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  • 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
  • Experience in the Application of Computer Modelling to Coal Mine Roadway Design in Weak Rock - Winton Gale

    A summary of the weak rock failure process is presented to demonstrate the application of computer modelling to coal mine roadway design. The weak rock failure mechanism was chosen because its discovery required a design tool (modelling) that was not bound by a preconception of the results. Modelling was used to decode the relative influence of the geological and geotechnical factors.

    It is emphasised that computer simulation techniques are best applied in a practical sense if accompanied by field measurement and observation. The field measurements are used as both a means of validating the initial model and to confirm that actual events are within design expectation. Experience-in-the-Application-of-Computer-Modelling-to-Coal-Mine-Roadway-Design-in-Weak-Rock-W.Gale.pdf1.2 MB
  • Experience in Modelling Longwall Support Behaviour - Winton Gale

    Recent advances in computer simulations of strata caving mechanisms and the response of longwall supports to strata behaviour has allowed much better understanding of longwall support requirements. The computational method allows the simulation of longwall support behaviour under a wide range of geological conditions with emphasis on comparing different support geometries and support loading conditions. This paper presents results of the computational trials to simulate various longwall support geometries including the comparison of the two leg and the four leg support options, the premature caving of strata at the canopy rear and its influence on roof falls at the longwall face.

    The rock fracture distribution and caving characteristics of a wide range of strata geologies has a significant influence on the longwall support behaviour. Underground measurements and computer simulations were undertaken to investigate the caving characteristics of strata and some of the common problems typically encountered at the longwall face. The computer simulations highlight the importance of the longwall support geometry and location of the applied roof loads to minimise potential problems leading to major roof falls at the longwall face. Experience-in-Modelling-Longwall-Support-Behaviour-W.Gale.pdf1.9 MB
  • Subsidence Mechanisms about Longwall Panels - Ken Mills

    This paper presents a summary of the components of subsidence about longwall panels that have been observed and inferred from subsidence and other monitoring. The essentially independent components that make up the total subsidence observed on the surface are isolated and discussed. The combination of these components are shown to generate the range of profiles observed at surface level as subsidence.

    Monitoring of displacements within the overburden section provide another dimension to the understanding of subsidence behaviour. The concept of an arch shaped zone of large downward movement over individual longwall goafs is developed in the context of observations of subsidence movements. This concept provides a framework within which to better understand sag subsidence and elastic compression of chain pillars in multiple longwall panels at depth. Subsidence-Mechanisms-about-Longwall-Panels-K.Mills.pdf120 KB
  • Hydraulic Fracturing to Induce Caving: Fracture Model Development and Comparison to Field Data - Rob Jeffrey - Ken Mills

    Hydraulic fracturing is used at Moonee Colliery to induce caving as part of the routine operation of this longwall mine. Measurements undertaken to successfully introduce hydraulic fracturing to Moonee and pressure records routinely obtained from each treatment provide a unique opportunity to develop and test a new model of hydraulic fracture growth near a free surface. This paper presents the results of the comparison for several fracture treatments, demonstrating that the model is able to match the treatment data. Hydraulic-Fracturing-to-Induce-Caving-Fracture-Model-Development-Comparison-to-Field-Data-R.Jeffrey-K.Mills.pdf395 KB
  • Successful Construction of a Complex 3D Excavation Using 2D and 3D Modelling - Yvette Heritage

    Austar Coal Mine (Austar) successfully constructed an underground coal storage bin at a deep mine in challenging conditions. SCT Operations (SCT) was involved in various geotechnical assessments related to the bin excavation including vertical separation of the bin drift and underlying seam roadways, bin top area roof design and support and seam roof support at the bin base. Traditional
    methods used for determining support recommendations can be difficult to apply to complex three dimensional excavations. SCT used a combination of two dimensional and three dimensional numerical modelling using FLAC 2D and FLAC 3D to understand the key drivers and modes of failure about the bin excavation.The staged process of construction and an interactive approach between Austar and SCT enabled review and validation of the modelling process to occur throughout the construction. A key lesson from this program of work is that there is value in an interactive approach whereby site monitoring and review of model properties during construction provides early validation of the model. This ensures that natural geological variability, which can have significant impacts on rock failure and deformation, can be incorporated into the model as an ongoing process. Successful-Construction-of-a-Complex-3D-Excavation-Using-2D-and-3D-Modelling-Y.Heritage.pdf1.7 MB
  • Understanding Fracture Distribution within Intrusive Sills the Cordeaux Crinanite a Case Example from the Illawarra Coal Measures - Luc Daigle

    Recent diamond drill hole coring by BHP Billiton Illawarra Coal was used to characterise the distribution of fracturing within the Cordeaux Crinanite intrusive body. Geological data obtained from recent exploration boreholes and surface outcrops provided sufficient information to determine the pattern and history of fracture emplacement within the intrusive body.

    The Cordeaux Crinanite is an intrusive sill complex consisting primarily of thick olivine rich dolerite (crinanite) sills and thinner olivine cumulate (picrite) sills. Outcrop exposures of the complex are present along parts of Cordeaux Reservoir and form the bedrock to the Upper Cordeaxu Number 1 Dam and Upper Cordeaux Number 2 Dam and much of their catchments.

    The intrusive body is roughly circular in plan with a domed top and largely planar base with local bowl shaped features. The intrusive is commonly referred to as a sill but drill intersections show that it gradually cross-cuts stratigraphy. The base ranges from approximately the Balgownie Seam to above the Bulli Seam and into the Coalcliff, the roof may extend up to the Stanwell Park Claystone. Understanding-Fracture-Distribution-within-Intrusive-Sills-the-Cordeaux-Crinanite-a-case-example-from-the-Illawarra-Coal-Measures-L.Daigle.pdf312 KB
  • Instrumentation Monitoring at an Underground Mine to Establish Failure Mechanisms, Confirm Numerical Modelling and Determine Safe Working Conditions - Stuart MacGregor

    A number of potential failure modes were identified by observation and numerical modelling in the underground operation at Telfer Gold Mine. In order to gain a better understanding of the mechanisms of failure an instrumentation programme was designed. Monitoring methods included closure monitoring using tape, rod and sonic probe extensometers, stress monitoring, reinforcement monitoring with strain gauges, prism monitoring in the open pit and observation using a borehole camera.

    The results from the monitoring instrumentation established local and regional failure mechanisms with greater certainty. The information allowed mining methods, extraction sequences and reinforcement requirements to be reliably designed, using numerical modelling as a tool.

    Instrumentation was also installed to determine the ongoing stability of excavations which allowed safe working conditions to be identified throughout the mine. The aim of each type of instrumentation method is presented along with the interpretation of the monitoring results. The practical implications of each set of results are discussed and a cost breakdown for all the instrumentation types is included. Instrumentation-Monitoring-at-an-Underground-Mine-to-Establish-Failure-Mechanisms-Confirm-Numerical-Modelling-and-Determine-Safe-Working-Conditions-S.MacGregor.pdf2.2 MB
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