• Methods of Interpreting Ground Stress Based on Underground Stress Measurements and Numerical Modelling - Winton Gale

    This paper presents several new methods to help interpretation and understanding of ground stress. The methods are based on data from 239 stress measurements conducted in the virgin ground in NSW and Queensland mines and computational models simulating large scale faulted ground behaviour. The underground stress regime plays an important role in mining profitability and safety however, understanding of the stress tensor is often difficult due to its mathematical complexities and non-intuitive behaviour.

    The aim of this study is to explain stress distribution in faulted ground, its origin and propose several methods of stress interpretation. Major findings presented in this study include: increase of maximum horizontal stress with depth based on underground measurements and numerical simulation of faulted ground, affect of faults on ground stress, normalisation technique that allows comparison of lateral stress magnitudes in rock of different stiffness, ‘Strain Tectonic Factor’ concept and its value in understanding stress components and its affect on rock strength. Methods-of-Interpreting-Ground-Stress-Based-on-Underground-Stress-Measurements-and-Numerical-Modelling-W.Gale.pdf429 KB
  • Remote High Resolution Stress Change Monitoring of Hydraulic Fractures - Ken Mills - Rob Jeffrey

    This paper describes the use of strain gauge based borehole instruments to monitor stress changes associated with the creation and extension of hydraulic fractures in massive rock strata at Northparkes Mine in Australia and Salvador Mine in Chile. This work was conducted as part of the International Caving Study ICSII.

    These instruments proved very sensitive to the stress changes induced by the hydraulic fractures close to the fracture plane. Analysis of the stress changes observed allowed the fracture orientation and non-symmetric fracture growth to be constrained sufficiently that a clearer insight into fracture behaviour could be obtained at both sites, particularly when combined with other observations. Recognition of the elastic stress reorientation about an opening mode hydraulic fracture has proved to be an important element in the interpretation of stress change monitoring data.

    The nature of the stress reorientation is useful in discriminating between opening and shearing mode fracture growth. A technique of identifying a range of possible solutions of fracture orientation and non-symmetric fracture growth consistent with the stress changes observed on multiple instruments has been developed. Unique definition of fracture orientation from the stress change instruments is possible if the instruments are sufficiently distributed relative to the hydraulic fracture plane. Remote-High-Resolution-Stress-Change-Monitoring-of-Hydraulic-Fractures-K.Mills-R.Jeffrey.pdf1 MB
  • Successful Use of a Stress Relief Roadway at Appin Colliery - Winton Gale

    High horizontal stress levels can lead to extensive roadway deformation requiring expensive secondary support to ensure stability; this is particularly the case with longwall installation faces. Longwall installation roadways are a critical construction within coal mines. The use of a purpose built ‘Stress Relief Roadway’ to minimise roof deformation in the nearby longwall installation roadway, by reducing stress impacts has been undertaken at Appin Colliery – BHP Billiton Illawarra Coal. Its use led to significant cost and operational benefits. This paper outlines the process used; from identifying horizontal stress as an issue, as well as generating computer models through the various options and culminating in ground monitoring of the constructed roadways to the successful start of the longwall panel. Successful-Use-of-a-Stress-Relief-Roadway-at-Appin-Colliery-W.Gale.pdf581 KB
  • Impact of Longwall Width on Overburden Behaviour - Ken Mills

    The longwall panels at Clarence Colliery have experienced intermittent sudden weightings on the face that have caused some production delays. These weightings have typically been more severe on the wider faces. A program of surface subsidence and extensometer monitoring was undertaken above Longwalls 4 and 5 to investigate the behaviour of the overburden strata during longwall extraction on two faces of different widths.

    The monitoring indicated that a dome shaped zone of large downward movement extends up into the overburden strata to a height equal to about the panel width. A major strata unit between 50 m and 70 m above the coal seam influences the behaviour of the overburden strata and may be a factor in the observed sudden loading of longwall face supporLo;. Downward movement of this major unit appears to concentrate on vertical fractures. Increased loading on the face supports could then be expected. The downward movement of this major unit appears to be more significant in the overburden behaviour above the 200 m wide longwall compared to the 160 m wide longwall face Impact-of-longwall-width-on-overburden-behaviour-K.Mills.pdf1.8 MB
  • Review and Estimation of the Hydraulic Conductivity of the Overburden Above Longwall Panels. Experience from Australia - Winton Gale

    The aim of this paper is to summarise the results and conclusions of Australian Coal Association Research Project (ACARP) Report C13013 which relate to water inflows into a mine which occur through the overburden above and adjacent to longwall panels. The study assessed available data of inflows into underground coal mines and utilised computer simulation of water flow through fracture networks.

