SCT Operations (Strata Control Technology)
  • Ken Mills

Welcome to SCT's own publications library which contains a collection of recent publications and other resources with reliable research about our technology. 

  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • Growth Analysis and Fracture Mechanics Based on Measured Stress Change near Full Size Hydraulic Fracture - Rob Jeffrey - Ken Mills

    This paper describes the successful measurement of stress changes induced in a crystalline rock mass adjacent to a full size hydraulic fracture. A hydraulic fracture was initially formed using water and subsequently 2-D numerical models. The full three dimensional stress changes were measured using four ANZI stresscells installed and tested in situ prior to the start of hydraulic fracturing. The instruments were installed in pairs in two boreholes located some 7-8mm laterally and 17-20m above the injection point. The in situ stressfield at the site was such that the hydraulic fracture passed within approximately 5m of the instruments allowing the stress changes associated wit the passage of the fracture tip to be monitored as well as the stressed induced in the rock by the hydraulic fracture once the fracture tip was well past. The instruments were logged at 15 second intervals throughout the hydraulic fracture treatments to provide a time history of the complete three dimensional stress changes that occurred as each hydraulic fracture grew toward and then passed close to the instruments.

    The monitoring was undertaken as part of a larger project aimed at preconditioning a rock mass with multiple hydraulic fractures. The result of only two of the fractures are considered in this paper. Analysis of the other fractures is ongoing. Analysis of the stress change data provides information about the fracture rate and mode of growth orientation, and about the excess pressure acting inside the fracture to open it. Growth-Analysis-and-Fracture-Mechanics-Based-on-Measured-Stress-Change-near-Full-Size-Hydraulic-Fracture-R.Jeffrey-K.Mills.pdf370 KB
  • Mechanics of Horizontal Movements Associated with Coal Mine Subsidence in Sloping Terrain Deduced From Field Measurements - Ken Mills

    The ground movements associated with underground coal mining and, in particular, longwall mining, are recognised to include horizontal subsidence movements, but the mechanics of the processes that cause these horizontal movements are not well understood. Over the last two decades, three-dimensional subsidence monitoring has become routine in Australia and has provided a wealth of measurements of horizontal movements caused by mining subsidence. These measurements and other subsurface observations allow the processes that cause mining-induced horizontal movements to be inferred and, subsequently, verified. In this paper, the mechanics of the processes that cause horizontal movements, particularly those in sloping topography, are described and discussed on the basis of field observations.

    There are several processes recognised to generate horizontal subsidence movements. In flat terrain, systematic horizontal movements cause the surface to move initially toward the newly created goaf and, subsequently, in the direction of mining. Tectonic energy stored as horizontal stress is released by mining, and, when the horizontal stresses are high, the magnitude of this horizontal stress relief movement is large enough to be perceptible for some kilometres from the panel. In sloping terrain, there is an additional component of horizontal movement that occurs in a downslope direction. This movement, sometimes referred to as valley closure movement, has a magnitude that is typically much greater than systematic or stress relief movements. Mechanics-of-Horizontal-Movements-Associated-with-Coal-Mine-Subsidence-in-Sloping-Terrain-Deduced-From-Field-Measurements-K.Mills.pdf1.8 MB
  • Hydraulic Fracturing Applied to Inducing Longwall Coal Mine Goaf Falls - Rob Jeffrey - Ken Mills

    This paper describes the first successful use of hydraulic fracturing to induce a goaf event and control the timing of caving events in Australia. Hydraulic fractures are initiated at 7 to 10m above the bottom of a thick conglomerate roof and, because of the low vertical stress magnitude relative to the other two principal stressed, grow as horizontal fractures. The fractures extend radially outward from the injection borehole into the rock until a goaf fall occurs. Hydraulic fracturing has provided a means to control the timing of windblast events and thereby significantly improved safety. The successful implementation of hydraulic fracturing at Moonee Colliery to control the timing of goaf events has enabled the mine to continue operating. Hydraulic-Fracturing-Applied-to-Induce-Longwall-Coal-Mine-Goaf-Falls-R.Jeffrey-K.Mills.pdf240 KB
  • Impact of Vertical Stress on Roadway Conditions at Dartbrook Mine - Ken Mills

    A program of stress change and roadway deformation monitoring was undertaken to measure the forward abutment load distribution about a retreating longwall panel at Dartbrook Mine. The results of this monitoring allow roadway conditions observed at various stages of mining to be ranked according to the estimated vertical stress they have experienced . This ranking also provides a means to predict and visualise future roadway conditions at various stages of mining. This paper presents the results of the monitoring and describes the approach developed to predict future roadway conditions.

