Search Documents

By Bruce H. Gardner

This paper describes the application procedure of the Stress Control mine design method. This procedure has evolved over the past 20 years of the practice of this Method in trona, potash, salt, and, most recently, in coal mining. The Stress Control Method of mine design has been defined and amply described in previously-published work (1,2,4) -- likewise, the associated system of in situ instrumentation (3,4) and the computer modelling technique (2,3,4) which are the principal tools of this method. The present paper defines the procedure by which the in situ instrumentation and computer modelling cools are combined in adapting the general concepts of Stress Control to produce site-specific design solutions. This procedure is illustrated by means of case examples from three mines. The Stress Control Method utilizes the geometry of underground excavations, particularly through the time sequence of the creation of yielding pillars, to control stresses. The time sequence is devised such that all the primary stress envelopes are transformed into a single secondary stress envelope, surrounding the group of rooms separated by the yielding pillars. The secondary stress envelope provides protection to the roof and floor boundaries from all the external stresses. This stress relief effect removes the cause of roof and floor failure -- high shear stress in the weakly confined boundaries of underground openings. It thereby replaces artificial support with ground self-support, enabling simultaneous in¬creases in stability and percent recovery. Within limits which are still being explored and extended, the Stress Control Method has proven capable of overcoming the trade-off between stability and extraction ratio which has afflicted conventional mining methods. The application procedure of the Stress Control Method is outlined below in terms of the objectives, structure, and stages of a generic mine rock mechanics program for the site-specific adaptation of the time control yield pillar design technique.

Jan 1, 1984

By David A. Newman

Severe roof control problems have plagued a West Virginia underground mine since its initial development in the late 1970's. Adverse roof conditions in the Eastern portion of the reserve result from a combination of mining: - perpendicular to the major principal horizontal stress direction, - near lineament intersections, - in areas where the ratio of horizontal stress to vertical stress exceeds 2.00 to 4.00, and - where laminated strata are present in the immediate roof. The focus of this study is to identify the engineering (mine orientation, roof safety factor) and geological parameters (lineament and horizontal stress orientation, horizontal to vertical stress ratio, and immediate roof lithology) causing poor roof conditions and to quantify their influence on projected mining conditions in the Western portion of the reserve Rose diagrams were used to illustrate the correlation between the orientation of eight hundred thirty-six (836) roof falls with that of forty-four (44) lineaments and the principal horizontal stress Similarly a composite map was used to demonstrate the relationship between roof falls and the ratio of horizontal stress to vertical stress The immediate roof strata across the property were categorized as either laminated or non-laminated based upon a thorough examination of one hundred sixty-two core logs. The presence or absence of fireclay, a rider coal, and carbonaceous shale was also noted because of the historic correlation with poor roof conditions. Rock strength and physical property tests were conducted on selected immediate roof strata recovered from coreholes drilled along the mains projected to the Western portion of the reserve Potential roof control problems in the Western reserve were quantified based upon the correlations developed by analyzing engineering and geological data The main conclusion is that laminated roof strata, the rider seam, and its associated fireclay will have the greatest influence on mining conditions. Roof problems attributable to horizontal stresses will not be as severe in the West due to an increase in overburden depth and corresponding decrease in the stress ratio. Recommendations are provided for mining orientation, the layout of mains and submains, panel location, and roof support based on intended service life.

