Geological Issues Relating to Coal Pillar Design - Winton GalePublished Sep, 2017The strength characteristics of coal pillars have been studied by many workers and the subject is well discussed in the literature (for example. Salamon and Monro, 1967; Wilson, 1972: Hustrulid, 1976). A range of strength relationships have been derived from four main sources:
- Laboratory Strength measurements on different-sized coal block specimens;
- Empirical relationships from observations of failed and unfailed pillars;
- A theoretical fit of statistical data and observations; and
- Theoretical extrapolation of the vertical stress buildup from the ribside toward the pillar centre, to define the load capacity of a pillar.
Hydraulic Fracturing Applied to Inducing Longwall Coal Mine Goaf Falls - Rob Jeffrey - Ken MillsPublished Sep, 2017This 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
Performance of Roof Support Under High Stress in a US Coal Mine - Winton GalePublished Sep, 2017The National Institute for Occupational Safety and Health's (NIOSH) Pittsburgh Research Laboratory (PRL), RAG Pennsylvania and Strata Control Technologies of Australia have collaborated to conduct an extensive study of roof bolt strata interaction at the Emerald mine in Southwestern Pennsylvania. The primary goal of the project was to obtain detailed data on the interaction between the mine roof and the support elements for use in modeling studies. The study site was a longwall tailgate subjected to high horizontal stress. Three arrays of instruments were installed at the site, one in the tailgate entry and two in an adjacent crosscut. Pumpable concrete cribs were present in the tailgate array, and cable bolts were installed in one of the crosscut arrays. The instruments included mechanical and sonic extensometers for measuring roof movement, instrumented roof bolts, and three-dimensional roof stress cells. The study was ultimately successful in determining the magnitude of the horizontal stress concentration, the height of roof failure and the roof failure sequence, and the loading history of the primary roof supports. Performance-of-Roof-Support-Under-High-Stress-in-a-US-Coal-Mine-W.Gale.pdf175 KB
Rock Fracture Caving and Interaction of Face Supports Under Different Geological Environments. Experience from Australian Coal Mines - Winton GalePublished Oct, 2017This paper is presents a summary of recent investigations into fracture and caving about longwall panels. The results of these investigations indicate that rock failure initiates well ahead of the longwall face. Rock fracture typically forms in response failure through the material and bedding planes. Tensile fractures also form in massive units. These fracture patterns typically create a fracture network which determines the caving characteristics encountered at the faceline. The action of longwall face supports under such conditions is to maintain confinement to the fractured ground and develop a consistent caving line. The confinement developed above the canopy under these conditions can be variable on a shear by shear basis and the operational face support procedures play an important role in stability about the face area. Rock-Fracture-Caving-and-Interaction-of-Face-Supports-Under-Different-Geological-Environments.-Experience-from-Australian-Coal-Mines-W-Gale.pdf2 MB
Impact of Vertical Stress on Roadway Conditions at Dartbrook Mine - Ken MillsPublished Aug, 2000A 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 MillsPublished Oct, 2017This 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 MillsPublished Oct, 2017Radar 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 MillsPublished Oct, 2017Descriptions 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 MillsPublished Oct, 2017Subsidence 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 MillsPublished Oct, 2017Longwall 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 MillsPublished Oct, 2017Marhnyes 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 2002Published Feb, 2002The 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 2003Published Feb, 2003The 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 GalePublished Oct, 2017Tahmoor 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 MillsPublished Oct, 2017BHP 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 MillsPublished Nov, 2017Ground 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 MillsPublished Nov, 2017The 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 GalePublished Nov, 2017This 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 MillsPublished Nov, 2017Metropolitan 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 MillsPublished Jul, 1998Moonee 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