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By Peter Cain

Longwall extraction in the Phalen Scam in the Sydney Coalfield of Nova Scotia is carried out under flooded abandoned workings in the Harbour Seam 140 m above, and at depth of up to 700 m beneath the Atlantic Ocean. Careful planning of panel and pillar dimensions is necessary to prevent inflows from the overlying workings and to prevent the development of excessive subsidence strains at the seabed. This paper briefly summarises the research undertaken over the past five years by CBDC and CBCRL into the problems associated with mining under water, and then describes the development and application of a design/planning code of practice developed jointly by CBDC, CBCRL and mine regulatory agencies. The design method employed in the Code of Practise is based on empirical data from the coalfield and parameters developed from subsidence modelling and previous water inflow experience. It includes precautionary measures to be adopted during development and mining, and monitoring procedures to be followed. It is flexible enough to allow future developments in the understanding of inflow mechanisms to be incoporated without major modifications, and represents a major step forward in reducing the potential hazards associated with coal extraction in the coalfield.

Jan 1, 1996

By Thomas M. Barczak

With the development of the shield support, the primary requirement for successful ground control in longwall mining is to provide stable gate road and bleeder entries. Wood cribbing has been the dominant form of secondary support. However, the cost and limited availability of timber, along with the poor performance of softwood crib supports, has forced western U.S. mines to explore the utilization of other support systems. The recent success of cable bolts has engendered much interest from western operators. Several innovative freestanding support systems have been developed recently including: (1) "The Can"' support by Burrell Mining Products International, Inc., (2) Hercules and Link-N-Lock wood cribs and Propsetter by Strata Products (USA) Inc., (3) Variable Yielding Crib and Power Crib supports by Mountainland Support Systems, (4) the Confined Core Crib developed by Southern Utah Fuels Corporation; and (5) the MEGA prop by MBK Hydraulik. This paper assesses design considerations and compares the performance and application of these alternative secondary support systems.

Jan 1, 1996

By Peter Cain

The paper presents previously unpublished results derived from an unique interaction moitoring program conducted in the Sydney Coalfield of Nova Scotia. An abandoned, sealed longwall gateroad was re-entered and instrumented prior to extracting a longwall face 140 m below it. Instrumentation included piezometers connected to a constant head tank to monitor subsidence, and horizontally mounted extensometers to measure strains associated with subsidence. Survey levelling data was also collected. The monitoring results have allowed a number of significant conclusions to be drawn, all of which were either unanticipated or contradict current mainstream thinking. Firstly, interaction subsidence was less than anticipated. Secondly, the effects of rate of face advance on the development of the subsidence profile are evident. Thirdly, compressive strains are shown to remain in the subsided strata. Finally, strata response to undermining was almost immediate. The impact of these findings on the extraction of longwall panels beneath flooded workings in the Sydney Coalfield are discussed.

Jan 1, 1996

By I. V. Baklashov

Geomechanical conditions of mining operation are determined by varying in time and space mechanical properties and initial state of the rock mass, by geological and technological factors. We define adaptive mining technology as a technology capable of adjusting to changing geomechanical conditions of mining operation. The ongoing research at Moscow State Mining University consisting of laboratory, theoretical, and field investigations aims at the development of theoretical basis and methodology of the system of geomechanical support of adaptive mining technology for the bedded salt deposit in the Urals, mined by room-and-pillar method. This system of geomechanical support is capable of - dynamically assessing mechanical properties and state of the rock mass varying within the panel; - mapping mechanical properties of the rock mass within the panel; - assessing and predicting mechanical condition of load-bearing elements of the rock mass and its changes due to the adjustment of the parameters of mining operation considering the in-situ geomechanical conditions.