Jan 1, 1999

By Takashi Sasaoka, Hiroshi Takamoto, Kikuo Matsui, Hideki Shimada

"Indonesia is the second largest coal exporter to Japan. Almost all the coal is produced from open cut mines. However, a great deal of coal is left in the highwalls of the mined-out pits. The use of highwall mining systems has increasingly come into play in the U.S. and Australia, when overburden depth exceeds economical recovery in an open cut mine. However, the stability of the highwalls is always a major safety concern in such mining. This paper describes the characteristics of the highwall mining system and discusses the appropriate highwall mining system for use in Indonesia.INTRODUCTIONIndonesia produces over 200 Mt of clean coal and is the second largest coal exporter to Japan, accounting for about 32 Mt of coal annually. Over 99% of the coal produced in Indonesia comes from surface mines. More open cut mines will be developed and more coal will be mined in order to fill the great demand both in Indonesia and the rest of the world.In open cut mines in Indonesia, there are many sites where mining operations have developed long highwalls that have been abandoned due to the current economics. Mining operations have been transferred to lower stripping ratio blocks of coal or overlying seams. In some cases, coal lies buried beneath spoil heaps or is covered with mud and water. It is estimated that there is a great deal of coal beneath abandoned and working highwalls. Considering these situations, it seems to be worth introducing highwall mining systems into Indonesian open cut mines.Final highwalls of open cut mines can form the starting point for other mining methods, such as highwall or underground mining. In its basic application, highwall mining is a technique utilized after an open cut portion of a reserve has been mined, sometimes prior to the introduction of underground mining (Seib, 1993). In this system, the coal seam is mined by remotely operated equipment, such as an auger machine or a modified continuous miner incorporated into a highwall mining system. The major problems with highwall mining systems are less coal recovery due to leaving coal as the pillars and the instability of openings and highwalls due to the pillar and roof failures."

Jan 1, 2010

By Denise Weber, Jürgen Kateloe, Marku Papst, Laurenz Preusse, Axel Preusse

"The angle of draw (in Germany, referred to as Limit Angle) plays a significant role in the field of mining subsidence engineering. This applies both to the assessment of the affected area by underground mining activities as well as applied methods for the prediction of ground movements.Recent observations using remote sensing show that ground movements outside the affected area defined by the angle of draw have been detected in German hard coal mining. Since the affected area by underground mining activities also plays an essential role regarding claims of landowners with damages to their houses and infrastructures, possible solutions for limiting the size of the affected area will be presented in this paper.Ground movements which exceed the affected area of an underground coal mine, as defined by the angle of draw will be shown in an exemplary manner. Present knowledge indicates an expectation that ground movements outside the affected area will not cause damage of major significance.This paper outlines the consequences for all affected parties regarding ground movements due to underground coal mining that exceed the extent of the area defined by the currently assumed angle of draw. This will be visualized in a case study.INTRODUCTIONGermany has a long tradition of coal mining, especially in the mining area of the Ruhr district. Following a long period of mining activity, the area is now highly populated and densely developed. In such an environment, it is of utmost importance to calculate and predict ground movements initiated by mining activities; therefore, the research here focused on the prediction of ground movement for the last decades. Special attention was given to the development of accurate ground movement models, which can be utilized in all mining areas. These models are based on different influencing factors such as depth of mining, worked thickness and the angle of draw.Depending on these models, the mining companies calculate the area of land subsidence; the predicted results are used for financial calculations and planning. For structures on top of zones with a high risk of deformation, structural measures can be applied, construction warnings can be given and, in cases of damage, compensation can be planned. The technical and economic planning was performed for all areas of mining subsidence. The most important factor for the outer limits of mining subsidence is the angle of draw (Ger. Grenzwinkel), which plots the border of the area of mining subsidence."

Jan 1, 2015

By Ali Naeimipour

Availability of accurate geological information about the rock mass surrounding a tunnel or mine working is the key to enhance and optimize the required ground support measures. Various approaches have been used to obtain data on ground conditions, including the possibility of characterizing the ground while drilling for roof bolts, by the Measurement While Drilling (MWD) system, via optical, sonic and acoustic televiewers, and using various types of gamma probes. While there are many different systems for characterizing the ground in longer, larger-diameter holes drilled downward, the options for using probes and logging systems in short, small-diameter holes drilled in horizontal or an upward direction are very limited. This paper will present the results of recent field tests using optical televiewer (OPTV) to analyze the joints and voids in boreholes drilled in different directions. The studies show that OPTV is a promising tool; however, getting a good quality image from the upward drill holes still need to be studied. Meanwhile, estimated in-situ strength of the intact rock is needed to complement the jointing pattern for rock mass classification. For this purpose, preliminary steps for the development of a borehole scratcher to estimate rock strength will be discussed in this paper.

