• Subsidence Impacts on River Channels and Opportunities for Control - Ken Mills

    Subsidence associated with longwall mining is recognised to cause ground movements that generate horizontal compression at topographic low points; a process referred to as valley closure. River channels in the Southern Coalfield of N.S.W. are typically located directly on rock strata that has the potential to be impacted by valley closure. This paper describes the nature of the subsidence impacts that have been observed in river channels in the Southern Coalfield, techniques for monitoring these impacts, and a range of strategies for prevention, and control of the impacts.

    The Australian coal industry has supported ACARP research Project C12016 - Damage Criteria and Practical Solutions for Protecting Undermined River Channels. The results presented in this paper are based on the outcomes of this project. Subsidence-Impacts-on-River-Channels-and-Opportunities-for-Control-K.Mills.pdf893 KB
  • Observations of Ground Movements within the Overburden Strata above Longwall Panels and Implications for Groundwater Impacts - Ken Mills

    Longwall mining is recognised to cause disturbance to the overburden strata as the overburden strata moves downward into the void created by mining. These ground movements have been observed as surface subsidence over many decades and by numerous researchers through numerous surface and sub-surface monitoring programs, in a wide variety of different geological settings, using a wide variety of monitoring techniques. This monitoring provides an excellent database of experience from which to characterise the nature and extent of disturbance within the overburden strata above longwall panels. This characterisation is intended to provide a basis for better understanding the effects of longwall mining on the surrounding strata and, particularly in the context of groundwater interactions, the formulation of hydrogeological models used to predict groundwater impacts about longwall panels.

    The extend and nature of zones within the overburden are characterised in this paper on the basis of the level of disturbance and the nature of this disturbance. Zones characterised by tensile changes or stretching behaviour are found to be located directly above each panel with the level of disturbance above the mining horizon graduated as a function of panel width from the mining horizon through to about three times panel width above each individual longwall panel. These stretching zones and their influence on the hydraulic conductivity of the overburden strata contrast with zones of increased compression located directly above the chain pillars that separate individual longwall panels. Observations-of-Ground-Movements-within-the-Overburden-Strata-above-Longwall-Panels-and-Implications-for-Groundwater-Impacts-K.Mills.pdf1.5 MB
  • Mitigation of Subsidence Impacts at Marhynes Hole Rock Bar - Ken Mills

    Marhnyes Hole is a local swimming hole located on the Georges River about one kilometre to the north and east of Appin township. To allow Longwall 5A4 at West Cliff Colliery to mine under Marhnyes Hole in September 2002, BHP Illawarra COal undertook a range of mitigation and remediation activities aimed to preserve the integrity of the rock bar and the amenity of the adjacent rock pools. This paper describes the main component of mitigation strategy, a stress-relief slot designed to limit subsidence impacts on the structural integrity of the rock bar.

    The stress-relief slot was some 28.5m long, 18-20m deep and up to 150mm wide. It was constructed adjacent to Marhnyes Hole rock bar to limit the impact of subsidence compression on the structural and aesthetic integrity of the rock bar to a level that would allow it to be restored to its original condition once mining was complete.

    The slot was successful in achieving its intended aims. The structural integrity of Marhnyes Hole rock bar was protected sufficiently to allow it to be subsequently remediated. Most importantly for the BHP Illawarra Coal, West Cliff Colliery was able to mine Longwall 5A4 without interruption and adverse publicity was controlled. The cost of the slot construction was less than 3% of the estimated cost of steeping the longwall panel around the site with consequential interruptions to longwall continuity. Mitigation-of-Subsidence-Impacts-at-Marhynes-Hole-Rock-Bar-K.Mills.pdf1002 KB
  • Acoustic Scanner Analysis of Borehole Breakout to Define Stressfield Across Mine Sites in Sydney and Bowen Basins - Stuart MacGregor - Published 2002

    The role of horizontal stress, its orientation and magnitude, in defining the behaviour of strata in underground coal mines has been well established. Poor panel layouts have led to gate end stress concentrations, roof falls and lost production. The ability to define the horizontal stress regime over a mine site has historically been limited to point measurements, in part due to technology and cost. Recent advances in the application of geophysical tools, notably the acoustic scanner (borehole televiewer) have resulted in a new technique to conduct stress measurements. By quantifying the nature of borehole breakout and the mechanical properties of rocks in which they occur, this technique provides the ability to:
    • obtain a vastly greater number of measurements, both at different depths and spatial distribution, than other techniques such as overcoring or hydraulic fracturing
    • readily obtain depth versus stress relationships
    • define geotechnical domains on the basis of stress direction and in-situ stress magnitude for mine planning purposes

    This paper presents an overview of the technique and presents case histories in its application at a mine site in the Sydney Basin, Australia. Acoustic-Scanner-Analysis-of-Borehole-Breakout-to-Define-Stressfield-Across-Mine-Sites-in-Sydney-and-Bowen-Basins-S.MacGregor.pdf666 KB
  • Definition of Stress Regimes at Borehole Mine and Regional Scale in the Sydney Basin through Breakout Analysis - Stuart MacGregor - Published 2003

    The role of horizontal stress in affecting strata behaviour in underground coal mines has been well documented (Siddal and Gale1, Hebblewhite2, Mark3). In Australia, the nature and depth of the underground coal resources has resulted in high levels of horizontal stress, typically 2-3 times the vertical stress, and up to 9 times that expected by lithostatic burial. Horizontal stress impacts on all facets of strata behaviour, and is a fundamental input into the geotechnical design process.

