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  • The Role of Gas Pressure in Coal Bursts Winton Gale 2018

    Rock and coal fractures and micro seismic vibration are common occurrences during development mining. It is very uncommon for coal and rock to be propelled into the roadway during normal mining operations. However, such occurrences do occur and appear to require significantly more energy than is available from strain energy release during coal cutting. The sources of energy, which can contribute to the propulsion of coal from the face or ribs, are typically strain energy from the surrounding ground, seismic energy from a rapid rupture of the ground in the vicinity, or rapid expansion of gas from within the burst source area.

    The aim of this paper is to briefly review the bursts that could be related to strain energy or seismic energy. However, the greatest emphasis is placed on the effect that gas within the coal could play in moderate to gassy mines.

    It has been found that the bursts related to the expansion of gas can occur in coal and stone. The volume of gas involved in coal bursts is typically lower than in gas outbursts; however, the process is generally similar. The-Role-of-Gas-Pressure-in-Coal-Bursts-Winton-Gale-2018.pdf4.2 MB
  • Insights into the Energy Sources of Bursts in Coal Mines and the Effective of Prevention and Control Measures - Mahdi Zoorabadi - Winton Gale

    Coalburst is a general term, which is commonly used in the coal mining industry for the violent failures of coal in the ribs and face of roadways and panels in underground coalmines. Due to lack of interest in the industry to reveal the causing source of the event, or due to uncertainty about the source, they happily use this term. The term by its own does not reveal the source of the energy, which causes the event. There are three sources of energy that can cause a burst event in underground coalmines: 1) store elastic strain energy, 2) seismic events and 3) gas expansion energy. This paper presents the fundamentals about these sources of energies and discusses our known and unknown facts about the mechanisms. Additionally, it discusses the reliability and effectiveness of stress relief holes and gas exhaust holes as controlling measures to prevent burst events. Insights-into-the-Energy-Sources-of-Bursts-in-Coal-Mines-and-the-Effective-of-Prevention-and-Control-Measures-M.Zoorabadi-2018.pdf1.4 MB
  • Connectivity of Mining Induced Fractures Below Longwall Panels A Modelling Approach - Yvette Heritage - Winton Gale - Adrian Rippon

    Gas make into active longwall panels is an important issue in ventilation and gas drainage design. A method of simulating the mining induced fracture network and associated increase in hydraulic conductivity is a necessity for improved mine design, hazard management planning and gas drainage efficiency. This paper identifies and illustrates the key components in determining the connectivity of lower gas sources to an active goaf. Computer modelling identifies the formation of cyclic fractures that form below the longwall face and extend down back below the goaf. These cyclic fractures form when the stress conditions are high enough and the strata properties allow for shear failure to extend down through the strata.

    The mining induced fracture formation and stress redistribution creates increased hydraulic conductivity of the floor strata below the active goaf. The stress redistribution and fracture volume also reduce the pore pressure below the goaf, allowing gas desorption to occur from lower seams. The combination of gas desorption and increased hydraulic conductivity allows gas connectivity from gas sources below the seam to the active goaf. A monitoring program at a NSW mine as part of ACARP Project C23009 allowed for preliminary validation of the concepts illustrated from the computer modelling. Preliminary field gas flow measurements are within the range of connectivity expectations based on rock failure modelling of longwall extraction. This report presents the first validation results for the modelling approach presented in this paper. Further results from ACARP Project C23009 on optimisation of gas drainage will follow in future publications. Connectivity-of-mining-induced-fractures-below-longwall-panels-A-Modelling-Approach-Y.Heritage-W.Gale-A.Rippon-2017.pdf1.3 MB
  • Analytical Procedure to Estimate the Horizontal Anisotropy of Hydraulic Conductivity in Coal Seams - Winton Gale - Mahdi Zoorabadi

    The horizontal hydraulic conductivity anisotropy of coal seams is a controlling parameter for designing gas drainage boreholes. The ratio between the maximum and minimum horizontal hydraulic conductivity (RkH-kh) and the orientation of maximum horizontal conductivity defines this anisotropy in horizontal plane.

