• Deformability Modulus of Jointed Rocks, Limitation of Empirical Methods and Introducing a New Analytical Approach-Mahdi-Zoorabadi

    Deformability modulus of jointed rocks is a key parameter for stability analysis of underground structures by numerical modelling techniques. Intact rock strength, rock mass blockiness (shape and size of rock blocks), surface condition of discontinuities (shear strength of discontinuities) and confining stress level are the key parameters controlling deformability of jointed rocks. Considering cost and limitation of field measurements to determine deformability modulus, empirical equations which were mostly developed based on rock mass classifications are too common in practice. All well-known empirical formulations dismissed the impact of stress on deformability modulus. Therefore, these equations result in the same value for a rock at different stress fields.

    This paper discusses this issue in more detail and highlights shortcomings of existing formulations. Finally it presents an extension to analytical techniques to determine the deformability modulus of jointed rocks by a combination of the geometrical properties of discontinuities and elastic modulus of intact rock. In this extension, the effect of confining stress was incorporated in the formulation to improve its reliability. Deformability-Modulus-of-Jointed-Rocks-Limitation-of-Empirical-Methods-and-Introducing-a-New-Analytical-Approach-M.Zoorabadi-2016.pdf943 KB
  • In-Situ Stress Measurements and Stress Change Monitoring to Monitor Overburden Caving Behaviour and Hydraulic Fracture Pre-Conditioning - Jesse Puller, Ken Mills, Rob Jeffrey

    A coal mine in New South Wales is longwall mining 300 m wide panels at a depth of 160–180 m directly below a 16–20 m thick conglomerate strata. As part of a strategy to use hydraulic fracturing to manage
    potential windblast and periodic caving hazards associated with these conglomerate strata, the in-situ stresses in the conglomerate were measured using ANZI strain cells and the overcoring method of stress relief. Changes in stress associated with abutment loading and placement of hydraulic fractures were also measured using ANZI strain cells installed from the surface and from underground. Overcore stress measurements have indicated that the vertical stress is the lowest principal stress so that hydraulic fractures
    placed ahead of mining form horizontally and so provide effective pre-conditioning to promote caving of the conglomerate strata. Monitoring of stress changes in the overburden strata during longwall retreat was undertaken at two different locations at the mine. The monitoring indicated stress changes were evident 150 m ahead of the longwall face and abutment loading reached a maximum increase of about 7.5 MPa. The stresses ahead of mining change gradually with distance to the approaching longwall and in a direction consistent with the horizontal in-situ stresses. There was no evidence in the stress change monitoring results to indicate significant cyclical forward abutment loading ahead of the face. The forward abutment load determined from the stress change monitoring is consistent with the weight of overburden
    strata overhanging the goaf indicated by subsidence monitoring. In-Situ-Stress-Measurements-and-Stress-Change-Monitoring-to-Monitor-Overburden-Caving-Behaviour-and-Hydraulic-Fracture-Pre-Conditioning-Jesse-Puller-Ken-Mills-Rob-Jeffrey-2015.pdf1.8 MB
  • EXPERIENCE OF USING THE ANZI STRAIN CELL FOR STRESS CHANGE MONITORING - KEN MILLS, DAVE SELMO, JESSE PULLER, JIM SIMONOVKSI

    This Paper describes the ANZI (Australia, New Zealand Inflatable) strain cell and some examples of its application for stress change monitoring. The instrument has been used over the past three decades to measure three dimensional in situ stresses using the overcoring method of stress relief and monitor three dimensional stress changes in a range of applications mainly associated with underground coal mining, but also with civil and metalliferous mining projects.

