modeling as columnization between Levels is infeasible: and ??Water inflow as the mine approached river. To meet the challenges, a testing program to determine rock strength and physical properties was implemented along with numerical (LAMODEL and FEM) modeling, and borescope inspections of the immediate back. The efforts to implement rock mechanics were re-focused as Level 2 approached its mineable limit and the initial attempt to drive a decline to Level 3 was unsuccessful. A second decline site was selected based on inter burden stability, a consistent shale parting for the Level 3 back horizon, results from numerical models of the immediate back and multiple level interactions, and the projected low water inflow. The mine is operating on Levels 2 and 3 with long-term planning and ground monitoring for Level 3.

Jan 1, 2014

By Dennis Dolinar

"Tailgate entries in U.S. longwalls can be subjected to high loads and deformations. Therefore, tailgate entries are generally supported with varying types and amounts of standing support. However, even with standing support, roof falls and blockages can occur in the tailgate when the support system is not properly designed. Safe and efficient operation of a longwall face requires the selection of the appropriate type and amount of standing support. Because of the importance of maintaining a safe and functional tailgate entry, the National Institute for Occupational Safety and Health (NIOSH) is currently developing a standing support design methodology based on the ground reaction curve. Using the ground reaction curve for design requires that the ground and support interaction be quantified. This can be accomplished by the installation and analysis of data from instrumented field sites. Although necessary, the field results cannot develop the complete ground reaction curve. Numerical models, calibrated to the field results, are used to establish the complete ground reaction curve. In the present study, data was obtained and evaluated to establish points for calibration of the ground reaction curve for two western longwall operations.One test site, Mine A, was located at a depth of 1,550 ft. At this site, the floor consisted of siltstone, while the roof was an interbedded siltstone and sandstone. Because of the depth, a double row of22-in diameter CANs1 were installed to support the tailgate. Both the first and second longwall panels have mined past the instrumented site.The second site in Mine B is at a depth of only 760 ft. The floor at this mine consists of a weak shale/mudstone and the roof consists of thick, top coal overlain by weak shales. At this site, a double row of pumpable cementitious cribs was installed. The second test site has only been subjected to side abutment loading from first panel longwall mining.The measurements indicate that the support at both mines was heavily loaded when subjected to the side abutment loading from first panel mining. At the deeper site, the support began to yield when the face was more than 300 ft outby during second panel mining. In comparison to results from previous test sites in the Pittsburgh Seam and Illinois Basin, the amount of support loading and roof-to-floor convergence was an order of magnitude higher at the western sites. This has resulted in a significant difference in the position and shape of the ground reaction curves."

Jan 1, 2010

By Gift Makusha

Total extraction mining of coal is being undertaken on a small scale by Anglo Coal South Africa. Longwall mining has been done in the group at New Denmark colliery since the 80s and recently the company started experimenting with pillar mining at Goedehoop colliery. Total extraction mining methods result in caving of the overburden strata that is often accompanied by displacement of air causing wind blasts. The windblasts can cause injuries to underground mining personnel, damage to mining equipment, and ventilation disruptions. Some incidents of windblast damage and injuries have been recorded at both New Denmark and Goedehoop Colliery that prompted mine management to experiment with the goaf warning device, an instrument developed by CSIR mining technology to give early warning of impending god. The initial version of the Goafwarn was originally developed to operate as a stand-alone unit, with LED lights mounted on the device that would flash if the unit was triggered'. That set up would be effective in small working areas where the flashing lights are visible to all workers. This requirement is more difficult to satisfy in longwall application of the unit where mining activities are spread over large linear distances of up to 250 m and the flashing lights would not be visible to all workers in the longwall face. It was also found that the single unit had limitations in distinguishing between man made vibrations and cracking in rockmass leading to an unacceptably high incidence of false alarms. The case studies describe the experiments that were carried out to ameliorate the two issues mentioned above through networking more than one unit. This was achieved through the development of radio communication from the warning devices to a central control (Trollex Data Acquisition Unit) from which the alarms are issued once the prescribed triggers are satisfied. The operating system was made up of 4 Goafwarn units installed 30 m apart with 2 in the tailgate and other 2 in the main gate. The alarm state was defined where 3 or more units were triggered simultaneously. The concept was applied successfully in real time during longwall mining, and management procedures have been developed to achieve safety through the system including switching off the shearer, and flicking the lights to warn people.

