• 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
  • Applications of Hydraulic Fracturing to Control Caving Events in Coal Mines - The Moonee Experience - Ken Mills

    Hydraulic fracturing involves the injection of high pressure fluid into a rock mass to form one or more fractures. Fractures are oriented perpendicular to the lowest principal stress acting at the time of injection. Hydraulic fractures can be extended considerable distances from one or more boreholes oriented in any convenient direction. The technique offers a method to control caving related phenomena such as inducement of caving, control of periodic weighting, initiation of first goaf fall, and preconditioning of longwall takeoff areas. This paper describes the successful application of hydraulic fracturing to control windblast hazard at Moonee Colliery and opportunities that emerge for other applications.

    Moonee Colliery extracts the lower 3m of the Great Northern seam using a 100m wide longwall panel. A 35m thick conglomerate strata immediately overlying the seam is able to temporarily bridge across the panel so that the goaf does not immediately cave.
    When the conglomerate strata does eventually fall, the bottom 10-15m collapses more or less as a single mass over an area 50-300m long by the full 100m panel width. The windblasts generated by these events present a very significant hazard to men working on the longwall face.

    Hydraulic fracturing has been successfully introduced at Moonee Colliery as a method to induce caving events “on demand”. The men are evacuated from the longwall face area prior to commencement of the hydraulic fracture treatment. After a treatment typically
    lasting 15 minutes to 2 hours, a goaf fall event is usually initiated and mining can be recommenced with the windblast hazard eliminated.

    The work at Moonee is believed to be the first successful use of hydraulic fracturing to induce caving events in Australia. Infusing water to weaken rock and small-scale hydraulic fracturing, ahead of or over longwall panels, has been tried previously in Australia and South Africa. Infusion is currently being used in China. Hydraulic fracturing has also been used in Poland to condition the roof over new panels and to modify the stiffness of rock around mine openings to reduce rock burst hazards. The application of hydraulic fracturing, described in this paper, to control the timing of caving events has not
    been used before.

    The technique also offers the potential to control periodic weighting events, induce caving at longwall startup, precondition pre-driven longwall take-off roads and take control of caving in other situations where it would be desirable to induce the goaf to cave. Application-of-Hydraulic-Fracturing-to-Control-Caving-Events-in-Coal-Mines-Ken-Mills-2002.pdf1.1 MB
  • 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
  • A Method of Determining Longwall Abutment Load Distributions for Roadway and Pillar Design - Ken Mills

    This paper describes a method to determine abutment loads on longwall chain pillars and adjacent roadways. The method is based on: observation of subsidence behaviour, field measurements of abutment load distributions, and considerations of total overburden load about one or more longwall panels.

    Surface subsidence data is used to deduce how far the overburden strata can transfer overburden weight and the total abutment load required to be distributed for any particular depth and longwall geometry. To be of practical use in roadway and pillar design, the shape of the abutment load distribution is also required as a function of distance from the goaf edge. Direct field measurement using high quality, three dimensional stress monitoring instruments is considered to provide the most reliable method of determining the magnitude and shape of the abutment load distribution at various stages of longwall mining.

    The abutment load distribution determined at any one site by field measurement can be scaled horizontally to account for changes in overburden depth and vertically to account for changes in total abutment load. Thus, within the limitations of extrapolating data from one site to another, the abutment load distribution can be estimated for different depth and longwall geometries. Pillar loading and the vertical stress acting on adjacent roadways can then be determined from the measured load distributions, or scaled versions thereof, for any particular stage of mining, longwall geometry or depth of overburden. A-Method-of-Determining-Longwall-Abutment-Load-Distributions-for-Roadway-and-Pillar-Design-K.Mills-2001.pdf1.2 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
  • Displacement Monitoring and Visualisation Using Terrestrial Photogrammetry - Ken Mills

    Digital photogrammetry has been to shown to be an effective and efficient method for visualising and measuring subsidence behaviour of sandstone cliff formations. The system described in this paper provides the capability to measure subsidence movement over areas that are inaccessible to conventional subsidence monitoring techniques. Preliminary comparisons with a more traditional survey technique (total station survey to prisms) has shown mean variations, between the techniques, in vectors of movement of 17 millimetres.

    They system described provides visualisation of the sandstone cliff formations through the generation of 3D images which can be rotated to be viewed from any angle. Visualisation provides a powerful tool for measuring, interpreting and understanding the shape, structure and geology. 3D images provide an advantage in tracking vectors of movement of natural features over a large area of inaccessible escarpment.

    This paper provides a description of the technique and results obtained to date. The photogrammetry techniques used are based on photogrammetry techniques developed for mine highwall mapping. The results obtained in mapping subsidence exceed expectation for the camera system used. Further development will be undertaken to develop this teqnique for commercial applications in landform stability monitoring.