    The monitoring results show the vertical stress distribution associated with the front abutment from a longwall goaf decreases exponentially away from the goaf edge. The vertical stress distribution peaks at the goaf edge at 5 to 6 MPa stress increase and decreases to 1 MPa within 50m becoming imperceptible beyond 100m. By scaling this measured distribution to reflect the loading environment, the vertical stress can be estimated for different overburden depths and different stages of mining. Photograph of roadway conditions at particular vertical stress levels provide a way to visualise roadway conditions at various stages of mining in the future. Impact-of-Vertical-Stress-on-Roadway-Conditions-at-Dartbrook-Mine-K.Mills.pdf2.6 MB
  • Successful Application of Hydraulic Fracturing to Control Windblasts at Moonee Colliery - Ken Mills

    This paper describes the first successful use of hydraulic fracturing to induce caving events “on demand” in Australia. Moonee Colliery operate a longwall immediately below a thick conglomerate strata. This strata temporarily bridges across the extracted longwall panel to create a large area of standing goaf. When this standing goaf eventually collapses, the windblast generated presents a significant hazard to men working on and around the longwall face.

    Hydraulic fracturing has been successfully introduced to take control of the timing of these caving events so as to eliminate the risk of windblast injury. The longwall face area is completely evacuated during the treatment. Water is pumped into an injection point located in the conglomerate strata above the standing goaf.
    A horizontal fracture is generated and grows outward from the injection point, separating the conglomerate strata below the fracture horizon. At some point the strata can no longer span and a goaf fall is initiated. After a treatment, mining can be recommenced with the windblast hazard eliminated. Successful-Application-of-Hydraulic-Fracturing-to-Control-Windblasts-at-Moonee-Colliery-K.Mills.pdf221 KB
  • Remote Monitoring of Mine Subsidence Using Radar Interferometry - Ken Mills

    Radar interferometry has been increasingly used to generate digital terrain models and to map ground surface displacement. The technology has the potential to monitor subsidence movements in three dimensions over entire coalfields every few weeks. This paper describes some of the experience and challenges associated with using radar interferometry for three dimensional subsidence monitoring. In conventional differential radar interferometry (DInSAR), the ground surface displacement can be measured along the looking direction of the radar system.

    DInSAR results of the same area are required from at least three different looking directions to measure vertical and horizontal displacements in three dimensions. DInSAR results generated from data acquired by the European satellite ENVISAT at three different look angles have been used to develop displacement vectors of mining deformation in three dimensions. Interpretation of the ENVISAT results has been complicated by what is called phase unwrapping errors caused by the high displacement gradients at the edge of the subsidence zone. Results derived from data acquired by the new Japanese satellite ALOS is also used here to demonstrate how the high phase gradient problem can be eased by having the interferometric data with longer wavelength and finer imaging resolution. Remote-Monitoring-of-Mine-Subsidence-Using-Radar-Interferometry-K.Mills.pdf344 KB
  • Review of Mechanisms Resulting in Observed Upsidence and Closure Movements - Ken Mills

    Descriptions of natural valley bulging movements as well as mining induced valley related upsidence and closure movements have been well documented by various authors. It is clear from this research that there are numerous factors which affect these movements and it is likely that the movements comprise a number of mechanisms. Several models have been forwarded to describe valley related movements resulting from longwall mining. A review of some of these models is presented in this paper. This review indicates that several mechanisms may be in operation at any given site depending on the actual conditions at that site. An empirical method to predict these movements was developed as part of two ACARP research projects during 2000 and 2002. This method has proven to provide conservative upper bound predictions in the majority of cases. As the mechanisms and the factors which influence valley related movements are better defined, refinements in the method of prediction can be made so that the level of conservatism in the current method of prediction can be reduced. Review-of-Mechanisms-Resulting-in-Observed-Upsidence-and-Closure-Movements-K.Mills.pdf636 KB
  • 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
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