Jan 1, 2005

By Eddie Martin

Jim Walter Resources, Mining Division, employs the retreat longwall mining system as the primary method of coal production. The strata control aspects of developing and maintaining roadways for retreat longwall mining have continued to improve and advance at Jim Walter Resources. New and innovative concepts in pillar design, roof and rib support, as well as longwall face support, have been combined to provide better strata control. Control of immediate roof breakup in the headgate and tailgate entries has been a problem at Jim Walter Resources' mines. The application of roof trusses as supplemental roof support failed to provide the desired strata stability. The immediate roof deterioration has been controlled through the installation of increased density of roof bolts in the affected roadways. Control of the rib sides has been a documented problem at Jim Walter Resources' No. 4 Mine since its start-up. The installation of 3/4 inch diameter roof bolts at 2 ft. spacing failed to control rib rolls. The development and application of a yieldable rib support system provided the desired results. Rib rolls have been virtually eliminated where the yieldable system of rib support has been installed. An evaluation of longwall panel development using the total yield pillar mining plan has been completed. Results of the strata stability and other operational data have been compiled. Coal production and gate entry stability were not adversely affected using the total yield pillar mining plan for longwall panel development. Strata Control on the longwall face is provided by the face support system. Excessive convergence and roof breakup along the longwall face have been experienced at several of the Jim Walter mines. In an effort to eliminate this problem, an evaluation of higher setting pressures for longwall face supports is being conducted. Results have been inconclusive to date. Each of the above strata control projects will be presented as case studies. Since Jim Walter Resources business is coal production, emphasis will be directed toward the application rather than the theoretical aspect of the projects.

Jan 1, 1988

By Karl Jr. Zipf

The famous Homestake gold mine in Lead, SD. closed recently alter 125 years of operation. However, the mine may receive a new lease on life as a National Underground Science Laboratory (NUSL) supported by the National Science Foundation (NSF) and dedicated to high-energy physics experiments involving neutrinos. These experiments require the construction of many large underground chambers tells of meters wide at depths of over 21 18 m. Construction of such large openings at such extreme depths has few precedents m civil-engineering-type construction. The Spokane Research Laboratory has a long history of ground control research at the Homestake Mine and has supported NSF and the physics community in their efforts to develop a NUSL at I Homestake or other suitable location. This paper will summarize underground science and describe proposed underground science laboratories ill the United States, in particular tile proposed NUSL at Homestake. Next, the paper describes an empirical design approach using the Q system of rock mass classification for these large Openings. .M initial site investigation that produced three good case histories is described. The actual support system used at the three sites agrees well with support recommendations from current support design charts, giving credibility to estimates of the classification parameters and Q. Feasibility of, and support requirements for, 25-, 50-, and 100-m spans arc estimated in the anticipated rock mass. This analysis indicates that 25- and 50-m spans are probably feasible, but a stable 100-nn span may be unachievable. The empirical predictions are then compared to detailed numerical analyses. The paper concludes with preliminary support recommendations and necessary site investigations needed to make a proposed NUSL a successful reality. These studies have been important input to NSF for their decisions on the technical feasibility of developing the proposed NUSL. If NUSL is developed, it could enable scientific experiments in areas beyond high-energy physics. 'the laboratory could also enable research in rock mechanics. ground water hydrology. mining engineering, nine safety and health, and ground control.

Jan 1, 2002

By C. Srinivasan

Microseismic emissions are generated by deformation and cracking of rock around an opening inside a mine. The rock tends to achieve equilibrium in a new geometry under changing stress conditions was found to provide useful clues to the impending roof falls and ground instability. The typical characteristics of these microseismic event patterns is shown to be of diagnostic value in sensing in advance rook fall that invariably follow such events. In most of the longwall faces, the main problem faced was due to strata behavior in the presence of massive sandstone bed, which is generally difficult to cave. Microseismic monitoring system was used to monitor strata behavior during longwall mining operation at Rajendra underground coal mine of South Eastern Coal Fields limited (SECL). More than 44,000 microseismic events with local magnitude between -3.4 and 0.5 were recorded during the monitoring period from September, 2001 to March 2003. Analysis of these events was correlated with respect to roof falls. The time series of range of source parameters such as Radiated energy, seismic moment, Energy Index, Seismic Schmidt number, time of occurrence and location of fractures before and during roof fall exhibit qualitatively distinct patterns about six hours before roof falls. &ong the several seismic parameters investigated the microseismic event rate as precursor was found to be most significant indicator of an impending roof fall. The results are discussed as case study in this paper.