Jan 1, 1996

By Christopher Mark

Knowledge of in situ stresses is fundamental to many studies in earth sciences, and coal mine ground control is no exception. During the past 20 years, it has become clear that horizontal stress is a critical factor affecting roof stability in underground coal mines. The theory of plate tectonics and the World Stress Map (WSM) project has been extremely helpful in explaining the sources and the orientations of the horizontal stresses observed underground. Recently, WSM geophysicists studying deep-seated stresses have developed a model of how stress magnitudes vary with depth in the crust. They have devoted relatively little attention to near-surface stresses, however. This paper explores the relationships between deep-seated and shallow in situ stresses in several of the world?s coalfields, using a data base of more than 350 stress measurements from underground coal mines. The analysis indicates that distinct regional trends exist, corresponding roughly to the regional stress fields identified by the WSM. The paper presents equations for estimating stress magnitudes that were developed by treating depth and elastic modulus as independent variables in regression analysis. The magnitude of the horizontal stress increases with depth, at rates that range from 0.8 to 2.0 times the vertical stress, just as the WSM ?critically stressed crust? model predicts. Overall, it seems that the stress regimes encountered in underground coal mines are closely linked to those that exist deep in the earth?s crust.

Jan 1, 2008

By Thomas M. Barczak

The intent of this paper is to make mine operators and engineers aware of parameters which affect the loading behavior of passive roof support systems. The strength of a passive roof support element alone is not a meaningful measure of support performance. Passive roof support elements and the surrounding strata act as a system, responding to changes in the physical mine environment due to the extraction of coal and associated redistribution of stresses. The load- ing of a passive roof support element is a function of the stiffness of the roof support structure; a stiffer structure will react a higher load to a converging mine roof than will a more flexible structure. If the stiffness if the support structure is not properly designed, excessive loading will result due to the displacement of the mine roof, causing premature failure of the support. Ideally, the yield strength of a passive support needs to be no greater than that required to support the dead weight of the immediate mine roof, providing the post yield characteristics of the support are compatible with the irresistible convergence of the overburden strata and the support system continues to provide this resistance after reaching its yield load. Idealized design considerations for passive roof support systems in terms load-displacement characteristics discussed. The stiffness and yield characteristics of wood concrete cribbing, as determined controlled tests in the Bureau's Roof Simulator, are compared preliminary recommendations passive roof support utilization also provided.

Jan 1, 1987

By Christopher Mark

Severe dynamic multiple seam interactions can occur when active mine workings are subsided by underlying mining activity. The most dramatic events are usually caused by longwall mining or pillar recovery beneath open, overlying entries. But pillars in abandoned workings can also fail and cause subsidence and damage an overlying mine. This paper describes a case history in which an apparent ?pillar squeeze? in the abandoned workings of a lower seam was initiated during retreat mining in an upper seam. The event subsequently extended more than 1,500 ft and ultimately closed the upper seam mine. Analysis indicated that the pillars in the lower mine were adequately sized for the lower seam mining and were not affected by the initial development above them. When retreat mining in the upper seam had progressed several hundred feet, however, the pillars located directly beneath the pillar line were overloaded, and an extensive squeeze initiated in the underlying workings. The squeeze, in turn, subsided the overlying workings, causing widespread rib falls and roof instability. Previous examples of dynamic multiple seam interactions resulting from delayed subsidence of underlying workings have been reported in the literature. This is, apparently, the first recorded incident in which pillar recovery in an active mine triggered pillar failure in an underlying mine, which then triggered a dynamic interaction that impacted the active overlying seam.

Jan 1, 2012

By Ihsan Berk Tulu

In the Analysis of Retreat Mining Pillar Stability (ARMPS) program, the magnitude of the abutment loading adjacent to a gob area is calculated using an ?abutment angle? concept. The extent of the abutment loading is determined solely as a function of depth from an empirically derived equation. In the recommended calibration method for LaModel, it was believed that the empirical equations for calculating the magnitude and extent of the abutment load, as used in ARMPS, were the best available methods for determining these critical overburden loading values. Therefore, similar calculations were implemented in the LaModel calibration method. However, the latest in situ stress measurements of abutment loading performed in the United States and Australia have shown that there can be significant deviations in the measured abutment magnitude and extent as compared to the predicted values from the empirical formulas used in ARMPS and LaModel. It seems reasonable that the overburden geology, depth, extraction panel width, and mining height should have a significant effect on the extent and magnitude of the abutment load. However, the current empirical formulas do not incorporate many of these significant parameters into determining the abutment load on the pillars. In this paper, stress measurements from US and Australian mines were back-analyzed by using analytical and numerical methods to investigate the measured abutment extent and loading. Ultimately, it was determined that the original empirical abutment extent formula in conjunction with the original ALPS square-decay stress distribution function was supported by the case histories in this paper. Also, for depths less than 900 ft, the average 21° abutment angle was supported by the case histories; however, at depths greater than 900 ft, the abutment angle was found to be significantly less than 21° and should be calculated with a new proposed equation.

