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By Lisa Burke

In underground coal mines, concrete block stoppings are widely used to control mine ventilation. Although stoppings are not intended as roof support, they are subjected to roof to floor convergence, and may fail as a result of this loading. Premature failure of stoppings can significantly affect mine ventilation, limiting the amount of fresh air reaching the working faces and increasing the risk for methane explosions. Softening materials are often used in stoppings to reduce the damaging effects of roof to floor convergence. However, to date, research has not focused on the behavior of stopping construction materials subjected to roof or floor loading, leaving the design process to trial and error. A combination of numerical simulations and large scale physical tests were employed to develop a scientific understanding of stopping performance. The first series of physical models were constructed using standard CMU (concrete masonry unit) blocks and tested in a load frame. The behavior of these walls was simulated with a distinct element model. A key finding was that very small variations in block size and irregularities in block shape significantly affect wall performance. These variations, which are inherent to the type of blocks typically used underground, create stress concentrations that initiate the failure of stopping walls. A second series of models employed concrete stoppings that incorporated woos planks, a shredded wood material, and foam planks like those commonly used underground. Numerical models were again able to match the physical test data. Analysis of the softening layers used in stopping construction indicated that the strength and stiffness of the material relative to that of the wall is crucial in determining the amount of roof to floor convergence the wall can withstand prior to failure. The product of this study is a numerical model that can be used to evaluate the performance of stopping materials and different wall geometries in a controlled environment. Testing was done in a laboratory setting with walls constructed on flat, parallel surfaces and subjected to uniform loading at a constant rate. Parametric studies with the model indicate that in this ideal environment it is possible to control the amount of vertical displacement the stopping can withstand by adjusting the number of softening layers built into the wall and the thickness of the layers. Additionally, under these conditions, the position of softening layers within the wall appears to have little effect on the amount of displacement walls can withstand prior to failure.

Jan 1, 2004

By Syd Peng

It is necessary to fully understand roof geological features in order to design a proper roof support system for underground coal mines. These geological features include: rock type, rock strength. rock layer interfaces, voids, cracks, and bed separations. Traditionally, engineers have relied heavily on the use of surface borehole log data to identify geological features. Since surface boreholes very expensive to be drilled in close spacing, log data are often insufficient to provide detailed information on the roof geology. Recent advances in control technology have made a large amount of roof bolter drilling data available to mine personnel. These data can be obtained during normal roof bolt installation cycle with little or no interruption in mining operations. This research project has developed several methods to identify geological features in a mine roof from measured drilling parameters. Power trend line analysis can be used to locate discontinuities and interfaces in mine roof. Power averaging can be applied to determine the relative strength of roof strata. Thrust trend line analysis can be used to locate fractures in mine roof. The results of these methods are combined to produce a mapped roof geology In a production environment, manually interpreting and managing collected drilling parameters is not practical since hundreds of holes will be drilled during a production day. A new software package, MRGIS, has been developed to allow mine engineers to make use of the large amount of roof drilling parameters for roof support design. An actual underground test site is presented to demonstrate the analysis and display capabilities of MRGIS.

Jan 1, 2003

By E. H. R. Schumann

The local stratigraphy and composition of the coal measures, including competent dolerite sill and massive sandstone lavers is shown to have a restricting influence on the development and magnitude of surface subsidence above total extraction panels in South Africa. The critical panel width necessary to induce nonviolent failure of a dolerite sill is quantified. Small subsidence due to the deflection of a 70 metre thick dolerite above subcritical panels is shown by example. Maximum surface subsidence tilt an; ground strain is compared to the NCB model. An exponential subsidence profile curve, SPC, describes a trough half-profile. A parabolic relationship between horizontal displacement and tilt, which has been discovered to exist can be substituted in the first derivative of the SPC in order to generate transverse displacement and strain profiles.

Jan 1, 1988

By Yi Luo

Regulating the face advance rate in longwall mining operation can be an effective means to reduce the disturbance potential to surface structures associated with a longwall subsidence process. However, there are two seemingly different views about the relation between the face advance rate and the disturbance potential associated with dynamic subsidence process. The US mining industry prefers a faster but a constant face advance rate while the German counter- part is forced to select a slower rate due to its mining conditions, especially while undermining sensitive objects. In order to gain a better understanding about these two different approaches, the experiences in longwall mining subsidence in both Germany and US are presented in this paper with the fundamental differences in geological conditions and mining practices.

