Coal Pillar Design Issues in Longwall Mining - Winton Gale - Published 1998Coal 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
Connectivity of Mining Induced Fractures Below Longwall Panels A Modelling Approach - Winton Gale - Yvette Heritage - Adrian Rippon - Published 2017Gas make into active longwall panels is an important issue in ventilation and gas drainage design. A method of simulating the mining induced fracture network and associated increase in hydraulic conductivity is a necessity for improved mine design, hazard management planning and gas drainage efficiency. This paper identifies and illustrates the key components in determining the connectivity of lower gas sources to an active goaf. Computer modelling identifies the formation of cyclic fractures that form below the longwall face and extend down back below the goaf. These cyclic fractures form when the stress conditions are high enough and the strata properties allow for shear failure to extend down through the strata.
The mining induced fracture formation and stress redistribution creates increased hydraulic conductivity of the floor strata below the active goaf. The stress redistribution and fracture volume also reduce the pore pressure below the goaf, allowing gas desorption to occur from lower seams. The combination of gas desorption and increased hydraulic conductivity allows gas connectivity from gas sources below the seam to the active goaf. A monitoring program at a NSW mine as part of ACARP Project C23009 allowed for preliminary validation of the concepts illustrated from the computer modelling. Preliminary field gas flow measurements are within the range of connectivity expectations based on rock failure modelling of longwall extraction. This report presents the first validation results for the modelling approach presented in this paper. Further results from ACARP Project C23009 on optimisation of gas drainage will follow in future publications. Connectivity-of-mining-induced-fractures-below-longwall-panels-A-Modelling-Approach-W.Gale-Y.Heritage-A.Rippon.pdf1.3 MB
Applications of Hydraulic Fracturing to Control Caving Events in Coal Mines - The Moonee Experience - Ken Mills - Published 2002Hydraulic 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.pdf1.1 MB
An Investigation into Underground Mine Interaction with Overlying Aquifers Huntly, East Mine, New Zealand - Winton Gale - Published 2006In recent years, Huntly East Mine has operated at a depth range of approximately 100 m to 220 m below a Quaternary aged clay, sand and silt aquifer that is connected to a nearby large river system (Waikato River). A key issue for mine planning and environmental management has been the development of mine design criteria to allow efficient mining of the reserves and to maintain the integrity of the aquifer.
A case study and back analysis at Huntly East Mine is presented, which investigates the overburden conductivity and the impacts caused by mining-induced caving. The case study includes: i. computer modelling of the mine geometry, caving and overburden fracture networks created; ii. field investigation to develop an engineering geological model of the overburden within the goaf to validate the goaf geometry as defined by the computer generated model; iii. in situ field measurement of overburden conductivity in the pre- and post-mining condition; iv. interference testing across the goaf to determine the level of interconnectivity; and v. measured water pressure profiles above the mine. An-Investigation-into-Underground-Mine-Interaction-with-Overlying-Aquifers-Huntly-East-Mine-New-Zealand-W.Gale.pdf822 KB
Application of Computer Modelling in the Understanding of Caving and Induced Hydraulic Conudctivity About Longwall Panels - Winton Gale - Published 2005Computer modelling is being used to simulate rock fracture, caving and stress redistribution about longwall panels with increasing confidence. The models are being assessed against field monitoring and have significantly increased the understanding of caving mechanics within the overburden. This paper discusses the modelling approach and provides some examples of its application to overburden damage and induced hydraulic conductivity. Computer models used in this study simulate the fracture process in the geological units throughout the overburden. Analysis of the mining induced fracture patterns and in situ joint patterns allows an estimation of the hydraulic conductivity within the overburden. The cubic flow relationship has been used in the examples presented. Application-of-Computer-Modelling-in-the-Understanding-of-Caving-and-Induced-Hydraulic-Conudctivity-About-Longwall-Panels-W.Gale.pdf271 KB
Combining Modern Assessment Methods to Improve Understanding of Longwall Geomechanics - Winton Gale - Published 1998Ongoing, 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
Developing Methods for Placing Sand Propped Hydraulic Fractures for Gas Drainage in the Bulli Seam - Ken Mills - Published 2006BHP Billiton Illawarra Coal is seeking ways to significantly increase gas capture rates from in seam drilling programs in its underground coal mining operations. Hydraulic Fracture Technology (HFT), a joint venture between SCT Operations Pty Ltd and CSIRO Petroleum, is working with Illawarra Coal to develop the capability to enhance gas drainage rates in the Bulli Seam using sand-propped hydraulic fracturing based on HFT’s experience at Dartbrook Mine where gas drainage rates were increased by 5 to 180 times. One of the principal challenges for implementing sand-propped hydraulic fracturing in the Bulli Seam is the high vertical stresses that cause borehole breakout in horizontal holes drilled in coal. Borehole breakout effectively precludes the use of open hole straddle packers which are a convenient tool for placing multiple sand-propped hydraulic fractures in in-seam holes.