Jan 1, 2014

By Anton Sroka, Antoni Tajdus, Axel Preusse, Krzysztof Tajdus

"In Europe, the magnitude and range of mining deformation predicted on the surface have been, in most cases, determined using methods based on Gauss' influence function (e.g., Knothe's theory, Ruhrkohle method, Stochastic method). The increase in the application of numerical methods (finite element method FEM, finite difference method FDM, etc.) provides an opportunity to offer a different approach in describing mining deformations. However, this phenomenon is very complex, and due to numerical limitations, a two dimensional analysis was made (in plane strain) in most cases. Such 2D models could not accurately describe the mining geometry and geological conditions. The authors of this paper carried out the calculations in 3D models which describe a mining situation and the character of the strata in one Upper Silesian Polish mine. Furthermore, the authors include some tips for three-dimensional modeling.INTRODUCTIONThrough the years, many numerical models have been used to describe a phenomenon of underground mining exploitation influence on the ground surface. However, such rock masses are highly disturbed (cracks, fractures, changes in water conditions, etc.), and it is very difficult to suitably describe, in a mathematical manner, such materials. Therefore, many of the numerical models created have been corrected in various degrees.At first, models widely used to describe the behavior of strata in the region of exploitation appeared as elastic models, where scientests assumed that a whole strata behaved in a linearly elastic manner, according to Hook's law. Those models have been modifed to received suitable VALUES (of settlement and range of subsidence troughs. Scientests created this amongst others: elastic models with contact between the rocks layers (Shippam, 1975; Salamon, Chugh and Yang, 1993), elastic models with no tension (Chrzanowska-Szostak, 1988), and elastic models with a contact on the subsidence limit plane (Tajdus K., Tajdus A., 2005). However, in most cases, those models did not bring the expected results and the differences between shapes of measured and the calculated subsidence trough were substantial."

Jan 1, 2010

By Michael Gauna, Christopher Mark

"Coal bursts involve the sudden, violent ejection of coal or rock into the mine workings. They are almost always accompanied by a loud noise, like an explosion, and ground vibration. Bursts are a particular hazard for miners because they typically occur without warning. Despite decades of research, the sources and mechanics of these events are not well understood, and therefore they are difficult to predict and control. Experience has shown, however, that certain geologic and mining factors are associated with an increased likelihood of a coal burst. A coal burst risk assessment consists of evaluating the degree to which these risk factors are present, and then identifying appropriate control measures to mitigate the hazard. This paper summarizes the U.S. and international experience with coal bursts, and describes the known risk factors in detail. It includes a framework that can be used to guide the risk assessment process.BACKGROUNDCoal bursts involve the sudden, violent ejection of coal or rock into the mine workings. They are almost always accompanied by a loud noise, like an explosion, and ground vibration. Bursts are a particular hazard for miners because they typically occur without warning. During the years 2012-2014, serious coal bursts occurred at three different U.S. room and pillar mines. These events resulted in three fatalities and two permanently disabling injuries (MSHA, 2014; MSHA, 2013). In all three instances, the events occurred during pillar recovery at depths exceeding 1,000 feet (300 m). None of these three mines had previously reported a burst to MSHA. Coal bursts also occurred at three longwall mines during this same time period.Despite decades of research, the sources and mechanics of bursts are not well understood, and therefore these events are difficult to predict and control. Experience has shown, however, that certain risk factors are associated with an increased likelihood of a coal burst. A coal burst risk assessment consists of evaluating the degree to which these risk factors are present. In addition, some control techniques are effective in reducing the likelihood of an event or protecting miners from their effects."