Jan 1, 2001

Virtually all mineable Appalachian coal seams exist in a multi-seam environment. This makes it inevitable that most seams will eventually experience interaction induced ground control and mining problems. Indeed, it has been estimated that every working face will experience interaction effects from previous or current workings at least once during its lifetime (1), and that these problems are considerably exaggerated when seams are in close proximity -- a distance of less than 110 feet. The continued increase in multi-seam problems has resulted in considerable research activities over many years (2,3,4,1). Theoretical analysis, numerical and physical model studies, statistical and empirical analysis of numerous interaction related field data have been applied to multi-seam mining situations and have identified many useful trends and design criteria (4,5,6,7). To fully utilize research results and transfer multi-seam mining technology to field engineers, a design model -- USEAM was constructed (8). This model, designed for close seam under-mining situations, was computerized to facilitate utilization. In the current research, a new computer program "MSEAM" was developed to produce a model for both under- and overmining close seam situations. It is hoped that both of these software packages will aid field engineers in using important interactive design criteria to gain a better understanding of interaction mechanisms. Through advance planning with these programs, detrimental interaction may be avoided and minimized, and occasionally a design can be selected which will allow interaction effects to be utilized to the benefit of mining operations.

Jan 1, 1987

By Thomas Barczak

Longwall shields provide essential ground control in Iongwall mining, yet a high percentage of shields are operating at less than peak capacity and many at well below the rated support capacity due to defective hydraulic cylinders or malfunctions in other hydraulic components. Leg pressure data are currently collected on state-of¬the-art longwall shields, but typically are not analyzed to evaluate shield performance. The National Institute for Occupational Safety and Health (NIOSH) Shield Hydraulics Inspection and Evaluation of Leg Data (SHIELD) program is a Visual Basic software system that is designed to analyze leg pressure data and identify shields that are not performing to rated specifications. The program analysis is configured to detect the following conditions: (I ) loss of leg pressure, (2) imbalance in leg pressure, (3) low set pressure, (4) low yield pressure, and (5) full extension of the bottom stage. Other performance assessment measures include the percentage of the support capacity utilized (ratio of peak load to yield load), the percent of time that a shield operates at yield load, the ratio of the set load to the yield load, and the amount of support capacity that is lost due to leaking cylinders. Historical record keeping will allow the user to select a particular shield and review the performance record as developed by the program for that shield. In addition to these performance assessment measures, the software will include an animated description of shield hydraulic systems that will describe the operation and significance of each hydraulic component and the impact of component failures on the shield's capability to provide the required roof support. A general overview of the SHIELD program is provided in the paper together with an example analysis of a 2.5-year-old Australian longwall face.

Jan 1, 2002

By Bruce Hebblewhite

An ACARP-funded industry research project in Australia recently investigated the problem of premature rock bolt failures in underground coal mines. The project carried out limited sampling from 9 different collieries in NSW and Queensland, followed by a more comprehensive, controlled sampling from one colliery. Over 50 broken bolts were collected and subjected to metallurgical analysis at UNSW to determine both the steel composition and to analyse the fracture surfaces. Prior to this study, there had been anecdotal evidence of similar bolt failures from a number of sites within the industry. The sampling and analysis conducted by this project - whilst being below a statistically representative sample of the entire industry bolt population - highlighted the fact that bolt failures were continuing to occur at a number of sites, in both older and recently installed bolts. The failures were almost entirely attributable to stress corrosion cracking (SCC). It appears that a combination of factors - including steel composition, bolt manufacturing process, loading environment, geological and hydrological environment, bacterial corrosion - can all contribute to this problem. There is no simple solution, however, as with similar research findings from the UK, it appears that steels with higher values of fracture toughness, or Charpy Index, are less susceptible to SCC. Testing to date has been limited to bolt failures below the resin encapsulation horizon. A real concern is the potential extent of the problem at higher horizons, within the encapsulation. These may require some form of non-destructive testing (NDT) for detection. Initial trials in Australia of the ultrasonic device developed by DMT in Germany have proved encouraging. Further testing using this device is anticipated in future research. This paper describes the outcomes of the project, completed in 2002.

Jan 1, 2003

By George J. Karabin

Since the inception of a qualitative pressure arch theory by Haak4 in 1911, much work has been done in an effort to apply this concept to underground mining. It has been used in both longwall and room and pillar mining to determine roadway and pillar stability (with varying degrees of success). This paper presents a summary of a case study where the pressure arch concept was extended in a totally qualitative manner to define the source of roof control problems in the mains of a Pennsylvania mine. It was concluded that, due to the method of mining employed, a pressure arch formed over a portion of the entries, and the high abutment stresses accompanying it resulted in extreme roof deterioration. The following discussion will include a general presentation of the arching mechanism, the physical response of the roof strata under mining-induced abutment stresses (both from the case study and in general), a computer simulation of ground stability under both "normal" and "abutment" stress conditions, and finally, potential means of alleviating such conditions.