Jan 1, 2005

By Daniel W. H. Su

"This paper presents results from a recent study conducted to evaluate the effects of longwall-induced stresses and deformations on the mechanical integrity of shale gas wells drilled over a longwall abutment pillar. The primary objective is to demonstrate that a properly constructed gas well in a standard longwall mining abutment pillar can maintain mechanical integrity during and after mining operations. Successful designs would allow for the safe recovery of both coal and gas reserves. Major elements of this comprehensive study include: (1) modeling of rock movements to predict impacts on gas wells, (2) measurements of actual rock movements at the surface and in the subsurface, (3) measurements of actual impacts on test wells designed similarly to real shale gas wells, ( 4) comparison of modeling results to actual rock movements to validate the model, and (5) employing the validated models to evaluate various casing and cementing designs to minimize casing deformations and absorb longwall-induced strains along the intermediate and production casings.A study site was selected over a southwestern Pennsylvania coal mine, which extracts 1,500-ft-wide longwall faces under about 600 feet of cover. Four test wells and four monitoring wells were drilled and installed over an abutment pillar with 125-ft by 275-ft centers. The four test wells were drilled to a depth of 642 ft, or 42 ft below the Pittsburgh seam, and were designed like a real shale gas well, except that only three casings (namely the surface, water protection and coal protection casings) were installed in each well. Casing steel grades, wellbore diameters and annular spaces were purposely varied in each of the four test wells to evaluate the effect of different well designs. The four monitoring wells - one vertical extensometer and three in-place inclinometers (IPI) - were drilled to different depths to measure the vertical settlement and horizontal displacements as a result of longwall mining on both sides of the abutment pillar. In addition to the test wells and monitoring wells, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the 1,500-ft-wide longwall panels on the south and north sides of the abutment pillar mined by."

Jan 1, 2016

By Francis Kendorski

A longwall coal mine in Appalachia about 1,500 ft deep encountered a fault while developing a new longwall panel. The fault extended from mining depth to the surface near a secondary road and drainage. The fault was located inside the anticipated angle of draw within the mined panel and gob. The fault extended vertically out and up, away from the panel, caved zone, and gob at nearly the angle of draw: the fault very nearly following the angle of draw. It was initially thought that "fault reactivation" could possibly occur. Fault reactivation is the phenomenon of having mining subsidence localized along a fault leading to a "reactivation" of the fault and shearing and displacement along the fault. Such fault reactivation would disrupt and deform the fault plane beyond the normal angle of influence of the subsidence trough, and may provide a conduit for any ground and surface waters to reach the mine. We contacted all operators of longwall mines in Appalachia to determine if any Appalachian longwall mines had ever experienced fault reactivation, and teamed that none had experienced the phenomenon. After studies of possible water intrusion quantities and rates based upon in-fault pump tests, which indicated that water intrusion rates should be manageable, and the prior experience that faults in this particular area were usually barriers to water flow, mining proceeded with caution and monitoring. Mining was successful with no noticeable increase in water inflow rates, and no measurable off-setting of the fault exposure on the surface. It can be concluded that the fault did not reactivate due to its relationship to the mining sequence.

Jan 1, 2003

By Keith Heasley

In this paper, a state-of-the-art, three-dimensional, full waveform, microseismic system was used to analyze the rock failure around a deep (> 750 in (2500 ft) of cover) bump-prone longwall panel. The microseismic system consisted of both underground and surface geophones coupled through radio telemetry and a fiber-optic network to produce pseudo real-time detection and location of seismic events surrounding the coal seam. This was the first microseismic installation of this scope at a U.S. coal mine, and the system was intended to help determine the exact strata mechanics associated with the redistribution of stress and the associated gob formation at one of the U.S.'s deepest longwall mines. Overall, 5,000 calibrated seismic events were recorded during the mining of one panel, including a Richter magnitude 4.2 event which occurred inside of the array. Analysts of these events provides a number of notable insights into the rock mass behavior. For instance, the longwall panel was widened during the retreat process, and the mining-induced seismicity shows a distinctly different behavior between the start of the panel, the mining of the narrow part of the panel and the mining of the wider part of the panel. Also, in itself, the acquisition of the ML 4.2 event was a major milestone in geomechanical and bump research. This is the first time that such an event has been recorded with this detail and accuracy at a bump-prone coal mine in the U.S. `The analysis of this single event has set distinct new limits on the relationship between mining seismicity and coal bumps.