Jan 1, 2015

By Bob J. MacDonald

The Cape Breton Development Corporation is a federally owned Crown corporation in Canada that operates two coal mines the Prince and Phalen Colliery in the Province of Nova Scotia. Retreat longwall mining was first introduced to the shallower workings (I 80m) of Prince Colliery in the early 1980's. Phalen Colliery started longwall retreating in 1987 and since that time has extracted twelve panels. In 1993 there had been a total of eight longwalls varying in widths from 140-230m mined in Phalen Colliery that is on the Phalen Seam. Up to this point there were only minor ground control problems on the faces except for one of the earlier longwalls that was mining a split seam. There is some interaction on Phalen workings from remnant structures in the overlying Lingan and No. 26 Collieries which worked the Harbour Seam. Induced abutments loads along pillar edges have been observed and resulted in poor face conditions when mining the Phalen Seam. The ninth panel, 6 East was the first panel to suffer a major roof fall from the First Major Weighting. This was also the first panel to undermine a paleo sandstone river channel named the Lower Sandstone Unit (LSU). For the first time in Phalen's history subsequent longwalls began to experience periodic weighting events. The 6 East panel suffered a number of periodic weightings and on two occasions had to use foam cement to fill the cavities that resulted. On the next east side panel, 7 East, there were numerous periodic weighting events, but interestingly there was no significant First Major Weighting. 7 East was the first 260m wide panel and all others were 230m or less. This longwall was 3,400m long and began producing in December of 1994 and was still cutting coal in May of 1997. There were five significant roof fall events on this panel, three of which required foam cement to fill the cavities. Of these three the interruption to production varied from four days to seven months. There were definite precursors to each of the weightings including: 1) an increase in audible noise (termed bumping) on the face, 2) supports went rapidly into yield, 3) roof convergence increased, 4) water percolated from the face roof strata, and 5) severe spalling of the coal occurred resulting in increased tip to face distance. Longwall face weightings are a result of geomechanical factors and cannot be avoided However, the loss of roof control is a different matter and highly subjective to operational procedures. The causes of weightings, their magnitude and impact on the mining at Phalen have been investigated. The following factors have been identified as relevant to these occurrences. 1. The influence of pillars in the overlying Harbour Seam workings. 2. Location of the LSU relative to the Phalen Seam. 3. Thickness of the LSU. 4. Pressure arch effect from the adjacent gobs. 5. Poor operational practices on the longwall including: a) slow rate of retreat; b) inadequate supply of pump pressure to the hydraulic supports; c) premature yielding of the yield valves; and d) poor face alignment practices. This paper will review the events that took place on the 7 East longwall and show that the Lower Sandstone Unit is the primary cause of the weightings. However, other factors contributed to the loss of roof control and the formation of the roof cavities.

Jan 1, 1997

By Keith Heasley, Christopher Newman, Zach Agioutantis

"Previously, in 2013, a laboratory version of the computer code ARMPS-LAM, introduced at the 32""d International Conference of Ground Control in Mining, successfully integrated the laminated overburden model of LaModel into an ARMPS-style pillar stability analysis. The ARMPS-LAM analysis has now been fully integrated into the ARMPS program allowing users to more accurately classify the stability of a given mine design utilizing a laminated overburden model for the determination of pillar loading within theAMZ. ,This paper introduces the new Windows-based ARMPS-LAM program. The program's ease of use, output result generation, and limitations are highlighted through the parametric analysis of three hypothetical case studies where traditional ARMPS output is compared to output generated by LaModel. As mining operations continue to produce at greater depths and in more complex geometric and geological conditions, flexible and efficient analyses of mine stability for underground room and pillar mining has become more and more essential. The ARMPS-LAM program provides the mining industry with an empirically backed, numerical design tool for the evaluation of underground room and pillar mine plans.INTRODUCTIONIn the Southern Appalachian coal fields of the United States, retreat room-and-pillar methods have allowed mining operations to maintain high rates of production in the face of adverse geologic conditions and shrinking coal reserves. Over the years, research has been dedicated to understanding and mitigating mining-related dangers due to second mining practices, such as: pillar squeezes, floor heave, roof falls, pillar bumps, etc. However, as mining operations continue at deeper depths and with more complex geometric and geologic conditions, there is an inherent industry need for more accurate, flexible, and faster evaluations of stability for underground room-and-pillar mining."

Jan 1, 2016

By Ihsan B. Tulu, Keith A. Heasley, Aanand Nandula

"The objective of this paper is to present the development of a software tool that will assist in an area calculation of the Analysis of Roof Bolting Systems (ARBS) support intensity while incorporating more detailed geology, stress, and intersection span input. In this area ARBS tool, a detailed CMRR distribution can be determined from the available geologic database at the mine. In addition, the overburden and multiple-seam stresses, as obtained from the boundary-element program LaModel, can be converted to a “pseudo-depth” which is then used as the depth input to the area ARBS calculation. Finally, the section-specific intersection spans can be determined from the mine design for input to the calculation. All this detailed area input information for ARBS is handled by the latest version of the Stability Mapping (StabMap) program, which now outputs an area ARBS suggested support density given the required input parameters. This final ARBS grid can be plotted, analyzed and overlaid on the mine map for optimum use in support design and for presentation to production personnel.INTRODUCTIONSince the inception of roof bolts in the 1940s, there has been a lack of a universally accepted roof bolting design method (Mark, 2002; Mark and Pappas, 2012). To address this situation, the National Institute of Occupational Safety and Hazard (NIOSH) developed roof support design guidelines, which were incorporated into a computer program called the Analysis of Roof Bolting Systems (ARBS). These guidelines were based on the effects of roof quality, stress, and mine geometry (Mark, Molinda and Dolinar, 2001). The roof rock quality in ARBS is represented by the Coal Mine Roof Rating (CMRR), the depth is used as a surrogate for the stress, and the intersection span serves to represent the mine geometry (Mark, Molinda and Dolinar, 2001). These three input parameters are used in the fundamental, empirically derived equation of ARBS to calculate a required support density factor, called ARBS, (Mark, Molinda and Dolinar, 2001):"