Jan 1, 2012

By Hani Mitri

Understanding the interaction between rock bolts and underground rock movement is critical for safe and cost effective underground excavation design. Although early research on this subject involved a balance of theoretical analysis and field measurement, recent work has been heavily focused on analytical and numerical studies. This paper describes technologies that have the potential to redress the balance through instrumentation of rock bolts deployed under routine operating conditions in underground mines. Two new products are presented in this paper. Firstly the U-cell coupler is a load cell device that can be routinely coupled to the threaded end of a bolt prior to installation. It measures the ?series? load at the head of the bolt. Secondly, the d-REBAR involves an array of small-diameter, long-base length displacement sensors recessed into grooves along the entire length of the bolt. Both new instruments are interfaced with on-board digital signal conditioning and telemetry. Methodologies for the deployment of the new instrumentation and guidelines are presented for the interpretation of results based on data obtained from field trials. Successful field installations demonstrate the viability and practicality of these new technologies and, moreover, provide important insight in to rock bolt/rock mass interaction.

Jan 1, 2012

By Arnold Hammons

Automated temporary roof support (ATRS) systems have been incorporated into roof bolters since the 1970?s. During roof bolting, many injuries have occurred from large pieces of draw rock falling inby the ATRS beam and tipping back into the operator?s area. Other injuries have occurred due to smaller, sharp rocks falling onto the operator?s hand and arm during initiation of roof bolt drilling. These injuries are attributed to weak immediate roof, sometimes called roof skin failure. Even though ATRS systems support the roof and provide a measure of protection against roof skin failure, additional hazards sometimes do exist immediately inby and outby the supported roof. Design innovations such as the ATRS mounted rocker pad deflector systems have been designed to improve the roof bolter operator safety. This paper will inform the reader of the hazards, machine evolution through different design solutions, and the current state of the art in preventing roof skin failure injuries when roof bolting.

Jan 1, 2009

By Axel Studeny

Geomechanical-numerical methods are an established tool for planning of underground excavations worldwide. By describing the interaction between heterogeneous layered strata and support elements it is possible to determine their suitability. The German hard coal mining industry has more than twenty years experience in investigation and using these parameters to carry out numerical calculations. Today, nearly all excavations (shafts, pit bottoms areas, bunkers, coal faces, development drivages, gate roads etc.) are calculated by geomechanical-numerical tools in combination with empirical planning methods. A high accuracy of planning results, which is a special requirement in the German coal industry with its deep-level longwalls, bedded and weak surrounding strata and multiple seam mining layouts, depends on the extensive calibration of the numerical models, which is achieved by: 1) Taking a large number of underground measurements with empirical evaluation. 2) Carrying out physical modeling. 3) Using the characteristic features of the installed support elements. This paper compares the planning approach used for multiple and single seam mining and presents the advantages of this planning tool. The main focus is on the feasibility of rockbolting as the sole means of roadway support for multiple seam mining operations, though the paper will also look at how the costs can be improved by using a low bolting density in single seam layouts.