Jan 1, 2001

By Jeffrey K. Whyatt

Estimation of the initial vertical stress carried in a coal seam is an important first step in virtually all methods of evaluating the required size of pillars in coal mines. Such estimates are a trivial exercise in coal fields overlain by gently undulating topography. The LaModel program is easily and routinely applied in such cases to efficiently estimate mining-induced stresses throughout the coal seam. However, application of LaModel to coal mines operating under rugged topography and/or in tectonically disturbed regions is more difficult. LaModel estimates stresses in a twostep process. Topographic and multi-seam mining effects on insitu stresses are estimated in an initial step, and then mininginduced stresses are estimated in an independent second step. A LaModel program function has recently been introduced that decouples these two steps, allowing in-situ stresses to be directly entered from a file rather than calculated. One application of this function is to streamline sensitivity studies by avoiding repeated recalculation of in-situ stress. The function also allows importation of in-situ stress estimates developed by other means. There are no constraints on the imported stress field, which may include significant stress perturbations due to topography, faults, etc. Hence, the range of geologic environments that can be examined by LaModel is significantly increased. This study uses the new decoupling function in an exploration of in-situ stress estimation in mountainous terrain. One option is to use the topographic stress estimation feature included within the LaModel program, and the influence of important parameters in this routine is explored. Another option explored is importation of results from a detailed volume element model. Results of this study suggest that the method used to estimate in-situ stress, and the details of its execution, can have a significant impact for mines in mountainous terrain with strong overburden. More generally, decoupling is an efficient and flexible function that should be considered for most design studies.

Jan 1, 2011

By Dion Pastars

Resin based grouts are the main form of rock bolt anchorage in the underground coal industry in Australia and New Zealand. To be effective, the system requires the mixing of the catalyst and mastic components of the resin, as well as shredding of the laminate cartridge that contains the resin. An unknown measure is the load transfer characteristics of a bolt where the resin is well mixed but remains encased in the cartridge (gloved). Laboratory and in situ field investigations have been undertaken to quantify the performance loss due to mixed gloved bolts. This work showed repeatable results, indicating serious performance loss of the gloved and mixed system, with load transfer approximately 10- 15% of a non-gloved system (MacGregor, 2004). The in situ testing has demonstrated the relationship between the adhesion qualities of the resin and the mechanical interlock generated by radial confinement with progressively increasing tensile load. Effective load transfer is defined by the ability of the system to sustain shear stress on the bolt hole wall.

Jan 1, 2005

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 Y. P. Chugh

The use of wooden supports, supplementary to the primary method of roof control using roof bolts in mines, consumes over 3 million cubic meters of hardwood in the U.S. annually. These supports arc primarily used in the form of cribs and posts. Along with a rapid depletion of high quality timber resources, supplies fluctuate seasonally, costs are escalating rapidly, and product quality is highly variable. Environmental considerations of global climate change put additional pressure on deforestation for utilization of timber for artificial supports. On the other hand, management of coal combustion byproducts (GCBs) from power plants has become a major issue to maintaining a healthy coal industry. The USA currently produces over 100 million tons of CCBs annually and the average cost for their management in the US is about $20/ton. This cost is expected to escalate rapidly with tougher environmental regulations. The principal author has been directing research, development, and demonstration studies for CCBs-based artificial supports and lightweight structural materials in mines for the past seven years and has made significant progress toward commercially viable products. More specifically, first and second generation crib elements and post elements have been developed, characterized analytically, and demonstrated in the field. It is estimated that about 2.5 x 10" tons of fly ash could be utilized for artificial supports in U.S coal mines and this would double if these materials were utilized in non-coal mines as well. The author believes that CCBs-based artificial supports have the potential to replace wooden supports and blocks with better performance and cost competitiveness while managing CCBs in an environmentally sound manner. This paper focuses on the development and field demonstration of CCBs-based artificial supports for use in underground mines, and its pilot scale fabrication facility. Recent analysis results on engineering performance evaluation of the developed crib under various loading conditions using finite element method are presented. The following comments highlight the developments: I ). Engineered artificial supports, crib and post elements, using CCBs as a major constituent, have significant potential for application in mines because their structural performance is significantly better than wooden cribs counterparts. 21. Engineered crib elements provide about 50% larger area for air-flow than a conventional wooden crib. 3). Engineered crib elements are not flammable. 4). Each engineered crib element utilizes about 10 kg of fly ash, about 40 % of the weight of crib element- Therefore, a typical crib 2.0 m high utilizes about 200 kg of fly ash. 5). Each 16.5 cm diameter engineered post utilizes 50 kg of fly ash. 6). Assuming 3.0 million cu. meter of wood consumed for post and crib support systems, it is estimated that about 4 - 5 million tons of fly ash may be utilized for supports fabrication in the USA. 7). An engineered post has excellent post-failure characteristics similar to wood. In addition, the engineered post also has excellent loading and unloading characteristics close to the yield point of about 75% of the ultimate load carrying capacity.