    This paper presents a new analytical procedure based on the field stress data and geometrical properties of coal cleats to calculate these two parameters. The application of this procedure for a real case in Eastern of Australia resulted in an average ratio of 20.9 for RkH-kh and orientation of NE for maximum horizontal conductivity. The comparison between these results with the measured values validated the accuracy of proposed procedure to estimate the anisotropy of horizontal hydraulic conductivity of coal seams. Analytical-Procedure-to-Estimate-the-Horizontal-Anisotropy-of-Hydraulic-Conductivity-in-Coal-Seams-W.Gale-M.Zoorabadi-2015.pdf377 KB
  • Investigation into Abnormal Surface Subsidence Above Longwall Panel Southern Coalfield - Winton Gale

    The subsidence over a longwall panel at Tahmoor Mine in the Southern Coalfield of NSW, Australia, was found to be approximately twice the size it had been in previous measurements. An investigation into the potential causes was conducted using computer modeling together with hydrological characterization and detailed geotechnical characterization of the strata.

    The abnormal subsidence was found to be consistent with localized weathering of joint and bedding planes above a depressed water table adjacent to an incised gorge. The study showed that other factors such as variation in stress field, joint zones, variation
    in rock strength and topographic factors did have sufficient impact to induce the abnormal subsidence.

    The results have significant implications to subsidence prediction in areas that may be prone to the phenomenon found at Tahmoor. Key indicators of the potential for this form of abnormal subsidence are presented. Investigation-into-Abnormal-Surface-Subsidence-Above-LW-Panel-Southern-Coalfield-2011.pdf2.6 MB
  • Using Helium as a Tracer Gas to Measure Vertical Overburden Conductivity Above Extraction Panels - Yvette Heritage - Winton Gale

    This paper investigates helium injection into the goaf as a tool to measure goaf to surface connectivity. Laboratory studies confirmed a relationship between gas velocity and fracture conductivity through helium injection. Field trials of helium injection into the goaf were successfully conducted to determine whether a connection exists between the surface and the goaf. A repeatable technique of borehole helium injection, with a borehole drilled into the highly permeable caved zone of the goaf, proved to demonstrate more quickly whether a connection to the surface exists. Using-Helium-as-a-Tracer-Gas-to-Measure-Vertical-Overburden-Conductivity-Above-Extraction-Panels-Y.Heritage-W.Gale-2009.pdf281 KB
  • An Investigation into Underground Mine Interaction with Overlying Aquifers Huntly, East Mine, New Zealand - Winton Gale - Published 2006

    In recent years, Huntly East Mine has operated at a depth range of approximately 100 m to 220 m below a Quaternary aged clay, sand and silt aquifer that is connected to a nearby large river system (Waikato River). A key issue for mine planning and environmental management has been the development of mine design criteria to allow efficient mining of the reserves and to maintain the integrity of the aquifer.

    A case study and back analysis at Huntly East Mine is presented, which investigates the overburden conductivity and the impacts caused by mining-induced caving. The case study includes: i. computer modelling of the mine geometry, caving and overburden fracture networks created; ii. field investigation to develop an engineering geological model of the overburden within the goaf to validate the goaf geometry as defined by the computer generated model; iii. in situ field measurement of overburden conductivity in the pre- and post-mining condition; iv. interference testing across the goaf to determine the level of interconnectivity; and v. measured water pressure profiles above the mine. An-Investigation-into-Underground-Mine-Interaction-with-Overlying-Aquifers-Huntly-East-Mine-New-Zealand-W.Gale.pdf822 KB
  • Water Inflow Issues above Longwall Panels - Winton Gale - published 2006

    The aim of this paper is to discuss the issues which relate to surface water inflow through the fractured overburden above longwall panels. The information used is a combination of field experience and computer modeling. Computer models used in this study simulate the fracture process in the geological units throughout the overburden. Analysis of the mining induced fracture patterns and in situ joint patterns allows an estimation of the hydraulic conductivity within the overburden. The cubic flow relationship has been used in examples presented. Water-Inflow-Issues-above-Longwall-Panels-W.Gale.pdf267 KB
  • Application of Computer Modelling in the Understanding of Caving and Induced Hydraulic Conudctivity About Longwall Panels - Winton Gale - Published 2005