    The study concluded that flow into mines is typically via an interconnected network of pre existing and mining induced fractures. The height above the coal seam that mining induced fractures extend is typically related to the width of the panel. However the potential for those fractures to form a connected network which can facilitate flow, is related to the amount of subsidence and the depth of mining. The study compares model simulations with measured data and provides guidelines to estimate the average hydraulic conductivity of the overburden above extracted longwall panels in Australia. Review-and-Estimation-of-the-Hydraulic-Conductivity-of-the-Overburden-Above-Longwall-Panels.-Experience-from-Australia-W.Gale.pdf952 KB
  • The Application of Field and Computer Methods for Pillar Design in Weak Ground - Winton Gale

    This paper describes the use of strain gauge based borehole instruments to monitor stress changes associated with the creation and extension of hydraulic fractures in massive rock strata at Northparkes Mine in Australia and Salvador Mine in Chile.

    This work was conducted as part of the International Caving Study ICSII. These instruments proved very sensitive to the stress changes induced by the hydraulic fractures close to the fracture plane. Analysis of the stress changes observed allowed the fracture orientation and non-symmetric fracture growth to be constrained sufficiently that a clearer insight into fracture behaviour could be obtained at both sites, particularly when combined with other observations. Recognition of the elastic stress reorientation about an opening mode hydraulic fracture has proved to be an important element in the interpretation of stress change monitoring data.

    The nature of the stress reorientation is useful in discriminating between opening and shearing mode fracture growth. A technique of identifying a range of possible solutions of fracture orientation and non-symmetric fracture growth consistent with the stress changes observed on multiple instruments has been developed. Unique definition of fracture orientation from the stress change instruments is possible if the instruments are sufficiently distributed relative to the hydraulic fracture plane. The-Application-of-Field-and-Computer-Methods-for-Pillar-Design-in-Weak-Ground-W.Gale.pdf2.4 MB
  • Experience in the Application of Computer Modelling to Coal Mine Roadway Design in Weak Rock - Winton Gale

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    The retreating longwall method using single entry gateroads is the predominant coal mining method in Chinese underground mines. Leaving only a very small pillar between adjacent panels has become the normal longwall mining practice.

    It has been found that, rock bolting can improve the roadway conditions, however the general stability against roadway collapse will be determined by the adjacent goaf fracture geometry and chain pillar strength. With the increasing percentage of gateroads supported solely by rock bolting, there is now a key requirement to reexamine chain pillar design issues and the implications for gateroad support practice in China.

    This paper describes the current industry practices and experiences with chain pillar design in China and attempts to define some of the chain pillar design implications for current Chinese practice. Considerations for future design of chain pillars with regard to roadway support practices and overall gateroad stability against major roadway collapses induced by re-mobilisation of adjacent goaf are provided. The-Implications-of-Chain-Pillar-Geometries-for-Chinese-Coal-Mines-W.Gale.pdf1 MB
  • Investigation Into the Extent and Mechanisms of Gloving and Un mixed Resin in Fully Encapsulated Roof Bolts - Stuart MacGregor

    Effective strata control, utilising fully encapsulated roof bolts is dependent on the installed quality of the reinforcement elements. One mechanism by which roof bolts may become less than fully efficient is by glove fingering (gloving) and un-mixing of the resin. Following a routine installed bolt quality audit and some small roof failures containing gloved bolts, a work programme was initiated to determine the extent of the gloving and un-mixing problem and to develop an understanding of mechanisms involved. Results have shown that gloving and un-mixing is a systematic and widespread pheno mena, occurring across the range of resin and/or bolt manufacturers, and in a variety of roof types. Gloving was found in bolts installed using either hand held pneumatic or continuous miner mounted hydraulic bolting rigs, under run of mine (ROM) condition s by operators, and under controlled manufactures “best practice” conditions.

    The mechanisms involved have been confirmed as being the development of a pressure front as the bolt encounters the resin cartridge and is spun up the hole, which in turn, leads to over -pressurisation and radial expansion of the resin cartridge. The result is an increase in the diameter of the plastic cartridge. Allowing the bolt to be spun up inside the cartridge without making sufficient contact to shred the cartridge or the hardener envelope, typically resulting in a portion of the cartridge enveloping the bolt and unmixed resin mastic and catalyst.

    Once the mechanisms involved and extent of the problem became clear, further research was undertaken to assess alternative bolt profiles and modifications in an effort to minimise and/or eliminate the gloving and unmixing phenomenon. Research has been undertaken using recovered bolts from various mine sites, as well as test bench trials and the quantification of the loading characteristics of gloved bolts using strain gauge roof bolts.

