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By David I. Hoyer

It is shown that by monitoring longwall leg pressures in real time, warning can be given for significant weighting events and the formation of roof instabilities, such as roof cavities, several hours in advance. Longwall Visual Analysis (LVA) is a software package that continuously monitors shield pressures and shearer position in longwall mines. LVA has been running on 19 Australian longwalls for up to four years, and as a result a very substantial database of shield pressure trends in a wide range of longwall situations has been collected. This database has been analyzed to develop indicators that will give operators and geotechnical engineers advance warnings of developing conditions such as weighting events and difficult roof conditions. LVA displays live charts of data, such as shield leg pressures, loading rates, and yield frequencies. Data analysis techniques were applied to historical data records from multiple longwall sites in order to develop a ?Cavity Risk Index? (CRI). The CRI indicates the risk of roof cavities developing, and it is based on pressure trends that indicate significant yielding and loading rates spanning a region of relatively low support. Case studies from different longwalls are given, showing how the CRI can give real-time advance warning of the formation of roof cavities with their anticipated location on the face. The limited number of case studies done so far indicate that the predictions have a high degree of reliability, but more work is required to quantify this.

Jan 1, 2011

By Mark Colwell

This paper summarizes the results of a research project whose goal was to provide the Australian coal industry with a chain pillar design methodology readily usable by colliery staff. The project was primarily funded by the Australian Coal Association Research Program and further supported by several Australian longwall operations. The starting point or basis of the project was the Analysis of Longwall Pillar Stability (ALPS) methodology. ALPS was chosen because of its operational focus; it uses tailgate performance as the determining chain pillar design criterion rather than simply pillar stability. Furthermore, ALPS recognizes that several geotechnical and design factors, including (but not limited to) chain pillar stability, affect that performance. There are some geotechnical and mine layout differences between United States and Australian coalfields that required investigation and, therefore, calibration before the full benefits offered by the ALPS methodology could be realized in Australia. Ultimately, case history data were collected from 19 longwall mines representing approximately 60% of all Australian longwall operations. In addition, six monitoring sites incorporated an array of hydraulic stress cells to measure the change in vertical stress throughout the various phases of the longwall extraction cycle. The sites also incorporated extensometers to monitor roof and rib performance in response to the retreating longwall face. The study found strong relationships between the tailgate stability factor, the Coal Mine Roof Rating, and the installed level of primary support. The final outcome of the project is a chain pillar design methodology called Analysis of Longwall Tailgate Serviceability (ALTS). Guidelines for using ALTS are provided.

Jan 1, 1999

By Chris Pritchard

Falconbridge Ltd., a major base metal producer in the Sudbury Basin of Ontario, Canada, identified a need to find a safe and cost effective replacement for welded-wire mesh used to support loose ground during underground nickel mining. Safer and lower operating costs are a must to maintain mine viability in lower base metal price environments. Stringent material strength parameters were put in place to determine if a spray-on liner was acceptable for testing and application trials. Falconbridge Ltd. took a methodical approach to the concept of a spray-on liner as a screen replacement proving the strength characteristics of the liner as a safe, cost effective, and suitable method of supporting ground. Materials testing was undertaken at Laurentian University to further understand the capacity of the material Laboratory and field tests indicated excellent adhesion and good tensile strength. Further, it maintains significant tensile strength even after elongation of over 25%. Compact, hydraulic mixing and spraying equipment have been developed and successfully utilized in the Fraser Mine of Falconbridge's Sudbury Operations Currently the Tekflex material has been approved as a wire-mesh replacement and is undergoing full-scale production trials in selected locations at Fraser Mine.

Jan 1, 1999

By Erdal Unal

In order to define the support parameters, by use of empirical approaches, it is particularly important to analyze the rock-load height that should be controlled by supports. The controlling mechanism however, depends on the type of support used there- fore, the rock loads should be treated differently. In this paper, an empirical equation is introduced which relates the rock-load height (ht) to a quantitative rock quality index (REIR), and roof span (B) as fallows:

Jan 1, 1986

By David H. Tang

Similar massive pillar failures were observed in two underground coal mines with different configurations of mine workings and overburden depths. Finite element models were used to analyze the causes and mechanisms of the pillar failure. The results of the analyses confirms the underground observation. For Case No. 1 the failure of coal pillar is mainly caused by a combined effect of smaller coal pillars under a large overburden and the interaction of multiple seam mining. For Case No. 2, the pillar failure was initiated by the weak roof rock but the ultimate pillar failure was caused by the splitting of pillars into very small size.