Results of an initial six week trial undertaken at Douglas Project pit-bottom are described, which is aimed to developing the capability to install, grout and perforate casing so that straddle packers can be used for sand-propped hydraulic fracturing in overstressed boreholes. The primary goals of the pitbottom trial at Douglas were to confirm that horizontal boreholes in Bulli coal at 500 m overburden depth are overstressed and unsuitable for use of open hole straddle packers, and to establish a method for installing, cementing and slotting casing so that straddle packers can be used to place hydraulic fractures. Both these goals were successfully achieved. Developing-Methods-for-Placing-Sand-Propped-Hydraulic-Fractures-for-Gas-Drainage-in-the-Bulli-Seam-K.Mills.pdf738 KB
Statistical Analysis of Undeground Stress Measurements in Australian Coal Mines - Winton Gale - Ken MillsThis 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 MillsDartbrook 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 GaleThis 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 JeffreyThis 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 GaleHigh 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
Water Inflow Issues above Longwall Panels - Winton Gale - published 2006The aim of this paper is to discuss the issues which relate to surface water inflow through the fractured overburden above longwall panels. The information used is a combination of field experience and computer modeling. Computer models used in this study simulate the fracture process in the geological units throughout the overburden. Analysis of the mining induced fracture patterns and in situ joint patterns allows an estimation of the hydraulic conductivity within the overburden. The cubic flow relationship has been used in examples presented. Water-Inflow-Issues-above-Longwall-Panels-W.Gale.pdf267 KB
Impact of Longwall Width on Overburden Behaviour - Ken MillsThe 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 GaleThe 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 GaleThis 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
A review of Recent in situ Stress Measurements in United Kingdom Coal Measures Strata - Paul Cartwright - Published 1997The 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
Three Decades of Measuring In Situ Stresses and Monitoring Stress Changes with the ANZI Strain Cell - Ken MillsThis paper describes the development of the ANZI (Australia, New Zealand Inflatable) strain cell over the past three decades and the operation of the instrument including some examples of its application. The ANZI strain cell is used for measuring strain changes in rock on borehole walls suitable for estimating in situ stresses and stress changes. The instrument comprises a pressuremeter design that allows electrical resistance strain gauges to be pressure bonded directly to the rock on a borehole wall. The strain gauges are monitored during overcoring to obtain stress relief strains for estimation of the in situ stress. In monitoring applications, strain changes within a rock mass induced by mining and other construction activities are measured over time.
The instrument's soft polyurethane membrane and hollow pressuremeter design have a number of characteristics that facilitate deployment, enhance data gathering, and simplify analysis. The membrane is soft enough to be ignored in any analysis and yet stiff enough to hold together even highly jointed rocks during overcoring. The pressuremeter design allows a pressure test to be conducted in situ after the instrument has been installed to confirm the correct operation of all the strain gauges, obtain an indication of the elastic properties of the rock in situ, and, in some circumstances, determine the direction of the in situ stresses acting across the borehole. The elastic properties of the rock are also obtained in a biaxial test conducted after overcoring from core collected from the pilot hole at the location of the instrument. Variations in the elastic modulus obtained during these various tests provide insight into the rock behaviour.
Recent developments in custom logging hardware have significantly improved the data density and the resolution of the strains able to be measured. For overcoring, strain changes are able to be recorded onto a laptop computer, processed, and displayed in real time during testing and overcoring. For monitoring, remote loggers are able to be deployed below ground at the borehole collar to take readings at intervals from a few minutes to a few days and remain unattended for six months or more. Three-Decades-of-Measuring-In-Situ-Stresses-and-Monitoring-Stress-Changes-with-the-ANZI-Strain-Cell-K.Mills.pdf1.1 MB
Building New Bridges on the Hunter Expressway over Abandoned Coal Mines - Ken Mills - Published 2012The industrial development of our societies over the past century used coal as the main source of energy which was mined from deep underground seams leaving voids below the ground surface. New urban development and transportation networks planned to meet the demand of future generations require roads and bridges to be built over these historical mining areas. Substantial mine related ground movement has been observed at the surface level above mining areas where standing pillars have become destabilised as a result of rising water levels within the mine, earthquakes, adjacent mining activity and the effect of sustained load in its supports. Building new bridges in such areas is a challenge for engineers.
This paper discusses the details of three prestressed concrete segmental balanced cantilever bridges having a combined total length of 850m with internal span lengths of 75m and pier heights varying up to 30m above the ground. Up to 500mm vertical and 450mm horizontal ground movements are predicted in the event of pillar instability in bord and pillar workings in the seams ranging from 65m to 170m below the surface. Ground treatment works have been undertaken to reduce the predicted vertical movements to 25mm while horizontal movements of 450mm remain to be considered in the design. The bridges have been articulated to accommodate the predicted ground movements due to mining subsidence. Various types of foundation have been adopted. Building-new-Bridges-on-Hunter-Expressway-over-Abandoned-Coal-Mines-K.Mills.pdf5.3 MB
A Method of Determining Longwall Abutment Load Distributions for Roadway and Pillar Design - Ken Mills - Published 2001This 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.pdf1.2 MB