Jan 1, 2018

By G. C. Xiao

A database, incorporating historical data from British mines, to study the factors affecting inflows from proximate aquifers into longwall workings, showed that a main casual factor was the immediate roof strata lithology. When this comprised principally stronger sandstone, there was a tendency for periodic inrushes associated with face weighting. This phenomenon is explained by the collection of water in bed sepa¬ration cavities, where free water connected to a high pressure head aquifer source, may be trapped.

Jan 1, 1990

By Andre Zingano

The coal seam strength and the failure criteria that represent the behavior of the coal seam are very important for mine section and pillar design. Additionally, mine section and pillar design take into account roof and floor layers, which form the roof-pillar-floor system. Therefore, underground ground control design begins, most importantly, with rock mass characterization. Another issue is the distribution or behavior of the coal seam strength for the whole mine area. Sometimes, the coal seam strength is assumed to be the same for an entire area, but this is not true because the coal seam is heterogenic and anisotropic, and also the coal strength is affected by faults and spacing between fractures. The objective of this paper is to demonstrate the use of the methodology for geomechanics characterization to determine the strength for the Bonito coal seam, and discuss the coal seam strength distribution around the entire mine area. The intact coal strength was determined by laboratory tests that were completed during the last 17 years, including triaxial and uniaxial strength tests. It was noted that there is a strong correlation between all laboratory strength tests with 90% confidence interval. The explanation for this coal seam strength variation is the presence of the discontinuities and their variation in quantity and quality at each location in the mine area, because the coal seam rock classification and its behavior will be affected by the quantity and quality of fractures.

Jan 1, 2014

By Dion Pastars

Kestrel Coal has undertaken a surface-to-seam consolidation program of faulted ground prior to longwall retreat in the 304 panel. Micro fine cement is used to improve the rock mass quality and therefore reduce the risk to personnel and equipment whilst increasing previously assumed sterilised reserves. The program is based on information attained from previously successful grouting campaigns in the Queensland coal area and follows the decision making process to ensure there is sufficient spatial permeation of cement across the faulted area. This data is used to assess the residual risk to the business in terms of health, safety and production

Jan 1, 2007

By Arka Jyoti Das, Prabhat Kumar Mandal

"Coal mining in India has a strategic importance for the energy security of the country. Presently, the majority of coal production is coming from open-cast mining. Due to the fast depletion of reserves amenable to open-cast mining, future coal production will depend primarily on underground production. In conventional bard-and-pillar/room and pillar mining as it is practiced in India, the percentage of extraction is very low. In fact, within most Indian coalfields, a large number of coal seams extract only 15- 20% of coal, but no pillar extraction is done due to the presence of surface or sub-surface structures leading to huge amount of coal being locked-up in underground. Spontaneous heating and fire, the accumulation of poisonous gases, severe stability issues, unsafe workings and environmental hazards are all major problems associated with these developed coal pillars, so there is a pressing need for technology in the mining industry to extract this huge amount of coal locked up under different constraints. In this study, we propose that the locked-up coal be extracted by replacing the developed coal pillars with artificial pillars. The paper briefly discusses the extraction methodology designed for this and its evaluation through numerical modeling for applied suitability underground.INTRODUCTIONDriven by rising populations and urbanization, an expanding economy and also a quest for improved quality of life, energy usage in India is expected to increase manifold in the coming years. As for India's energy scenario and its projected future plans, there is no doubt that coal will remain a prime source of energy, even over the next few decades.The majority of coal production in India comes from open-cast mining because of large-scale mechanization, but the coal reserves amenable to open-cast mining are gradually depleting - which may impose adverse impacts on the energy sector if underground production does not increase. Underground coal extraction, meanwhile, is going to become more complicated in near future due to the fast exhaustion of easily mineable coal reserves."