Jan 1, 1984

The applicability of four empirical strength criteria for rock masses, namely, Hoek and Brown, Bieniawski - Yudhbir et al., Rarnamurthy - Arora and Johnston - Sheorey et al. has been tested against the triaxial test data for jointed plaster of Paris of Arora. The following modified Bieniawski criterion shows best agreement with the triaxial test data for jointed plaster of Paris:- where st = axial compressive stress at failure, MPa; s3 = confining pressure, MPa; scm = uniaxial compressive strength of jointed plaster of Paris, MPa; and B = - 3.988 + 9.674 (scm )-0? 185 The modified Bieniawski strength criterion can be used to estimate the strength of a coal pillar, based on a knowledge of stress distribution in it.

Jan 1, 1992

By Richard E. Thill

Two Cross-borehole systems were developed by the Bureau of Mines, one operating at a central frequency of 20 kHz for close-in spacing between boreholes (2-20m) and another operating at a dominant frequency between 1-2 kHz for more distant range (40-300+m) between boreholes. The systems are specially designed for the expected ranges and high resolution required in mining applications. Potential applications of the crosshole systems are presented for a variety of geophysical measurements related to underground mining problems.

Jan 1, 1989

By John C. Stankus

From research started six years ago, a new design criteria has been developed for mine roof bolt systems This design criteria precisely and quantitatively determines parameters such as bolt length, installed tension, and spacing that will produce the Optimum Beaming Effect (OBE) OBE, a new roof control term, is defined as the roof beam that has no separation above and within the bolted range with the shortest bolt passible Utilization of this design criteria has shown that in many cases shorter roof bolts with higher installed tension can produce a much stronger beaming effect (Stankus, 1995) This paper will first present a brief deception of how the design criteria was formulated From there, actual case studies where the design criteria was utilized will be discussed In conclusion, current and future research efforts will be presented

Jan 1, 1997

By Rene Rodriguez

The initial results of our investigations on the applicability of geophysical exploration methods to map coal seam inclusions were summarized in a paper submitted at the Thirteenth Conference on Ground Control in Mining in 1994. These investigations were continued under a variety of conditions at a number of underground coal mining operations. The results have been encouraging and have fostered a much more detailed understanding of the geophysical principles at work in underground mine mapping. This report summarizes a particularly interesting application, that is, the use of Underground In-Seam Seismic exploration methods to identify and to approximately delineate coal seam inclusions within longwall panels. The methodology relies on the excitation of seismic sources in the coal seam along one side of the panel and the recording of seismic waves by geophones secured to the coal rib on the opposite side. Head-Body waves (compressional and shear), direct and channel waves are generated in the process. While the head waves are critically refracted waves traveling at the coal floor and roof, direct and channel waves are transmitted through the coal body and are dispersive in character. Tomographic techniques applied to the velocity inversion of first arrivals permits a fast mapping of coal seam inclusions in the panel. The process incorporates path velocity and geostatistical intersect analysis. Full wave analysis of the dispersive channel waves, provides an independent and confirmatory information of the coal geometry, including the inclusions within. The inversion technique in this case is the modal analysis of the wave trains. The application of this method to three different problems was confirmed by subsequent mining of the predicted anomalies, proving the accuracy of the technique within limits of experimental error.

Jan 1, 1995

By Wm. Mark Hart

Under a specific geological condition, roof supporting method, and pillar-entry system, an entry convergence concept may be the most effective means that can be used to effectively indicate the longwall entry stability (i.e. pillar bumps, roof falls, or floor beaver This concept may also be used to verify the effectiveness of the current pillar-entry system and roof supporting method. Furthermore, convergence measured under different mining phases may lead to a better understanding of the longwall mining-induced abutment loads and may be used to estimate stress change in pillars and thus pillar stability. In this paper, a remote entry activity monitoring system was designed; a comprehensive entry convergence monitoring strategy was proposed; thee abutment loading phases in longwall mining were defined; the relationship between roof deflection and stress change in a pillar was derived, and finally the extent and magnitude of the mining-induced abutment loads as well as their impact on entry convergence were discussed using the field data obtained so far.

Jan 1, 1995

By D. N. Cleaver

Like other influence function methods, the SWIFT subsidence prediction program, developed within the Mineral Resources Engineering Department at the University of Nottingham, requires calibration to regional data in order to produce accurate predictions of ground movements. Previously, this software had been solely calibrated to give results consistent with the Subsidence Engineer's Handbook (NCB, 1975). This approach was satisfactory for the majority of cases based in the United Kingdom, upon which the calibration was based. However, in certain circumstances within the UK and, almost always, in overseas case studies, the predictions did not correspond to observed patterns of ground movement. Therefore, in order that SWIFT, and other subsidence prediction packages, can be considered more universal, an improved and adaptable method of regional calibration must be incorporated. This paper describes the analysis of a large database of case histories from the UK industry and international publications. Observed maximum subsidence, mining geometry and Geological Index for several hundred cases have been statistically analysed in terms of developing prediction models. The models developed can more accurately predict maximum subsidence than previously used systems but also, are capable of indicating the likely range of prediction error to a certain degree of probability. Finally, the paper illustrates how this statistical approach can be incorporated as a calibration system for the influence function program, SWIFT.