    Borehole breakout analysis, particularly using high resolution acoustic scanner images, provides the ability to collect large data sets that have significant depth and spatial coverage. In real terms this provides the ability to investigate a range of stress phenomena at different scales, and assess the factors controlling in situ and mining induced stress regimes.

    This paper highlights a range of stress phenomena that have been observed through breakout analysis in the Sydney Basin and outlines the impact these have on underground mining operations. Definition-of-Stress-Regimes-at-Borehole-Mine-and-Regional-Scale-in-the-Sydney-Basin-through-Breakout-Analysis-S.MacGregor.pdf1.1 MB
  • Investigation into Abnormal Increased Subsidence above Longwall Panels at Tahmoor Colliery NSW - Winton Gale

    Tahmoor Colliery, located in the Southern Coalfield of NSW, has been in operation for over 30 years. Longwall extraction of 23 successive longwall panels has occurred with subsidence occurring within predictions and resultant impacts to natural and built environments occurring to expectations.

    Subsidence over a recent longwall panel of approximately twice that previously measured occurred at Tahmoor Colliery. An investigation of the potential causes was conducted using computer modelling together with hydrological characterisation and detailed geotechnical characterisation of the strata.

    The abnormal subsidence was found to be consistent with localised 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 not sufficiently induce the abnormal subsidence.

    The results have significant implications to subsidence prediction in areas which 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-Increased-Subsidence-above-Longwall-Panels-at-Tahmoor-Colliery-NSW-W.Gale.pdf2.4 MB
  • Successful Management Strategy for Mining Adjacent to a Sensitive Natural Feature - Ken Mills

    BHP Billiton Illawarra Coal operates Dendrobium Mine in an area 10-20 km west-northwest of Wollongong in NSW, Australia. The mine recently completed mining in Area 3A adjacent to an overhanging natural rock feature known as Sandy Creek Waterfall. Illawarra Coal undertook measures to protect the waterfall and the section of Sandy Creek immediately upstream of the waterfall from the effects of longwall mining using an innovative management process and an array of high resolution monitoring systems. This paper describes the management options that were considered and the processes that were adopted to successfully protect the waterfall from the valley closure effects of mining four adjacent longwall panels in close proximity to the waterfall while continuing to maximise recovery of the coal resource in the area.

    The management structure adopted involved a Technical Committee, a Steering Committee, and an external independent reviewer. The Technical Committee comprised senior representatives from Illawarra Coal, three external specialists in rock mechanics and subsidence, and a government observer. The Technical Committee was responsible for design of the monitoring systems, interpretation of the monitoring results, and the provision of recommendations to the Steering Committee suitable to guide decisions on when to cease mining each adjacent panel. The Steering Committee comprised Illawarra Coal management and technical personnel. Although the Steering Committee took advice from the Technical Committee, all decisions relating to mining were made by the Steering Committee. An external reviewer was engaged by the Steering Committee at the end of each longwall panel to review the results, interpretation and management decisions. This management structure was effective in successfully protecting the very sensitive structure of Sandy Creek Waterfall from potential impacts of nearby mining in the midst of active ongoing natural erosion processes. Successful-Management-Strategy-for-Mining-Adjacent-to-a-Sensitive-Natural-Feature-K.Mills.pdf2.2 MB
  • Developments in Understanding Subsidence with Improved Monitoring - Ken Mills

    Ground movements associated with coal mining have been occurring since coal mining was first practiced but the ability to interpret these movements and develop and understanding of the mechanics involved was initially limited by irregular mining geometries and the vagaries of pillar behaviour. The introduction of longwall mining to Australia with its regular geometries, full extraction, and single seam extraction has provided opportunities to eliminate many of the mining related variables that are present in pillar extraction operations and so provide a much more controlled environment in which to conduct measurements and develop understanding of the mechanics of overburden caving and subsidence processes. The understanding of these processes has developed as a result of improvements in surveying and monitoring techniques and the application of these techniques to satisfy the requirements of regulatory authorities in response to changing community expectations. This paper presents as overview of the developments in monitoring technique for characterising subsidence and sub-surface ground movements and the developments in understanding of subsidence related ground behaviour that have been possible as a result. Developments-in-Understanding-Subsidence-with-Improved-Monitoring-K.Mills.pdf690 KB
  • Experience of Monitoring Subsidence at Ulan Coal Mine - Ken Mills