Jan 1, 2016

By Tom Blevins, Roland Walker, Jinsheng Chen

"The Dywidag-Systems International DSI Tandem Anchor bolt, with an improved expansion shell-washer design, can substantially reduce potential entry roof falls caused by twisted or deformed expansion shells of the traditional mechanical anchor bolts during installation. In addition, the improved anchorage system can also ensure that a consistent and high installed load is achieved, which is critical for effective beam building under thinly laminated or layered roof conditions. This paper discusses the genesis and anchorage performance of the Tandem Anchor bolt, and analyzes the tension/torque relationship of different mechanical shell anchors using data obtained from both lab and field tests. It also discusses roof bolting mechanisms, roof bolting systems, and factors influencing effective beam building. Finally, numerical modeling was used to analyze the effect of bolt pretension magnitude on entry roof stability.INTRODUCTIONThroughout the history of underground mining, keeping the roof from uncontrollable collapse has been the greatest concern of the miner. Tremendous strides have been made in better understanding, predicting, and supporting the mine roof. This conference has spent the last twenty-eight years sharing information and distributing it to the mining community. A paraphrased statement from former Secretary of Defense Donald Rumsfeld adequately describes the challenges to the mining engineer as it pertains to roof control: it is a study of known knowns, known unknowns, and unknown unknowns. It is the hope of the authors to address some of these unknowns.The paper discusses the development of bolt anchorage systems from dry shells to shells with resin-assisted anchorage. It looks at some of the issues faced with the anchorage shells being used when introducing the insertion pressures caused by resin cartridges. The rationale behind the shell design changes was to compensate for this resin induced stresses.An improved expansion shell assembly (a slotted expansion shell base, a tapered cramming plug, and a novel support nut with tapered top) for mine roof bolts makes up the Tandem Anchor (U.S. Pat. No. 6742966); this minimizes the possibility of twisting seen with traditional mechanical shells and thus prevents springy bolts and shell leaves from breaking. A fully grouted Tandem Anchor bolting system has the advantages of both the pure mechanical point-anchor and the non-pretention fully grouted bolting systems. Once installed, the installed load or pretension will be locked in and never bleed off. Compared with the fully-grouted (2-speed resin) tension rebar bolting system, the fully-grouted Tandem Anchor bolting system can (a) reduce bolting cycle time (about 15 seconds per bolt), boosting coal production and productivity; (b) provide a greater installed load or pretention; and (c) achieve a consistent tension/torque ratio, which gives better bolting quality control. Therefore, a fully grouted Tandem Anchor bolting system is the most effective means to reinforce a weak rock mass intrinsically by the theory of beam building. For thinly laminated or layered roof conditions, a higher installed load or pretention (50% or more of the bolt's yield strength) can make a roof bolting system much stiffer, which results in less roof movement. Numerical modeling results show that the magnitude of the roof's tensile stress and relaxation zone can be substantially reduced under a higher installed load or pretention."

Jan 1, 2010

By John Shepherd

Pillar extraction carried out using various methods (split and lift, Wongawilli and old Ben) involves the formation of narrow fenders (commonly 7 m of coal) formed by driving "splits" for lifting off with a continuous miner. The strata above a split and fender adjacent to the goaf consists of a cantilever beam (usually >3 m thick) that extends from the solid coal and bridges across the fender. Lifting off the fender from the split in safe conditions depends upon the bridging capability of the cantilever and the fender strength. The nature and competence of the immediate and near roof, the support capability of the coal fender and the presence of faults and or joints can adversely affect the bridging behaviour of the cantilever. Insufficient knowledge of these parameters can lead to extraction under dangerous conditions. Remnants of fenders ("stooks") also support the cantilever and their dimensions are critical to obtain regular caving and goaf falls. Stook area, goaf roof span and roof stand-up time after lifting can be related and only a relatively small increase in Btook area of 10-15 m2 can lead to a significantly increased stand-up time, especially under a massive roof. Such conditions strongly influence the redistribution of abutment stresses and can create increased outbye pillar loading. Based on detailed underground geotechnical investigations, it is concluded that strata caveability, roof cantilever behaviour, fender H:W ratio and remnant stook size are all critical parameters for successful and safe pillar extraction.