Jan 1, 1982

By P. E. Christensen

Due to ever-increasing stripping ratios associated with the No. 8 Seam at the San Juan Mine surface operation, BHP Minerals decided to implement longwall mining for the extraction of the substantial future reserve base. In preparation for longwall developments, BHP opted to first develop a small-scale pilot operation within the No.8 Seam on an adjacent lease to address geotechnical/geological concerns related to ground control, subsidence, caving, panel and entry design, and operational issues related to entry development and proper face equipment selection. Room-and-pillar mining methods are currently being utilized to develop a full extraction panel for a wide range of geological, geotechnical, and geomechanical evaluations aimed at characterizing and assessing the mineability of the No.8 Seam. Variable roof and floor conditions, coupled with a multi-bench coal seam, require that a number of mining horizons, opening geometries, and roof and rib support fixtures be evaluated relative to expediting gate and panel developments. Issues related to development of timely caves in areas subject to overlying sand channel deposits will also be addressed through underground in-seam stress and deformation measurements, and interburden and surface subsidence monitoring. This information will ultimately be used to evaluate face support and gate and panel pillar/entry development requirements. The current depositional model for the property, developed from an aggressive drilling, highwall mapping, and lineament evaluation program, will also be refined during the Pilot Mine study through extensive in-trine mapping, additional roof and floor drilling and physical property testing, and determination of the in situ principal horizontal stress field.

Jan 1, 1998

By Stephen Tadolini

Reduction in the time required and improvements in safety during a longwall face move have compelled coal mine operators to examine and use pre-driven longwall recovery rooms. While this concept is not without controversy and risk, the rewards can result in substantial reductions in longwall face move/set-up times and provide safer working conditions. Both of these factors can reduce mining costs and enhance profitability in a competitive market. This project was even more complex because the longwall system operates under a weak roof condition. To examine the feasibility of this concept, a 16-ft wide by 170-ft long experimental pre-driven recovery room was designed using analytical, empirical and 3-dimensional finite element modeling techniques. Based on the projected loading conditions, immediate and main roof geological and strength characteristics, a variety of roof bolt types were selected and installed in the recovery room and adjacent chutes. To supplement the intrinsic supports, high-capacity roof-to¬floor cuttable cribs were strategically placed to reduce the potential of a weighting failure before the longwall shields safely entered the room. To evaluate the behavior and final support performance, the recovery room and chutes were instrumented to monitor the physical response during various loading phases. Additionally, the cuttable cribs were instrumented to evaluate the loading and stiffness characteristics during the extraction process. This paper describes the successful design process, support methodology, monitoring results, final observations and recommendations.

Jan 1, 2002

By James Arthur

In deep coal mining the support of the underground roadway is fundamental and without knowledge of the mechanics of ground control a mine is unlikely to be operating safely and efficiently. It is therefore important for the support system to be designed sat¬isfactorily for the size of the excavation required and to do so an understanding of the behaviour of the surrounding strata is re¬quired. Over the last 15 years there has been a major change in the industry and the support systems adopted. The industry has been privatised, legislation has been updated, a comprehensive research programme has been carried out and guidance docu¬ments produced. The paper will show how privatisation has not allowed support standards to deteriorate, highlight research, the legislation and guidance documents updated and how this has led to a reduction in accidents and falls of ground..