Jan 1, 2004

By John L. Ellenberger

The National Institute for Occupational Safety and Health (NIOSH) has continued the research role of the former U.S. Bureau of Mines to develop techniques that will reduce the hazards in the mining work place associated with coal bumps. Current research focuses on both analyzing historical seismic data from bump-prone operations and utilizing a mine-wide seismic network to investigate the exact strata failure mechanics associated with bump-prone geology. The anticipated outcome of this research will be reduced bump incidences through advanced engineering concepts and designs which implement the new understanding of strata behavior. The analysis of the historic seismic data consists of correlating observed mining seismicity with the geologic and geometric parameters at the sites. The primary seismic parameters are the timing, location and magnitude of a recorded seismic event. These parameters are correlated with such mining parameters as: the overburden, the size of the immediate gob, the size of the district gob area, etc. This detailed analysis of historical seismic data has provided an informative quantifiable relationship between many of the specific mining parameters and the induced seismicity. The second aspect of the coal bump research is the instrumentation of an appropriate field site to determine the main roof, floor, and gob behavior associated with hump behavior. The chosen field site is a deep-cover longwall mine with competent geology in a historically bump-prone area. The primary field instrumentation is a three-dimensional, full-waveform, seismic array with both surface and underground sensors surrounding an active multi-panel district. The purpose of this seismic array is to determine the timing, the exact location, and the mechanism (tensile fracture, bedding plane slip. etc.) of the failure of the strata surrounding the active and multi¬panel gobs. The preliminary results presented in this paper help to define the strata failure areas around the longwall panel.

Jan 1, 2000

By Peter Zhang

Room and pillar mining under shallow depth is usually conducted with a high extraction ratio, which leaves small pillars with a low safety factor in some old workings in abandoned mines. Pillar sloughage, squeezing or even collapse may have occurred in those old workings with small pillars, either at the time of mining or later. The conditions of the pillars in abandoned mines can affect mining above them. Subsidence fractures and abutment pressure are indications of multi-seam interaction caused by squeezing or collapse of small pillars. To mine over areas with potentially unstable small pillars, the panel layout in the upper seam should be such that multi-seam interaction can be minimized and proper roof and rib support should be installed in the influence zones. This case study presents an example of multi-seam mining above small pillars under a shallow depth of cover. Though the interburden is relatively thick and competent, interaction is still present over the influence zones. Pillar stability and multi-seam interaction analysis, numerical modeling and in-situ monitoring are presented to demonstrate how the upper seam can be mined safely over the small pillars.

Jan 1, 2014

By Winton Gale

"Rock and coal fractures and micro seismic vibration are common occurrences during development mining. It is very uncommon for coal and rock to be propelled into the roadway during normal mining operations. However, such occurrences do occur and appear to require significantly more energy than is available from strain energy release during coal cutting. The sources of energy, which can contribute to the propulsion of coal from the face or ribs, are typically strain energy from the surrounding ground, seismic energy from a rapid rupture of the ground in the vicinity, or rapid expansion of gas from within the burst source area.The aim of this paper is to briefly review the bursts that could be related to strain energy or seismic energy. However, the greatest emphasis is placed on the effect that gas within the coal could play in moderate to gassy mines.It has been found that the bursts related to the expansion of gas can occur in coal and stone. The volume of gas involved in coal bursts is typically lower than in gas outbursts; however, the process is generally similar.INTRODUCTIONA coal burst is defined as a rapid expulsion of coal (and potentially gas) from the boundary of the roadway. The volume of a burst can be variable, but volumes above 10-50m3 are noted and cause significant disruption to operations.Rock and coal fracturing is common about roadways, particularly under the influence of elevated stresses, either tectonic or related to mining abutment stress. Shear fracture is very common; however, it is very uncommon that the coal is propelled from the face or ribs as a result of fracture of the material. The nature of rock fracture about roadways under elevated stress is presented in Figure 1.A burst requires energy to propel the coal or rock from the rib or face of the roadway. The depth of material impacted may vary from 0.5m to greater than 3m inside the roadway boundary. The strength of the ground in this zone is variable depending on the nature of the coal seam (or surrounding material). Typically, this material has significant residual strength as confinement increases into the ribside.The energy required to propel this material is typically in addition to that which is associated with normal fracturing about the roadway."

Jan 1, 2018