Jan 1, 1992

By Pierre-Yves Declercq, Andre Vervoort

"After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—an upward movement. Although most surface movement (i.e., subsidence) occurs in the months and years after mining by the longwall method, surface movement still occurs many decades after mining is terminated. After the closure and flooding of underground excavations and surrounding rock, this movement is reversed. This paper focuses on quantifying the upward movement in two neighboring coal mines (Winterslag and Zwartberg, Belgium). The study is based on data from a remote sensing technique: interferometry with synthetic aperture radar (INSAR). The results of the study show that the rate of upward movement in the decade after closure is about 10 mm/year on average. The upward movements are not linked directly to the past exploitation directly underneath a location. The amounts of subsidence at specific locations are linked mainly to their positions relative to an inverse trough shape situated over the entire mined-out areas and their immediate surroundings. Local features, such as geological faults, can have a secondary effect on the local variation of the uplift. The processes of subsidence and uplift are based on completely different mechanisms. Subsidence is initiated by a caving process, while the process of uplift is clearly linked to flooding. INTRODUCTION After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—upward movement in the area above the past exploitation and in the immediate surroundings. Of course, most surface movement (i.e., subsidence) occurs in the months and years after mining by the longwall method (Peng, 1986). This downward movement continues to occur at a smaller rate many decades after mining has been terminated when the water in the underground excavations is pumped to the surface. This is the case as long as a mine is in operation. However, after the closure and flooding of the underground excavations and the surrounding rock, this movement is reversed, and the surface is uplifted. A total uplift of about half a meter has been recorded to date. This phenomenon has been described in several different coal basins in Europe, for example, in Belgium (Devleeschouwer et al., 2008), France (Samsonov, d’Oreye, and Smets, 2013), Germany (Baglikow, 2011), the Netherlands (Caro Cuenca, Hooper, and Hanssen, 2013), and Poland (Preusse, Kateloe, and Sroka, 2013). To date, research has focused mainly on understanding the phenomenon (e.g., Herrero et al., 2012) and identifying general trends, whereby the link with the rise in water levels is an important issue (Caro Cuenca, Hooper, and Hanssen, 2013). The most accepted explanation is linked to the swelling of clay minerals in the argillaceous rocks in the coal strata (Herrero et al., 2012). Without doubt, the full explanation is complex, and swelling is probably not the only cause of the residual movement. Most coal strata rock is weakened upon contact with water. The increase in water pressure also results in a decrease in the effective stress. Both aspects facilitate the fracturing of rock, which normally would result in further subsidence. However, decreases in the effective stresses also result in the relaxation or expansion of the rock, which induces an upward movement (Fjar et al., 2008). All of these aspects, including the long-term residual subsidence due to compaction under dry conditions, result in a net upward movement. The phenomenon discussed here is clearly different from the upsidence, sometimes observed on other continents (e.g., in Australia, Galvin, 2016). When upsidence occurs, the rock near the surface bends and buckles upwards due to the overstressing of the floors of the valleys."