Jan 1, 2007

By Peter Zhang

Shield recovery in a longwall move is normally conducted from tailgate to headgate. However, if the face is wide and the immediate roof is weak, difficult conditions with considerable roof sag, excessive shield load, and local roof falls could occur as the shield recovery proceeds to the middle of the face, causing delays in shield recovery. The situation could be aggravated when a pre-driven longwall recovery room is employed. Alternatively, a bi-directional approach for shield recovery, i.e. shield recovery starting from the middle of the face towards both tailgate and headgate, can alleviate the roof sag significantly by quickly withdrawing the shields towards the lower abutment pressure zones at the headgate and tailgate sides. In this case, the bi-directional shield recovery approach was used in a longwall move for a 1450 ft wide face under weak roof conditions. Shields were successfully withdrawn within five days without experiencing significant roof sag. The time-dependent roof-to-floor convergence, shield pressure, and roof conditions were monitored during shield recovery. 3-D finite element modeling was conducted prior to longwall move to evaluate the difference between a uni-directional and a bi-directional shield recovery during a longwall face move. Vertical stress, yield zone in and around the recovery room, load on the shields, and convergence at the center of the shield canopy were obtained from computer modeling, and compared with the data from field monitoring. Finally, the advantages of the bi-directional shield recovery are discussed.

Jan 1, 2007

By Victor V. Nazimko

Multiple mining generates gob-interaction displacements. They redistribute stress m strata and obey laws of irreversibility thermodynamics. An irreversible state causes inhomogeneity of the stress distribution on the mass. A destressed zone develops along a working panel, and worked out panels are deprived of the destressing. The inhomogeneity is smoothed out during elapsed time.

Jan 1, 1996

By Stephen C. Tadolini

The U.S. Bureau of Mines, in cooperation with the Cyprus- Plateau Mining Company, has conducted research to provide an alternative to traditional secondary support systems in a two-entry, yield pillar gate road. As recent as two years ago, it was impossible to imagine a gate road supported solely with internal high-strength resin-grouted cable supports that would virtually eliminate the necessity for crib or concrete external supports. The support system evaluated consisted of 1.6-m (5-ft) full-column resin-grouted bolts and 4.8-m (164) long cable supports installed in conjunction with wire mesh and "monster-mats." Cable loading and roof deformations were monitored to evaluate the behavior of the immediate and main roofs during first and second panel extractions. The stress and loading histories for the panels and yield pillar were monitored to evaluate the stress transfer and pillar performance in conjunction with the roof and floor behavior. The test results indicated that the designed support system successfully maintained the roof during the extraction of two longwall panels and dramatically reduced the cost of secondary support. This paper will describe the theory of a cribless support system, the advantages of cable supports, and present the pillar, roof, and floor measurements made to assess the support performance during longwall retreat mining.

Jan 1, 1995

By Dan Payne

The Crinum mine has recently completed workings in the southern area of the mine. Over the 10 years of longwall production 40 million tonnes have been extracted and the mine has experienced over 40 longwall face, 3 main gateend, and 2 tail gateend roof falls as well as 5 roadway roof falls away from the longwall. Weak roof (less than 10MPa (1450psi) in the bolted horizon) has been the principal roof control issue at the mine. However, weak, highly cleated coal, water inflow from overlying aquifers, some minor structure and a diatreme in the main headings have also contributed to the challenges at the mine. This paper describes the geotechnical experience over the 10 years, the mine?s approach to addressing the issues and the relative success of these approaches.

Jan 1, 2008

By Axel Preusse

On the basis of the Law on Financing Hard Coal Mining dated 28 December 2007, the subsidized German hard coal mining industry will be shut down to the end of the year 2018 in a socially acceptable manner; in 2012 this decision will be reassessed. Even after the abandonment of all mining activities subsequent burdens are inevitable. In this paper, the possible long-term effects resulting from a mining industry history, which is spanning more than 200 years, and their possible consequences are pointed out, in particular in connection with the abandonment of mine water drainage measures which is economically meaningful in the long term. According to the decision situation as it presents itself today, this point in time will be far beyond 2018. In preparation for this, an action framework is to be developed in areas possibly affected.