Jan 1, 1995

By S. K. Sharma

The present study attempts to critically review the pillar-design as required by Coal Mines Regulations of India vis-a-vis latest rockmechanics formulae, Equivalent material modelling was the approach adopted for testing the safety and stability of the pillar sizes predicted by various formulae. Two parameters, considered here for calculating pillar sizes was the stresses acting over the pillars and the strength of the pillars. Stresses acting on the pillars were calculated by the tributary area method. Based on the studies of Coates and Hustrulid, the stress obtained was reduced by 40 percent. For calculating the strength of the pillars, three formulae, namely Holland and Gaddy, Salamon and Munro and Central Mining Research Station, India, were used. The pillar widths were then determined by equating the product of recommended factor of safety and pillar stress with the pillar strength.

Jan 1, 1992

By L. Munsamy

The bulk of primary mining at the Anglo Coal's Bank Colliery was carried out by drill and blast method, in a bord and pillar layout exploiting the No. 2 Seam. Secondary extraction, top/bottom coaling, also took place using both the drill and blast (D&B) method and continuous miners (CM). Extensive mining in this seam resulted in the risk of sterilising the No. 4 Seam reserves. Therefore, multiseam design guidelines established by Salamon and Oravecz (8) were re-evaluated in light of the low safety factors and the low pillar width to mining height ratios of pillars in the No. 2 Seam. Two and three dimensional (2D & 3D) numerical modelling was carried out to quantify and verify the parameters that influence the multiseam mining. The modelling was conducted primarily in two dimensions (plane strain), with 3D models to confirm the validity of the 2D results. A panel risk assessment flow chart was developed to assess the panels with respect to pillar failure and multiseam mining. This flow chart was extended into the evaluation of risk of surface subsidence using the current rock engineering knowledge. The aim of this risk assessment tool is to assess all underground workings at Anglo Coal collieries simply and rapidly. The identification of high-risk areas would warrant further investigation and possible backfilling to reduce the risk of pillar failures ensuring that reserves in the No. 4 and 5 Seams above are intact.

Jan 1, 2004

By David P. Conover

The effects of stress distribution on pillar stability were evaluated through rock mechanics studies conducted in three underground room-and-pillar coal mines. Results of the three field instrumentation programs are presented to illustrate the use of borehole pressure cells to determine vertical stress distribution in coal pillars at various stages of mine development. Vertical stresses and stress changes are determined using methods developed at the Bureau of Mines. Measured stress distributions are compared against theoretical distributions and possible explanations for observed behavior are discussed. Results indicate that stresses and stress changes calculated from pressure cell data are inconsistent with theoretical values; however, the data are useful for qualitative evaluation of trends over time. Calculated values are found to be sensitive to coal properties and initial setting pressures of the cells.

Jan 1, 1989

By Nikolaos Polysos

To drive and utilise gate roads economically requires accurate planning and risk assessment considering the variable geomechanical requirements. The geomechanical part of roadway planning is secured by geotechnical investigations of drill cores and geophysical borehole logging. The rock specific parameters determined in this way allow a statement about the quality (defomtation and strength characteristics) of the rock that has to be cut across. They set up the basis for a standardised rock assessment. Combined with the prevailing rock mechanical situation, technical boundary conditions for advance and support techniques can be identified already in the planning phase. Detailed geotechnical investigations prior to the planning and driving of roadways can lead to a technically and economically optimised roof support system. Geotechnical core logging and a standardised assessment scheme based on the data acquired will be introduced. Geophysical borehole logging methods provide additional physical data. Different methods and their validity, which may lead to a further improvement of the planning reliability, will also be presented. The geomechanical information gained and their relevance fir the planning will he illustrated. Keywords: rock classification, geotechnical rock parameters. geophysical well logs. support planning

Jan 1, 2002

By Anthony T. Iannacchione

Longwall mining produces surface subsidence basins that affect streams by water diminution and contamination. These effects can ultimately impair the biological character of the plants and animal communities living in the streams. The combination of flow, water quality, and ecological effects threaten the environmental sustainability of longwall mining and, ultimately its social acceptability. Pennsylvania has developed new standards, released in 2005 and implemented in 2007, to help protect the commonwealth?s valuable streams from the detrimental effects of subsidence. These standards are aimed at eliminating environmental problems and will therefore help to make underground bituminous coal mining more sustainable. The engineering solutions necessary to comply with these standards are fundamentally affected by the geologic nature of coal-bearing strata, the existing in-situ stress conditions, and the ability of selected species to survive and adapt to changes in their environment caused by longwall coal mining subsidence. Needless to say, geologists, biologists, and engineers are all playing a pivotal role in developing prevention controls and recovery measures necessary to protect streams and sustain underground coal mining. The challenges facing scientists and engineers in sustaining the development of underground coal mining in Pennsylvania are examined through case studies. These challenges all arise from the subsidence effects of longwall mining. The key effects of subsidence, discussed through case studies, are: a) tension cracks, b) springs in headwater stream areas, c) stream gradients and subsidence basins, and d) stream compression ruptures and horizontal stress. All of these potential threats demonstrate the need for high-level scientific knowledge and innovative engineering solutions. One could argue that Pennsylvania?s stringent stream protection standards put the coal mining industry at a competitive disadvantage compared to other coal producing states. It is equally likely that compliance with these standards help to mitigate the effect of subsidence, thereby helping the coal industry achieve a sustainable future.