Jan 1, 2017

By Muhammad Zaka Emad, Rangzeb Goraya, Muhammad Usman Khan

"Numerical modeling has been used extensively to assess the stability of surface and underground mines. Tabular, steeply dipping ore bodies are generally mined out by open stoping methods with delayed backfill. Cemented rockfill (CRF) is employed as a backfill material for narrow vein deposits with low production rates. Sublevel stoping methods with delayed backfill are employed to mine out steeply dipping ore bodies. Backfill is exposed to blast-induced vibrations and dynamic loading due to loss of confinement, which could lead to backfill failures. Backfill failures could influence the operating cost of a mining stope and the productivity of the entire mining project. This work shows a detailed overview of the factors contributing to backfill failure. A detailed literature review is presented along with case studies of longhole stoping operations from the Canadian Shield region for this work. The backfilled stopes are analyzed with the finite difference code FLAC3D. The literature review shows that the factors primarily responsible for fill failure are the strength of backfill, stope geometry, mining sequence, and blast-induced vibrations. INTRODUCTION Sublevel stoping methods with delayed backfill are popular in Canadian hardrock and metal mines. Backfill facilitates ground control and greater recovery of ore with pillar mining. It reduces wall slough, thus decreasing ore dilution from adjacent walls. The production of cemented rockfill (CRF) is simple; rock aggregates are mixed together with cement slurry and then placed in a mined-out stope. CRF has a low water content, thus it requires no additional drainage unlike some other backfill. Low water-to-cement ratio enables faster curing rate, hence a faster mining cycle can be achieved (Annor, 1999; Hassani and Archibald, 1998; McKay and Duke, 1989). CRF is generally placed in stopes from the upper sill drift using a truck or a load haul dump machine. Some of the problems associated with CRF are the moderate to high operational cost, the difficulty to get a tight filling, and segregation. Backfill failure is a product of many factors, such as cementation, particle size distribution, placement method employed, water quality, segregation, and blast-induced vibrations. The problem of segregation in aggregates has been observed when aggregate is dumped from a backfill raise (Meech, 2012; Indiana DOT, 2013). Segregation becomes unavoidable when the aggregates are not properly sized (Yu and Counter, 1983; Stone, 2007; Yu, 1989). The exposed backfill zones will contribute to ore dilution due to backfill when subjected to dynamic loading due to fill failures. This paper presents a detailed literature review and numerical modeling results of a mine backfill, along with case studies from longhole stoping operations. The backfilled stopes are analyzed with the finite difference code FLAC3D (Itasca, 2017). The literature review shows that the primary factors responsible for fill failure are the strength of backfill, geometry of stope, sequence of mining, and the vibrations induced by blasting operations."

Jan 1, 2017

By Essie Esterhuizen, Michael M. Murphy, Chris Mark

"The response of the surrounding rock mass to the creation of mining excavations determines the ultimate load on a pillar support system. In conditions where the ground is relatively soft and weak, the full overburden weight can be transferred to the pillar system. However, in stiffer and stronger rocks, a greater portion of the overburden load is transferred to the unmined coal barriers or abutments, and the pillar stress is reduced. This paper makes use of numerical models to examine the interaction between typical pillar systems and the surrounding rock mass for weak and strong geological conditions at various spans and depths of cover. The concepts of structural failure and functional failure of pillars are used to assess pillar performance when pillars are deformed beyond their peak resistance. The results show that the span-to-depth ratio is an important factor in determining the pillar stress and the ultimate deformation of pillars. The ultimate pillar strain appears to be closely related to the functional success of pillar systems.INTRODUCTIONThe National Institute for Occupational Safety and Health (NIOSH) was requested by Congress to study the safety of retreat pillar mining following the Crandall Canyon Mine disaster that took place near Price, Utah in August, 2007. As a part of this study, an investigation was made to better understand how the rock mass responds to the creation of mining excavations and how the loads are distributed among the pillars and the surrounding abutments or barriers. The outcome of these investigations contributed to the development of a modified loading model for the ARMPS-2010 (Mark, 2010) method ofretreat mining pillar design.Structural and Functional Failure of PillarsPillar design is typically conducted by estimating the pillar strength and stress, and then sizing the pillars so that an adequate margin of safety exists between the expected strength and stress. The pillar strength can be defined as the maximum resistance of a pillar to axial compression (Brady and Brown, 1985). If a pillar is loaded beyond its peak strength, load shedding or yielding can occur, and the pillar is considered to have failed as a structure. Structural failure generally refers to a loss of loadcarrying capacity."

Jan 1, 2010

By Susan Robertson

Many injuries are caused each year by rock falls in coal mines. Most of these injuries are not caused by major roof collapses, but from falls of smaller rocks from the immediate top or roof skin. Various surface controls are used in mines to control this surface rock. One that has been found to be very effective is roof screening. Depending on the size of the screen, roof coverage approaching 100% can be achieved. Many mines are reluctant to use screen for primary skin control because of additional costs of time and materials, but others are having great success at both controlling costs and surface rock. Data is presented from two mines that show a dramatic reduction in roof skin injuries when screening is used. Much of this success is due to innovations in roof bolting machines. Four case studies of roof screen experience are presented along with associated costs of materials, impact on bolting advance rates, and potential ergonomic risks. The effects of roof screening on skin control and safety are also included. Finally, this paper will provide information about features of different roof bolting machines that affect production and safety.

Jan 1, 2002