Jan 1, 2008

By Theresa M. Bodus

The strength of roof shales is, in part, a function of the preferred orientation of clay minerals within them. Therefore, analysis of clay fabric under both air-dried and hydrated conditions should be helpful in understanding roof shale failure. Core samples were collected from the Energy, Anna, and Lawson shales, which are locally present as roof shales of the Herrin No. 6 coal of southern Illinois. The Clay Fabric Index (CFI) was measured for thirty core samples, using X-ray diffractomtry, to investigate the relationship between clay fabric and roof collapse in coal mines. Greater CFI values indicate a weaker preferred orientation among clay minerals, which generally results in a weaker inter-grain bond. Average CFI values for air-dried samples were: 0.15 for Anna shale; 0.20 for Energy shale; and 0.37 for Lawson shale, .After the introduction of water, average CFI values increased to: 0.28 for Anna shale; 0.22 for Energy shale; and 0.43 for Lawson shale. The laboratory results indicate the following: [l] under air-dried conditions, Anna shale is the most stable, followed by Energy and Lawson shales; and 121 under hydrous (mine) conditions, Energy shale is the most stable, followed by Anna and Lawson shales. In mine environments where roof failures involving Energy shale have occurred, planar trends delineated by a relatively high density of small (0.2-4 cm diameter) iron concretions have been reported along the failure surface. Radial cracks have been observed around unsealed anchor bolt holes in the Anna shale. Instability of the Lawson shale is ubiquitous in coal mines. In the laboratory, similar physical changes were noted in the shale samples after the introduction of water, including [l] increased hydration around the perimeter of iron concretions in Energy shale, [2] radial extension cracks in Anna shale, and 131 extreme slaking of Lawson shale. Measured increases in the CFI reflect the observed physical changes of the samples upon the introduction of water, which probably reflect changes in strength that may result in roof failures in hydrated mine environments.

Jan 1, 1989

By K. H. Brandt

A standard planning system for the development of roadways was adopted for German coal mines based on the results of geotechnical and rockmechanical investigations. Its goal is to provide for sufficient support systems under conditions of multiple seam mining at great depth. The report points out the required determination of geotechnical parameters and the basis for achieving a high level of planning reliability and resulting high performance in roadway development and longwall mining. The instrumentation and the planning procedure for selecting the support design most appropriate for the geotechnical conditions are described and demonstrated by examples.

Jan 1, 2004

By D. J. Reddish

An understanding of the mechanisms that cause large seismic events in a deep mine provides a foundation for designing effective measures to reduce the incidence of these events, as well as reduce the potential for damage arising from these events. The work described in this paper focuses analytical efforts on major recognizable and controllable factors that influence mining-induced seismicity and serves to screen out many of the more random aspects of individual events. Information used in this study was developed in the course of a long-standing rock burst research program conducted by the U. S. Bureau of Mines in cooperation with mines in the Coeur d'Alene Mining District and regional universities. Information was collected on 39 seismic events having local magnitudes greater than 2.5 that occurred between 1989 and 1994. Five major types of events were developed from these data that describe all but two of the 39 large events. The characteristic mechanisms, tint-motion pattern, damage pattern, and relationships to mining and major geologic structures were defined for each type of event. This information provides a foundation for assessing whether or not changes in mining activity aimed at reducing one type of rock burst will increase the incidence of other types.

Jan 1, 1996

By P. Trueman

A project is underway to convert a validated methodology to a system that will permit the near real time interpretation of loading data from longwall powered supports, allowing rapid adaptation of mining strategies, maintenance scheduling and set pressures for optimal roof control. Such a system has the potential to significantly reduce roof control problems and is a pre-requisite to successfully fully automating a longwall face. The development depended upon a robust methodology for interpreting the pressure sensor data coming from the face in the context of support strata interaction using a novel characteristic load cycle analysis concept. New software was developed capable of handling the large quantity of data emanating from a modern longwall with the current level of signal corruption and loss, accurately delineating load cycles and extracting the critical load cycle features essential for interpreting how the support and strata are interacting. Existing software being used to access the pressure data were found either to be incapable of delineating load cycles at all or, where they could, the accuracy was low, making it impractical to use them in combination with the new advances in load cycle analysis. The software is currently being extended from ?off line? to ?on line? in order to provide a real time response to changing strata conditions.

Jan 1, 2006