    Computer modelling is being used to simulate rock fracture, caving and stress redistribution about longwall panels with increasing confidence. The models are being assessed against field monitoring and have significantly increased the understanding of caving mechanics within the overburden. This paper discusses the modelling approach and provides some examples of its application to overburden damage and induced hydraulic conductivity. Computer models used in this study simulate the fracture process in the geological units throughout the overburden. Analysis of the mining induced fracture patterns and in situ joint patterns allows an estimation of the hydraulic conductivity within the overburden. The cubic flow relationship has been used in the examples presented. Application-of-Computer-Modelling-in-the-Understanding-of-Caving-and-Induced-Hydraulic-Conudctivity-About-Longwall-Panels-W.Gale.pdf271 KB
  • Numerical Modelling of Floor Deformation Mode at Longwall Face - Winton Gale - Published 2005

    High stress concentrations ahead of the longwall face often exceed the floor strength and induce fractures in the floor strata. While concentrations of the vertical stress alone induces fractures in the roof ahead of the longwall face, combinations of the vertical and horizontal stress appear to be the dominant factor in formation of floor fractures. These fractures develop in response to the triaxial stress conditions exceeding rock strength. In the immediate floor, fractures appear to form at frequent intervals dipping under the goaf at a steep angle while more complex bedding shear appears to dominate the floor failure at a greater depth. In a stronger floor the fractures appear to occur less frequently. If weak bedding planes are present in the floor, shear failure along these beddings can occur far ahead of the longwall face. The post failure displacements along the fractures and the formation of new fracture surfaces often occur in response to the stress relief, bending or buckling of thin bedded layers in the floor. The post failure displacements can be
    large and may interfere with mining operations.

    This paper presents the computational approach using FLAC to model the development of fractures in the floor strata. The model uses programmable “fish routines” that allow simulation of failure modes that may occur in response to the changing stress field ahead of the longwall face. Continuous monitoring of the two dimensional stress field is used to predict the fracture types and the direction at which the fractures may propagate. The fractures are then simulated using FLAC ubiqitous elements that allow to assign the joint direction and the reduction of joint strength in the direction of the calculated fracture. The stress state is tested continuously during the execution of the program and fractures are simulated when the stress exceeds the rock strength. This procedure can simulate the progressive development of fractures during the longwall advance. The method is particularly helpful to estimate the type of fractures and their frequency that depend on the strength of floor strata and stress build up during a longwall advance. The depth of floor failure can have a significant influence on the gas release from the floor strata in gaseous mines. The type of fractures and the fracture orientation that is computed can be presented in the movie files to view the development of fractures in the floor during the longwall advance. Numerical-Modelling-of-Floor-Deformation-Mode-at-Longwall-Face-W.Gale.pdf1.5 MB
  • Computer Simulation of Ground Behaviour and Rock Bolt Interaction at Emerald Mine - Winton Gale - Published 2004

    A collaborative project between RAG Emerald Mine, NIOSH, and SCT Operations was conducted to investigate ground behaviour, reinforcement performance, and stress redistribution in a coal mine entry subjected to a severe horizontal stress concentration. Field measurements indicated that the stresses applied to the study site nearly doubled during longwall mining, resulting in roof deformations extending to a height of 4.8 m (16 ft) above the entry.

    This paper focuses on the computer simulation that was undertaken to provide more insight into the roof behaviour and rock bolt interaction during mining. The model’s input rock properties were derived from extensive laboratory testing, and the model itself simulated a broad range of failure mechanisms. The effects of different bolt patterns on roadway behaviour were evaluated. Comparison between the model results and the field measurements indicated that that the model effectively simulated the critical elements of the actual roadway’s behaviour. With the confidence gained, the model was used as a baseline for additional simulations that evaluated the expected performance of alternative roof support systems. The study will also provide a benchmark data set for future applications of numerical modelling to U.S. coal underground mining. Computer-Simulation-of-Ground-Behaviour-and-Rock-Bolt-Interaction-at-Emerald-Mine-Winton-Gale.pdf697 KB
  • Experience in Computer Simulation of Caving Rock Fracture and Fluid Flow in Longwall Panels - Winton Gale - Published 2002

    Recent advances in computer simulation together with field measurements of caving and microseismic activity about longwall panels, has allowed a much better understanding of the caving process and the variability due to geology. Research between SCT Operations and CSIRO Division of Exploration and Mining has initiated new methods of computational modelling predicting various caving patterns and strata failure far ahead of the longwall face.