    Through the generation of a large number of vector movements and visualisation a more complete knowledge and understanding of subsidence behaviour of sandstone cliff formations is derived. MSTS-Conference-2001-Displacement-Monitoring-and-Visualisation-Using-Terrestrial-Photogrammetry-K.Mills.pdf2.1 MB
  • Caving Induced by Hydraulic Fracturing at North Parkes Mine - Rob Jeffrey - Published 2000

    This paper describes the first use of hydraulic fracturing for cave inducement in a block caving mine. As of September 1999, several hundred hydraulic fracture treatments have been performed at Northparkes and are attributed with inducing about 7 million tonnes of ore to cave. Caving-Induced-by-Hydraulic-Fracturing-at-North-Parkes-Mine-R.Jeffrey.pdf267 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
  • A review of Recent in situ Stress Measurements in United Kingdom Coal Measures Strata - Paul Cartwright - Published 1997

    The in-situ stress regime is recognised by United Kingdom coal mine operators as a significant design parameter related to efficient mine design. The results obtained from recent overcore stress measurements undertaken in Coal Measures strata are analysed and presented. A relationship has been deduced which relates the maximum horizontal stress to the depth and to the elastic properties of the rock. This relationship is considered more suitable for estimating the maximum horizontal stress magnitude in Coal Measures strata than existing methods based solely on the depth of cover. This study also indicates that all in-situ stress determinations in sedimentary strata should be quoted with the elastic properties of the test horizons. A-review-of-recent-in-situ-stress-measurements-in-United-Kingdom-Coal-Measures-strata-P.Cartwright.pdf2.1 MB
  • In Situ Stress Measurement Using the ANZI Stress Cell - Ken Mills

    This paper describes the operation of the ANZI (Australia, New Zealand Inflatable) Stress cell. Laboratory and field measurements are used to illustrate the instrument's operation. The ANZI stress cell has a pressuremeter design that enables 18 electrical resistance strain gauges to be pressure bonded directly to the rock of a borehole wall. The strain gauges are monitored during overcoring to obtain stress relief strains.

    An up hole pressure test is undertaken prior to overcoring to obtain the elastic properties of the rock in situ and to confirm the correct operation of all the strain gauges. The elastic properties of the rock are also obtained after overcoring in a biaxial test. The ANZI stress cell is widely used for routine three dimensional stress measurement in underground coal operations in Australia. It is being increasingly used in the United Kingdom, China, Japan and Vietnam in coal mining, civil and hard rock applications. In-Situ-Stress-Measurment-using-the-ANZI-Stress-Cell-K.Mills.pdf1.2 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
  • Investigations Aimed to Improve Tailgate Serviceability at Dartbrook Mine - Ken Mills

    Dartbrook Mine has experienced rib control difficulties because of deterioration in the tailgate corner of the longwall face as overburden depth has increased. This paper summarises an investigation to optimise support and develop strategies to improve the serviceability of the tailgate roadways. Field measurements undertaken in the tailgate of Longwall 6 identified roadway softening mechanisms, deformation characteristics and factors controlling deformation. This provides the basis for optimising the reinforcement system as part of an ongoing Strata Management Plan at the mine. Investigations-Aimed-to-Improve-Tailgate-Serviceability-at-Dartbrook-Mine-K.Mills.pdf397 KB
  • 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
  • Remote High Resolution Stress Change Monitoring of Hydraulic Fractures - Ken Mills - Rob Jeffrey

    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. Remote-High-Resolution-Stress-Change-Monitoring-of-Hydraulic-Fractures-K.Mills-R.Jeffrey.pdf1 MB
  • 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
  • Impact of Longwall Width on Overburden Behaviour - Ken Mills

    The longwall panels at Clarence Colliery have experienced intermittent sudden weightings on the face that have caused some production delays. These weightings have typically been more severe on the wider faces. A program of surface subsidence and extensometer monitoring was undertaken above Longwalls 4 and 5 to investigate the behaviour of the overburden strata during longwall extraction on two faces of different widths.

    The monitoring indicated that a dome shaped zone of large downward movement extends up into the overburden strata to a height equal to about the panel width. A major strata unit between 50 m and 70 m above the coal seam influences the behaviour of the overburden strata and may be a factor in the observed sudden loading of longwall face supporLo;. Downward movement of this major unit appears to concentrate on vertical fractures. Increased loading on the face supports could then be expected. The downward movement of this major unit appears to be more significant in the overburden behaviour above the 200 m wide longwall compared to the 160 m wide longwall face Impact-of-longwall-width-on-overburden-behaviour-K.Mills.pdf1.8 MB
  • 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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