    The ANZI strain cell has a pressuremeter design that allow 18 electrical resistance strain gauges at various orientations to be pressure bonded directly to the rock on a borehole wall. The instrument's soft polyurethane membrane and hollow pressuremeter design have characteristics that facilitate deployment, enhance data gathering, and simplify analysis. Further recent developments that improve deployment and monitoring have increased the capability of the instrument. Automatic, remote, and high speed monitoring at resolutions of just a few microstrain has significantly improved the capability to measure and thereby understand the nature of changes in the three dimensional stress in rock strata around excavations in rock. EXPERIENCE-OF-USING-THE-ANZI-STRAIN-CELL-FOR-STRESS-CHANGE-MONITORING-KEN-MILLS-DAVE-SELMO-JESSE-PULLER-JIM-SIMONOVKSI-2015.pdf1.1 MB
  • Analytical Procedure to Estimate the Horizontal Anisotropy of Hydraulic Conductivity in Coal Seams - Winton Gale - Mahdi Zoorabadi

    The horizontal hydraulic conductivity anisotropy of coal seams is a controlling parameter for designing gas drainage boreholes. The ratio between the maximum and minimum horizontal hydraulic conductivity (RkH-kh) and the orientation of maximum horizontal conductivity defines this anisotropy in horizontal plane.

    This paper presents a new analytical procedure based on the field stress data and geometrical properties of coal cleats to calculate these two parameters. The application of this procedure for a real case in Eastern of Australia resulted in an average ratio of 20.9 for RkH-kh and orientation of NE for maximum horizontal conductivity. The comparison between these results with the measured values validated the accuracy of proposed procedure to estimate the anisotropy of horizontal hydraulic conductivity of coal seams. Analytical-Procedure-to-Estimate-the-Horizontal-Anisotropy-of-Hydraulic-Conductivity-in-Coal-Seams-W.Gale-M.Zoorabadi-2015.pdf377 KB
  • A Combined 2D and 3D Modelling Approach to Provide Adequate Roof Support in Complex 3D Excavations - Yvette Heritage

    Traditional methods for assessing effective roof support can be difficult to apply to complex 3D excavations. Through worked examples, this paper illustrates the successful approach of combined 2D and 3D numerical modelling to understand the mechanisms of rock failure for unique excavation geometries. The modelling approach provides adequate roof support recommendations for complex 3D excavations in Australian coal mines. A-Combined-2D-and-3D-Modelling-Approach-to-Provide-Adequate-Roof-Support-in-Complex-3D-Excavations-Y.Heritage-2015.pdf2 MB
  • Monitoring of Ground Movements at Sandy Creek Waterfall and Implications for Understanding the Mechanics of Valley Closure Movements - Ken Mills

    BHP Billiton-Illawarra Coal operates Dendrobium Mine in an area 10-20km west-northwest of Wollongong in New South Wales, Australia. The mine recently completed mining the Wongawilli Seam in Area 3A adjacent to a natural rock overhang known as Sandy Creek Waterfall. Illawarra Coal undertook to protect the waterfall and the section of Sandy Creek immediately upstream of the waterfall from the effects of adjacent longwall mining using an innovative management process and an array of very high resolution monitoring systems. This paper describes the results of the high resolution monitoring systems and the implications of these results for general understanding of natural and mining induced ground movements around valleys.

    The program of monitoring conducted at Sandy Creek Waterfall measured closure, stress changes, microseismic activity and shear movements adjacent to the waterfall during mining of Longwalls 6, 7 and 8. These measurements provided insights into the mechanics of both mining induced valley closure and natural erosion processes. At the completion of Longwall 8, the monitoring strategy and the management decisions based on this monitoring have been effective in protecting the overhanging sandstone rock structure that forms Sandy Creek Waterfall and the upstream section of Sandy Creek, as required by the NSW Department of Planning and Infrastructure.

    The measurements and observations made at Sandy Creek Waterfall and the interpretation placed on these results are considered to provide a coherent understanding of the relatively complex deformation mechanics at this site. These mechanics are consistent with measurements and observations made at other sites. Monitoring-of-Ground-Movements-at-Sandy-Creek-Waterfall-and-Implications-for-Understanding-the-Mechanics-of-Valley-Closure-Movements-K.Mills-2014.pdf3.4 MB
  • Experience of Using the ANZI Strain Cell for Three Dimensional In Situ Stress Determinations in Deep Exploration Boreholes - Ken Mills - Jesse Puller

    This paper describes the Australia, New Zealand Inflatable (ANZI) strain cell, its operation, and recent development for overcoring in exploration boreholes. The ANZI strain cell is an instrument system that uses the overcoring method of stress relief to determine the three-dimensional, in-situ stresses in rock. The instrument has been used successfully for over three decades in numerous underground mining and civil projects, but technical advances over the last decade or so have allowed the system to be deployed in surface exploration boreholes to greater depths than was previously possible. Recent development of a downhole electronic data logger, a wireline-enabled drilling system, and an instrument deployment system has simplified the process of obtaining three-dimensional overcore measurements at depths approaching 1km to a single shift operation.