Jan 1, 2002

By Madan M. Singh

Mining under bodies of water such as oceans, lakes, rivers, streams, ponds, and other impoundments, is not a new technique. However, in the United States it has been fairly restricted to date. In order to recover the maximum amount of the nation's reserves, in the future, it is important that further mining under such bodies of water be planned. However, it is imperative that this is not done at the expense of danger to the nation's miners or appreciable property damage. If all the bodies of water are considered large, and it is assumed that these constitute a hazard when mining in their vicinity irrespective of their size and geometry, a significant percentage of the coal reserves are likely to be sterilized. How- ever, by adopting special mining plans and procedures, it may be possible to release reserves for exploitation that would otherwise be unmineable since the surface of bodies of water below which they lie are relatively small in size. In 1977, the Bureau of Mines issued Information Circular (IC) 8741, entitled "Results of Research to Develop Guidelines for Mining Near Surface and Underground Bodies of Water". This document was based on the work done by Skelly and Loy (under Contract No. H0252083) and K. Wardell and Partners (under Contract No. H0252021). How ever, these guidelines have limited application, because these did not (1) define the bodies of water of concern, and (2) take into account the nature of the strata being encountered. These guidelines assumed that all bodies of water which would be undermined were large. For the purposes of this discussion, water bodies may be divided into three categories, depending upon their potential to cause damage to the underlying mining activity: ? catastrophic ? major ? limited Bodies of water which pose the threat of catastrophic destruction in the mine, should there be an inrush, include oceans, large lakes, big inland reservoirs, and rivers that could flood the mine complete- ly in a short period. Those with major potential for damage would include lakes and streams which might present a danger to both life and property if not considered in the mining plans. Finite bodies of water which have a volume considerably smaller than the mine volume offer a limited potential for damage in the case of an inrush. These include flowing streams which have a relatively small rate of replenishment and which can be dealt with by pumping or isolation in the event of a breakthrough into the mine. Such bodies of water may be inconvenient and cause a temporary disruption of production, and could even cause some damage to equipment, but only in rare instances would personal injuries occur. The scope of this paper is limited to the exploitation of stratified mineral deposits with overlying -face bodies of water. Vein and dessiminate ore deposits are not discussed, nor are breakthroughs into adjacent mines in the same horizon. Overlying, water-logged strata also pose a serious threat to mining, but are not treated in this discussion. Pending further investigation these strata may be considered as bodies of water with catastrophic potential for damage, with the base of the strata being considered the bottom of the water body. When the body of water is large, so that it may cause catastrophic damage to the mine in the event of an inrush, the guidelines suggested in IC 8741 are applicable. In order to develop criteria and procedures for mining under smaller bodies of water, however, it is necessary that the nature of the strata disturbance due to underground mining be examined.

Jan 1, 1981

By David B. Plumeau

The Cayuga Mine, a deep rock salt mining, operation, changed from a standard room and pillar mine design to a yielding pillar/pressure arch design five years ago. In general the roof conditions improved. However, entries adjacent to the abutment pillars continued to fail. It was noticed that where abutment pillars were accidentally "notched" (by driving cross- cuts one cut into the pillars), the abutment entries had better roof conditions than where there were no "notches". Experiments with intentionally notched abutment pillars showed that notching did improve abutment entry conditions. During the past four years notch configuration has been varied to suit local ground conditions. Recently, "pre-cutting" of notches has proven itself as a means of protecting abutment entries under severe ground conditions. As an added bonus, the notches improve productivity by providing more places to work and reduce costs because it is not necessary to bolt the notches.

Jan 1, 1981

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 Robert L. Schmidt

The support of underground openings by water is a natural phenomenon. Surface sinkholes, such as those that occur with some frequency in Florida, are attributed to a lowering of the water table resulting in the draining of existing limestone cavities. This withdraw the natural support and causes the surface subsidence. The role of water as a ground support medium was demonstrated in a mining situation during Bureau of Mines research on borehole mining of phosphate in St. Johns County, Florida. The borehole mining tool (BHT) incorporates a water jet nozzle and slurry eduotor in a single down- hole tool. The tool is lowered through a bore- hole into the ore zone, and an underground cavity is eroded by the motion of the water jet with the slurry simultaneously pumped to the surface. In addition to phosphate, the system has been successfully used in coal, oil sands, and uranium. The Florida phosphate ore is located at a depth of approximately 250 ft in strata that produce high water inflows. These factors preclude extraction by conventional methods, either surface or underground, but they present an ideal setting for borehole mining. Previous borehole mining research had led the researchers to believe that the system would only be effective in a void cavity, that is, a cavity where the water level is maintained below the water jet nozzle, so that the water does not impede the action of the let. It was soon discovered, however, that the void cavity presented ground control problems. The overlying strata started to cave soon after the support provided by the water was removed. This was an unacceptable situation as it adulterated the ore and threatened surface subsidence. Further tests wherein the size of the cavity -as reduced by cutting only a 30° arc also resulted in caving. A new concept, air shielding, was then introduced and tested. With this system, the water jet cutter was surrounded with a shroud of compressed air, thereby gaining the advantages of cutting in air while retaining the roof support afforded by a water-filled cavity. This arrangement proved to be highly successful. The air-shrouded jet was able to cut a cavity radius of about 18 ft, and the system production was 35 tph, which compared favorably to the production previously attained in void cavities. The borehole phosphate experience proves the efficacy of remote mining in a water filled opening. This system neutralises the hazards of heavy ground, and may, as in the case of the phosphate deposits, be the only viable alternative. It opens up the possibility of application in a wide range of deposits not only those amenable to hydraulic mining. Remote mechanical System might be developed using present robotics technology and then operated in water-filled openings.