    To understand the impacts of gloved and un-mixed bolts on roof control, failure pathways and reinforcement requirements a FLAC 2D numerical simulation was undertaken, with the results being incorporated into the strata management plan for a particular operation. Laboratory data has been collected and analysed to assess magnitudes of resin pressure as the bolt encounters the cartridge1 and the effects of gloving and un-mixing on the load transfer characteristics of the resin bolt system. Investigation-Into-the-Extent-and-Mechanisms-of-Gloving-and-Un-mixed-Resin-in-Fully-Encapsulated-Roof-Bolts-S.MacGregor.pdf502 KB
  • Sand Propped Hydraulic Fracture Stimulation of Horizontal In seam Gas Drainage Holes at Dartbrook Coal Mine - Rob Jeffrey

    Longwalls 107, 108 and 109 at Dartbrook Coal Mine contained coal with a high gas content and low permeability. Horizontal in-seam drain holes were found to have low gas production rates compared with drainage rates in previous panels. Hydraulic fracture stimulations, using water and sand, were therefore carried out in three boreholes in Longwalls 109 and 108 at Dartbrook to assess the effectiveness of sand propped fractures in stimulating gas drainage from in-seam boreholes. Boreholes 108-10-10 and 108 -7-1 were stimulated with 20 and 10 fractures respectively and, on average, 100 kg of sand was placed into each fracture. The fractures placed into LW 109 were to be mined and mapped, but operational constraints precluded mapping of these fractures.

    The stimulations produced a significant increase in gas drainage rates from the two boreholes. Hole 108-10-10, which ran perpendicular to the major joint system in the seam, increased its early gas rate by a factor of about 180 while hole 108-7-1, which was drilled parallel to the joint set, increased its rate by about 22 compared to pre-stimulation rates. The stimulated gas rates continuously increased for several weeks and the higher rates were sustained for the entire period the holes were monitored. Based on the higher stimulation effect achieved in hole 108-10-10 (drilled perpendicular to the jointing) compared with hole 108-7-1 (drilled parallel to the jointing), target drainage holes drilled perpendicular (northsouth) to the jointing are better stimulation candidates.

    Fracture modeling suggests the sand proppant bank may extend to 15m from the borehole. The unpropped portion of the fracture may extend to more than 40m. A purpose-built fracturing system was developed and used at Dartbrook to stimulate holes that covered most of LW109. This full-scale enhancement of gas drainage was successful and allowed efficient mining of that panel.

    [Coal Operators' Conference, University of Wollongong & the Australiasian Institute of Mining and Metallurgy, 2004] Sand-Propped-Hydraulic-Fracture-Stimulation-of-Horizontal-In-seam-Gas-Drainage-Holes-at-Dartbrook-Coal-Mine-R.Jeffrey-2004.pdf265 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
  • Mapping Techniques for Determining Sandstone Roof Channel Paleodrainage Direction in Coal Mines - Luc Daigle

    Sandstone channels are constant hazards to coal mining operations, the presence of a channel over a seam can result in thinning of the seam, changes in stress orientations, variation in stress intensity, abrasive cutting, roof hangup, faulting, jointing, loading of the face and even wind blasts. Prediction of the trend of a channel is usually difficult as channels tend to meander and follow complex orientations. However, geological mapping of development roadways can provide very good information on where to expect channels.

    Mapping paleodrainage indicators at Yancoals’ Ashton Coal Mine in the Hunter valley demonstrates channel trends can be projected through analysis of these indicators. Recognising and recording oriented indicators on hazard plans can assist in forewarning where channels may be anticipated. The experience at the Ashton Coal Mine shows how recording these features provide a means of predicting channel trends. Drainage pattern interpretation of the sandstone channels was possible due to excellent exposure of coarse grain channel sediment, levee, and overbank facies deposition. Comparison of the plotted drainage trends with paleodrainage direction indicated by oriented plant debris demonstrates direct correlation, using paleodrainage indicators in the absence of an exposure of the channel base can be used to predict the trend of the channel at a specific location. Mapping-Techniques-for-Determining-Sandstone-Roof-Channel-Paleodrainage-Direction-in-Coal-Mines-L.Daigle.pdf1.2 MB
  • Rock Damage Characterisation from Microseismic Monitoring - Winton Gale

    This paper outlines the concepts used to correlate rock failure with microseismic events and presents examples of microseismic monitoring together with associated computer modelling of the rock failure. This study is motivated by the need to develop improved ways to reduce ground control hazards in underground mining. Toward this end we present and compare results from numerical modelling and microseismic monitoring studies conducted at several different mine sites. Emphasis is on integrating results obtained with these tools to characterize, and thus increase our understanding of, important mine deformation processes. The ultimate goal is to use this knowledge to design mine structures, and develop mitigation
    measures, that minimize specific ground control hazards. Rock-Damage-Characterisation-from-Microseismic-Monitoring-W.Gale.pdf700 KB
  • Hydraulic Fracturing Applied to Stimulation of Gas Drainage From Coal - Rob Jeffrey

    Hydraulic fracturing is routinely applied to stimulation of oil, gas, and coalbed methane wells around the world. The stimulation effect is achieved in coal seams as in other reservoirs, by producing a conductive fracture, connecting the well to the coal reservoir. The conductivity of the fracture is usually maintained by placing a round and sieved sand proppant in the fracture channel. The proppant prevents the fracture faces from closing back completely on one another after the treatment. The design of the fracture treatment, therefore, centers on selecting fluids, injection rates, and slurry concentrations that will produce the desired propped fracture channel. Hydraulic-Fracturing-Applied-to-Stimulation-of-Gas-Drainage-From-Coal-R.Jeffrey.pdf501 KB
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