    The capability to deploy ANZI strain cells in surface exploration boreholes represents a significant breakthrough for the design of underground mines and underground excavations generally. Highconfidence characterisation of the in-situ stresses at the design stage provides the opportunity to design key infrastructure and mining systems to take advantage of the in-situ stress field from the outset before mining begins. Understanding the three-dimensional, insitu stress field not only provides a measure of the magnitude and direction of loads acting within the rock mass, it also provides insight into the mechanics of all the various processes driving ground deformations, including which geological fault structures are at limiting equilibrium. Experience-of-Using-the-ANZI-Strain-Cell-for-Three-Dimensional-In-Situ-Stress-Determinations-in-Deep-Exploration-Boreholes-2014.pdf2.6 MB
  • A Review of the Accuracy and Reliability of Empirical Subsidence Predictions - Ken Mills

    The prediction of subsidence effects resulting from the underground extraction of coal is undertaken prior to commencing mining operations in order to assess the likely consequences and impacts of subsidence on the natural and built environment above and in the vicinity of the mining operations. Often subsidence predictions are also undertaken for many alternative mine layouts before the appropriate layout is chosen. These subsidence predictions are used by the mine owners, consultants and stakeholders to manage the subsidence impacts on the natural and built features by providing a basis to:
    • develop appropriate management plans; and
    • assess whether the observed subsidence movements are developing as expected.

    With a continuing increase in the awareness of and the need to protect the natural environment, and with an increasing need to extract coal beneath the built environment, there has also been an increasing demand for greater detail in the provided predictions and assessments of the effects, consequences and impacts of underground mining on the natural and built features. With this increased demand for greater detail, there must also be an understanding of the background to, and the accuracy and reliability of, the subsidence predictions that are being used for the impact assessments on the natural and built features and for the management plans developed.

    This paper provides a discussion on the background to the commonly used empirical methods of subsidence prediction in NSW and provides an assessment of the accuracy of two commonly used empirical subsidence prediction methods, using monitored data from the Southern Coalfield of NSW. A-Review-of-the-Accuracy-and-Reliability-of-Empirical-Subsidence-Predictions-K.Mills-2014.pdf6.1 MB
  • ACG Deep Mining 2012 Stress Measurement Workshop - Ken Mills

    This paper describes the development of 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. ACG-Deep-Mining-2012-Stress-Measurement-Workshop-K.Mills-et-al-2012.pdf1.1 MB
  • Three Decades of Measuring In Situ Stresses and Monitoring Stress Changes with the ANZI Strain Cell - Ken Mills

    This 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-2012.pdf1.1 MB
  • Building New Bridges on the Hunter Expressway over Abandoned Coal Mines - Ken Mills

    The 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-2012.pdf5.3 MB
  • Subsidence and Mitigation Strategies Hunter Expressway - Ken Mills

    The Hunter Expressway Alliance has been commissioned by the Roads and Traffic Authority to design and construct a new dual carriageway motorway between the F3 at Minmi and Buchanan in the lower Hunter Valley. The route of the proposed motorway passes over an area where coal has been mined for over a century and further mining is proposed in the future. Subsidence movements associated with the sudden collapse of standing pillars and proposed future mining have potential to impact on some of the major bridge structures and sections of pavement. This paper presents an overview of the mining hazards identified and the various mitigation strategies that have been implemented to protect the project against these potential hazards.