Jan 1, 2011

By Alexander Kattner

Surface fractures in urbanized areas show the risk potential of near-surface mining. This kind of mining subsidence damage occurs mainly in areas where hard coal was mined near the surface with very little overlying rock strata. In North Rhine-Westphalia (NRW), the mining authorities assess areas that are at risk for caveins by means of envelopes constructed according to Hollmann and Nürenberg (1972). Drafting abandoned underground structures depends on the assessment of historical documents such as mine plans, mine titles, and operation plans of prior mines in this area. A research and development project establishing a comprehensive Geo Information System (GIS) for hazard prevention was undertaken in cooperation with the mining authorities of NRW. It aims to display the areas endangered by abandoned, shallow underground mines for the purpose of hazard prevention. A tool was programmed using ArcGIS as a result of this project. It is used to digitize underground workings from preserved mine plans, calculate outcrop lines of coal seams and display hazardous areas. In the future, a database of shallow mine workings will be established. The tool will be used for a systematical mapping of shallow workings and assessment of endangered areas.

Jan 1, 2011

By Dave Forrester

Mining subsidence over shallow abandoned coal mine workings is unfortunately not an uncommon experience in the coalfields of North America and there is a significant knowledge and literature base addressing the phenomenon and related issues. From time to time, however, a case history arises which is outside the more routine occurrence. This paper addresses one such case and presents an overview of a subsidence Event above abandoned coal workings, which badly damaged a two-story High School building. The subsidence at the Cape Breton & Victoria Regional School Board (CBVRSB)'s MacDonald High School in Dominion, N.S., Canada on October 30, 2002 led to a detailed investigation. It was concluded that the ground movement and associated building damage had been caused by mining subsidence in underlying old underground coal mine workings of the 1890s, only 150 ft deep at the School. The Event was not isolated and a similar but smaller event happened nearby a few months later. The paper describes the events and summarizes the subsequent investigations and findings. The 17-year old High School building was eventually demolished.

Jan 1, 2004

By D. J. Reddish

This paper illustrates how numerical modelling has been utilised to predict water and gas pathways around active longwall panels using two case studies. This information has proved invaluable m the design of methane drainage schemes and for risk assessments undertaken at the mining planning stage. The modelling approach described has now been applied successfully to several situations in UK coalmines in order to optimise the position, length and orientation of methane drainage boreholes. The first case study describes the numerical modelling of the stress and shear plane development mund a longwall panel. The simulation was used to predict potential gas pathways through regions of low stress confinrment which are conducive to fluid flow. The conclusions from the modelling were utilised to allow an increase in the volumes of gas drainage and allow the efficient mining of the panel. A detailed discussion is provided on the model construction, determination of input paramters and optimisation of the borehole positions Details are provided on how the models allow a prediction of the fracture patterns, stress distributions and zones of reduced permeability around an active longwall panel. The second case study describes the numerical modelling of a fault adjacent to a face line drivage of a panel that had just started to be retreated. An inrush of water had occurred and a concern was whether further mining would lead to more extensive inrushes or whether the water inrush was associated with more local, limited capacity water bearing features. The role of a fault that was positioned close to the locality of the mine workings was examined. Fracture patterns stresses, strata permeability and water flows were predicted using mechanical and hydro-mechanical coupled models.

Jan 1, 2003

By Qiang Fu

ABSTRACT This paper aims to identify the main issues concerning top-coal loss and the optimum drawing interval of LTCC (Longwall Top-coal Caving) mining by underground tests and DEM (Distinct Element Method) modeling. By extracting a new roadway in the goaf of the mined LTCC Panel No.4318 in Wang Zhuang coal mine of Lu-an CMA (Coal Mining Administration), and underground observation of the distribution of loss coal and broken rock in different sections of the roadway, this paper analyses the composition and presents a 3-D distribution map of the loss coal in the exploration roadway. This study indicates that the DEM modeling of broken top-coal is reasonable and scientific as it can simulate the movement of broken coal and waste rocks and determine the optimum drawing interval of top coal.

Jan 1, 2002