    The rock fracture distribution and the caving characteristics of a range of strata sections have been simulated by computer methods. The computer simulation of strata behaviour includes coupled fluid and mechanical behaviour. Validation studies of the method were addressed together with case studies. The method allows the simulation of longwall support behaviour and fluid pressure distributions about longwall panels under various geological conditions. The system also allows a prediction of the monitoring data, which is best suited to give an early warning of weighting events or signal various key caving characteristics. Experience-in-Computer-Simulation-of-Caving-Rock-Fracture-and-Fluid-Flow-in-Longwall-Panels-W.Gale.pdf4 MB
  • Application of Computer Modelling in the Understanding of Subsidence Movements - Winton Gale - Published 2001

    Computer modelling is being used to simulate rock fracture, caving and stress redistribution about longwall panels with increasing confidence. The models are being assessed against field monitoring and have significantly increased the understanding of caving mechanics within the overburden.

    The modelling supports the concept that the ground subsides from the seam and progresses upwards and as such subsidence is the end point in a failure pathway within the overburden. The nature of the strata, in situ stresses and the mining geometry will influence the subsidence. The nature of the fractures created and the enhanced permeability will influence the interaction of mining with aquifers and surface water. Application-of-Computer-Modelling-in-the-Understanding-of-Subsidence-Movements-W.Gale.pdf425 KB
  • Combining Modern Assessment Methods to Improve Understanding of Longwall Geomechanics - Winton Gale - Published 1998

    Ongoing, collaborative research between CSIRO's Exploration and Mining and Strata Control Technology has resulted in a better understanding of rock failure mechanisms around longwall extraction. Failure has occurred further ahead of the retreating face than predicted by conventional longwall geomechanics theory. In some cases significant failure has been detected several hundred metres ahead of the face position with demonstrated influences of minor geological discontinuities. Shear, rather than tensile failure has been the predominant failure mechanism in the environments monitored. Validating technologies of microseismic monitoring and new face monitoring techniques have assisted the development of predictive 2D computational modelling tools. The demonstrated 3D consequences of failure has assisted in the ongoing direction of the project to further investigate these effects. Combining-modern-assessment-methods-to-improve-understanding-of-longwall-geomechanics-W.Gale.pdf2.9 MB
  • Coal Pillar Design Issues in Longwall Mining - Winton Gale - Published 1998

    Coal pillar design has been based on generalised formulae of the strength of the coal in a pillar and experience in localised situations. Stress measurements above and in coal pillars indicate that the actual strength and deformation of pillars varies much more than predicted by formulae. This variation is due to failure of strata surrounding coal. The pillar strength and deformation of the adjacent roadways is a function of failure in the coal and the strata about the coal.

    When the pillar is viewed as a system in which failure also occurs in the strata, rather than the coal only, the wide range of pillar strength characteristics found in the UK, USA, South Africa, Australia, China, Japan and other countries are simply variations due to different strata-coal combinations and not different coal strengths. This paper presents the measured range of pillar strength characteristics and explains the reasons. Methods to design pillar layouts with regard to the potential strength variations due to the strata strength characteristics surrounding the seam are presented. Coal-Pillar-Design-Issues-in-Longwall-Mining-Winton-Gale.pdf3.3 MB
  • Statistical Analysis of Undeground Stress Measurements in Australian Coal Mines - Winton Gale - Ken Mills

    This paper presents a summary of 235 underground stress measurements conducted in the virgin ground of NSW and Queensland mines. The main objective of this study is to analyse the statistical information from the measurements that are relevant to strata control and mine planning with a view to estimate the risk involved with strata failure.

    Major findings include the statistical increase of maximum horizontal stress with depth in Queensland and NSW mines, a comparison of normalised lateral stress magnitudes and measurements in rock of a different stiffness, ‘Tectonic Factor’ concept, and maximum lateral stresses and their directions in NSW and Queensland coalfields. These findings can provide a valuable benchmark for mine planning and strata control with potential savings in mine operating costs. Statistical-Analysis-of-Underground-Stress-Measurements-in-Australian-Coal-Mines-W.Gale-K.Mills.pdf2 MB
  • Methods of Interpreting Ground Stress Based on Underground Stress Measurements and Numerical Modelling - Winton Gale

    This paper presents several new methods to help interpretation and understanding of ground stress. The methods are based on data from 239 stress measurements conducted in the virgin ground in NSW and Queensland mines and computational models simulating large scale faulted ground behaviour. The underground stress regime plays an important role in mining profitability and safety however, understanding of the stress tensor is often difficult due to its mathematical complexities and non-intuitive behaviour.