    The Hunter Expressway Alliance has been commissioned by the Roads and Traffic Authority to design and construct a new dual carriageway motorway between the F3 at Minmi and Buchanan in the lower Hunter Valley. The route of the proposed motorway passes over an area where coal has been mined for over a century and further mining is proposed in the future. Subsidence movements associated with the sudden collapse of standing pillars and proposed future mining have potential to impact on some of the major bridge structures and sections of pavement. This paper presents an overview of the mining hazards identified and the various mitigation strategies that have been implemented to protect the project against these potential hazards. Experience-of-Monitoring-Subsidence-at-Ulan-Coal-Mine-K.Mills.pdf225 KB
  • Reinforcement Design of Cataract Water Tunnel to withstand Longwall Subsidence - Winton Gale

    This paper presents the procedures followed in the design of the reinforcement system for Cataract Tunnel. The Tunnel is a 19th century unlined tunnel under which a series of longwalls will be extracted at Appin Colliery. Based on the prediction of subsidence
    levels, three dimensional stress analyses were conducted to investigate the range of additional stresses which were likely to be induced around the tunnel. The impact of the induced stresses on the tunnel were then analysed in detail and a reinforcement system designed.

    It should be noted that, as in most geotechnical situations it was not possible to define all the relevant variables. Stress variations outside those used in this analysis are possible. In recognition of this and the uncertainty of the actual stress redistributions, the design has been used as the basis for a minimum support pattern. It is envisaged that this may be supplemented by additional reinforcement such as additional bolts or cable tendons as determined by a planned program of monitoring and observation. Reinforcement-Design-of-Cataract-Water-Tunnel-to-withstand-Longwall-Subsidence-W.Gale.pdf446 KB
  • The Effects of Mining Subsidence on Rockbars in the Waratah Rivulet at Metropolitan Colliery - Ken Mills

    Metropolitan Colliery is currently longwall mining below a section of Waratah Rivulet in the Woronora water supply catchment. The effects of mining subsidence on the quantity and quality of water and the ecological integrity of the rivulet are of interest to the Sydney Catchment Authority. The Colliery has undertaken a significant program of work aimed to assess the likely impact of mining on the rivulet. The program includes subsidence monitoring, instrumentation to monitor the effects of mining subsidence on rockbars, flow and water quality monitoring and monitoring of vegetation, aquatic ecology and archaeological sites.

    This paper describes the results of subsidence and rockbar monitoring up to the completion of Longwalls 9 and 10. The-Effects-of-Mining-Subsidence-on-Rockbars-in-the-Waratah-Rivulet-at-Metropolitan-Colliery-K.Mills.pdf3.9 MB
  • Pillar Design to Control Subsidence at Moonee Colliery - Ken Mills

    Moonee Colliery are longwall mining in the Great Northern seam at depths ranging from 90m to 170m. Surface infrastructure above the first four longwall panels includes the Pacific Highway and several residential and commercial properties.

    This paper describes the pillar design approach used to manage surface subsidence in the area. The approach is based on previous detailed subsidence and pillar monitoring in nearby Wallarah Colliery and measurements of subsidence throughout the Lake Macquarie area for a wide range of pillar sizes and overburden depths. Undermining the Pacific Highway requires consideration of not only the amount of subsidence but also the timing and nature of subsidence. Various options were considered and a design developed to control surface subsidence to acceptable levels. This paper summarises the results of previous monitoring and outlines the issues considered in the longwall panel design for subsidence control at Moonee Colliery. MSTS-Conference-1998-Pillar-Design-to-Control-Subsidence-at-Moonee-Colliery-K.Mills-1998.pdf271 KB
  • Prediction of strata caving characteristics and its impact on longwall operation - Winton Gale

    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. The joint 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. This work was validated by field measurements of caving and microseismic activity at the longwall face.

    The rock fracture distribution and the caving characteristics of a range of strata sections have been simulated by computer methods. Validation studies of the method were addressed together with case studies. The interaction of caving with support convergence and face control is presented. The method allows the simulation of longwall support behaviour 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. Prediction-of-strata-caving-characteristics-and-its-impact-on-longwall-operation-Winton-Gale-1998.pdf2.9 MB
  • Investigation into Temporary Roof Support Principles - Winton Gale

    An investigation was undertaken in regard to Temporary Roof Support (TRS) systems incorporated into miner bolters. Two and three dimensional modelling was utilised to document the effects a TRS system may have on the surrounding underground environment. TRS systems were found to have little if any effect on strata stability about a simulated roadway but are considered capable of providing protection against minor roof falls where the operator is within the defined protection zone. Each system requires the combination of mesh, bolts and support points to provide a protective zone to the operators. In difficult conditions there is no substitute for bolting as close to the face as possible and the concept of a zone of influence about a support point, as indicated by industry guidelines is not considered as a suitable protection zone. Investigation-into-Temporary-Roof-Support-Principles-W.Gale.pdf1.8 MB
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