    [Proceedings of the 8th Triennial Conference on Mine Subsidence - 2011] Subsidence-and-Mitigation-Strategies-Hunter-Expressway-K.Mills-2011.pdf1.4 MB
  • Investigation into Abnormal Surface Subsidence Above Longwall Panel Southern Coalfield - Winton Gale

    The subsidence over a longwall panel at Tahmoor Mine in the Southern Coalfield of NSW, Australia, was found to be approximately twice the size it had been in previous measurements. An investigation into the potential causes was conducted using computer modeling together with hydrological characterization and detailed geotechnical characterization of the strata.

    The abnormal subsidence was found to be consistent with localized 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 have sufficient impact to induce the abnormal subsidence.

    The results have significant implications to subsidence prediction in areas that 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-Surface-Subsidence-Above-LW-Panel-Southern-Coalfield-2011.pdf2.6 MB
  • Using Helium as a Tracer Gas to Measure Vertical Overburden Conductivity Above Extraction Panels - Yvette Heritage - Winton Gale

    This paper investigates helium injection into the goaf as a tool to measure goaf to surface connectivity. Laboratory studies confirmed a relationship between gas velocity and fracture conductivity through helium injection. Field trials of helium injection into the goaf were successfully conducted to determine whether a connection exists between the surface and the goaf. A repeatable technique of borehole helium injection, with a borehole drilled into the highly permeable caved zone of the goaf, proved to demonstrate more quickly whether a connection to the surface exists. Using-Helium-as-a-Tracer-Gas-to-Measure-Vertical-Overburden-Conductivity-Above-Extraction-Panels-Y.Heritage-W.Gale-2009.pdf281 KB
  • An Investigation into Underground Mine Interaction with Overlying Aquifers Huntly, East Mine, New Zealand - Winton Gale - Published 2006

    In 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
  • Water Inflow Issues above Longwall Panels - Winton Gale - published 2006

    The 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
  • Developing Methods for Placing Sand Propped Hydraulic Fractures for Gas Drainage in the Bulli Seam - Ken Mills - Published 2006

    BHP 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
  • Application of Computer Modelling in the Understanding of Caving and Induced Hydraulic Conudctivity About Longwall Panels - Winton Gale - Published 2005

    Computer 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
  • Development and Application of Strata Management in Coal Mines - Stuart MacGregor

    The continuing need to improve productivity and safety requires mine operators to both successfully manage the hazards associated with strata control whilst optimising mining practices. Recent experience in Australian coal and metalliferous mines has seen the introduction of legislation to ensure that adequate consideration is given to geotechnical design and strata control.

    This paper outlines a rational approach for the development of a Roadway Strata Management System that is based upon the systematic assessment of strata behaviour during all stages of a roadways use and describes its application by Strata Control
    Technology Pty. Ltd. at Ulan Coal Mines Limited. Development-and-Application-of-Strata-Management-in-Coal-Mines-S.MacGregor-2005.pdf519 KB
  • Determination of Load Transfer Characteristics of Gloved Resin Bolts from Laboratory and In-Situ Field Testing - Stuart MacGregor - Published 2005

    Resin based grouts are the main form of rock bolt anchorage in the underground coal industry in Australia and New Zealand. To be effective, the system requires the mixing of the catalyst and mastic components of the resin, as well as shredding of the laminate cartridge that contains the resin.

    An unknown measure is the load transfer characteristics of a bolt where the resin is well mixed but remains encased in the cartridge (gloved). Laboratory and in situ field investigations have been undertaken to quantify the performance loss due to mixed gloved bolts. This work showed repeatable results, indicating serious performance loss of the gloved and mixed system, with load transfer approximately 10-15% of a non-gloved system (MacGregor, 2004).

    The in situ testing has demonstrated the relationship between the adhesion qualities of the resin and the mechanical interlock generated by radial confinement with progressively increasing tensile load. Effective load transfer is defined by the ability of the system to sustain shear stress on the bolt hole wall. Determination-of-Load-Transfer-Characteristics-of-Gloved-Resin-Bolts-from-Laboratory-and-In-Situ-Field-Testing-S.MacGregor.pdf1.3 MB
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