    The aim of this study is to explain stress distribution in faulted ground, its origin and propose several methods of stress interpretation. Major findings presented in this study include: increase of maximum horizontal stress with depth based on underground measurements and numerical simulation of faulted ground, affect of faults on ground stress, normalisation technique that allows comparison of lateral stress magnitudes in rock of different stiffness, ‘Strain Tectonic Factor’ concept and its value in understanding stress components and its affect on rock strength. Methods-of-Interpreting-Ground-Stress-Based-on-Underground-Stress-Measurements-and-Numerical-Modelling-W.Gale.pdf429 KB
  • Successful Use of a Stress Relief Roadway at Appin Colliery - Winton Gale

    High horizontal stress levels can lead to extensive roadway deformation requiring expensive secondary support to ensure stability; this is particularly the case with longwall installation faces. Longwall installation roadways are a critical construction within coal mines. The use of a purpose built ‘Stress Relief Roadway’ to minimise roof deformation in the nearby longwall installation roadway, by reducing stress impacts has been undertaken at Appin Colliery – BHP Billiton Illawarra Coal. Its use led to significant cost and operational benefits. This paper outlines the process used; from identifying horizontal stress as an issue, as well as generating computer models through the various options and culminating in ground monitoring of the constructed roadways to the successful start of the longwall panel. Successful-Use-of-a-Stress-Relief-Roadway-at-Appin-Colliery-W.Gale.pdf581 KB
  • Review and Estimation of the Hydraulic Conductivity of the Overburden Above Longwall Panels. Experience from Australia - Winton Gale

    The aim of this paper is to summarise the results and conclusions of Australian Coal Association Research Project (ACARP) Report C13013 which relate to water inflows into a mine which occur through the overburden above and adjacent to longwall panels. The study assessed available data of inflows into underground coal mines and utilised computer simulation of water flow through fracture networks.

    The study concluded that flow into mines is typically via an interconnected network of pre existing and mining induced fractures. The height above the coal seam that mining induced fractures extend is typically related to the width of the panel. However the potential for those fractures to form a connected network which can facilitate flow, is related to the amount of subsidence and the depth of mining. The study compares model simulations with measured data and provides guidelines to estimate the average hydraulic conductivity of the overburden above extracted longwall panels in Australia. Review-and-Estimation-of-the-Hydraulic-Conductivity-of-the-Overburden-Above-Longwall-Panels.-Experience-from-Australia-W.Gale.pdf952 KB
  • The Application of Field and Computer Methods for Pillar Design in Weak Ground - Winton Gale

    This paper describes the use of strain gauge based borehole instruments to monitor stress changes associated with the creation and extension of hydraulic fractures in massive rock strata at Northparkes Mine in Australia and Salvador Mine in Chile.

    This work was conducted as part of the International Caving Study ICSII. These instruments proved very sensitive to the stress changes induced by the hydraulic fractures close to the fracture plane. Analysis of the stress changes observed allowed the fracture orientation and non-symmetric fracture growth to be constrained sufficiently that a clearer insight into fracture behaviour could be obtained at both sites, particularly when combined with other observations. Recognition of the elastic stress reorientation about an opening mode hydraulic fracture has proved to be an important element in the interpretation of stress change monitoring data.

    The nature of the stress reorientation is useful in discriminating between opening and shearing mode fracture growth. A technique of identifying a range of possible solutions of fracture orientation and non-symmetric fracture growth consistent with the stress changes observed on multiple instruments has been developed. Unique definition of fracture orientation from the stress change instruments is possible if the instruments are sufficiently distributed relative to the hydraulic fracture plane. The-Application-of-Field-and-Computer-Methods-for-Pillar-Design-in-Weak-Ground-W.Gale.pdf2.4 MB
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