Jan 1, 1986

By Asmaa Yassien

Finite element models were developed to study the effect of tensioned and fully-grouted bolts on the stability of the mine opening in a three-entry development system using a typical geological column for the Pittsburgh Coal Seam. The models were used to study the effect of each bolt type on the stress distribution in the immediate roof, yield zone development and the vertical deformations. Comparison of all of those features show that the effect of a tensioned bolt on the stress distribution in the immediate roof is restricted to limited areas, whereas the fully-grouted bolt has a much wider effect on the stress distribution. Both types of bolts can reduce the yielding and deformation in the immediate roof to different degrees. The fully-grouted bolt reduces the sliding along the bedding plane more than the tensioned bolt. Both types of bolts were able to maintain the bedding planes without separation. The induced forces in both types of bolts are analyzed. A rapid increase in the shear force and bending moment occurred in the fully¬grouted bolt due to the relative motion of bedding planes.

Jan 1, 2002

By Pier Paolo Manca

The investigated area lies close to a large town and covers an area of about lkm2. The mine was worked at depths ranging from 5 to 100 meters but mining operations actually ceased some 50 years ago. There are seven coal seams, each one to two meters thick, overlain by sedimentary rocks in a strongly faulted area. Different kinds of subsidence have occurred, crown holes, shallow depressions and hummocky ground. The results of rock mechanical characterisation (RMR method) in the area have been used to define three different types of numerical model (FLAC 2D). A large numbers of simulations has been performed to solve three different problems at three different scales: to determine the, mechanical properties of the roof rock and overburden involved in the subsidence process, to assess the crown hole subsidence potential and to define the properties of the beam foundations.

Jan 1, 1999

By Jaco J. van Vuuren

Modikwa Platinum Mine, in the Eastern Bushveld of South Africa's Limpopo Province, is a new operation employing a hybrid mining method where mechanised methods are used for the main developments and the narrow, 60cm reef is conventionally stoped using a down-dip conventional mining method. Throughout the mine, the hanging wall immediately above the reef comprises 3 jointed and laminated layers of varying thickness, commonly known as the triplets, below a non-cohesive leuconorite parting plane. At North shaft, using pre-stressed standing support and mechanically anchored rockbolts, it was found difficult and hazardous to hold all three triplets in the hanging wall, however bringing them down caused a significant increase in dilution and therefore cost of mining. A novel solution was conceived whereby two of the triplets are kept up using short, stiff resin anchored rockbolts installed systematically within 0.5 m of the face. This resulted in both safety and productivity benefits. The rockbolts create a stable and safe hanging wall and allow the mine to extract UG2 reef more economically. New drilling equipment, developed locally from available drilling technology, can be operated from a safe position, and in situ tests and measurements are made to confirm effective support. The technology and underlying concepts were successfully tested in a trial stope prior to rolling out the new support system across all the stopes at North Shaft. This has involved an extensive training programme to ensure that the rockbolt system is installed correctly and that the conditions are appropriate for this technology. The paper describes the motivation behind the project, the underlying technology and its application to Modikwa, and the benefits now being realised.

Jan 1, 2004

By André Vervoort

To learn more about the fundamental roof behaviour in South African coal mines, accurate underground measurements of roof deflection were conducted in bord and pillar panels during development and pillar extraction. To conduct these measurements a survey levelling technique was adapted. The overall system reading accuracy was established as being within 0,4 mm on a 15 m sighting distance. The underground sites were modelled and, for a roadway development, similar symmetrical curvatures were calculated by a linear elastic model, as were measured underground. In a pillar extraction panel, a significant amount of bed separation was measured and the roof underwent an a-symmetric movement due to the close presence of the mined out area at one side of the roadway.

Jan 1, 1992

By Olayemi Akinkugbe

As coal production in the United States continues to increase, the availability of unexploited or virgin fields continue to diminish. As a result, mine operators are forced to mine in unfavorable or multiple seam conditions. The safe, productive exploitation of coal in multiple-scam conditions requires specific design technology. At present, there are two general design approaches that the mining engineer can use to analyze the multiple-seam problem in question, either simple empirical relationships or complex numerical models. Often, the empirical approaches are too general for the specific problem and the numerical models require too much time and expertise for the practicing engineer. This paper introduces a new computer program, Lam2D, designed specifically to enhance multiple seam mining operation by quickly and easily calculating, stress, displacement, subsidence, horizontal strain and safety factors associated with multiple seam mining situations. The program implements a simplified, two-dimensional boundary-element method in order to model the complex multiple-seam stress and displacement interactions. Most importantly, the program is greatly simplified by incorporating automatic overburden, interburden, coal and gob property generation thereby reducing typical modeling and solution time to a few minutes.

Jan 1, 2004