• STRESS MEASUREMENT
  • Impact of bedding plane and laminations on softening zone around the roadways - 3D Numerical Assessment - Mahdi Zoorabadi

    When the distributed rock stress around the roadways exceeds the strength of the rock, the rock is failed and a softening zone is formed. Roof deformation developed in the roof and ribs of the roadways are highly controlled by the depth of softening zones. The rock failure process starts from a point ahead of the face and grows into the roof, floor and ribs by advancing roadway. The
    maximum stress that can be transferred through the failed rocks would be equal to its residual confined strength. Therefore, rock stress is moved above failed zone and will create new failure zone if it is higher than the confined strength of rock at that depth. This process continues until the confined strength of the rock becomes higher than stress components. Bedding and lamination planes play a big role into the failure pathway of rocks around roadways. The thickness of softening zone is significantly influenced by the shear and tensile strength of bedding planes and laminations. This paper presents a 3D numerical assessment of the bedding and lamination planes impacts to the forming and extension of the softening zones. It highlights the requirements for better characterisation of bedding and lamination planes for reliable simulation of roadways. Impact-of-bedding-plane-and-laminations-on-softening-zone-around-the-roadways-3D-Numerical-Assessment-M.Zoorabadi-2017.pdf1.5 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
  • 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
  • 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
  • Computer Simulation of Ground Behaviour and Rock Bolt Interaction at Emerald Mine - Winton Gale - Published 2004

    A collaborative project between RAG Emerald Mine, NIOSH, and SCT Operations was conducted to investigate ground behaviour, reinforcement performance, and stress redistribution in a coal mine entry subjected to a severe horizontal stress concentration. Field measurements indicated that the stresses applied to the study site nearly doubled during longwall mining, resulting in roof deformations extending to a height of 4.8 m (16 ft) above the entry.

    This paper focuses on the computer simulation that was undertaken to provide more insight into the roof behaviour and rock bolt interaction during mining. The model’s input rock properties were derived from extensive laboratory testing, and the model itself simulated a broad range of failure mechanisms. The effects of different bolt patterns on roadway behaviour were evaluated. Comparison between the model results and the field measurements indicated that that the model effectively simulated the critical elements of the actual roadway’s behaviour. With the confidence gained, the model was used as a baseline for additional simulations that evaluated the expected performance of alternative roof support systems. The study will also provide a benchmark data set for future applications of numerical modelling to U.S. coal underground mining. Computer-Simulation-of-Ground-Behaviour-and-Rock-Bolt-Interaction-at-Emerald-Mine-Winton-Gale.pdf697 KB
  • 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
  • A review of Recent in situ Stress Measurements in United Kingdom Coal Measures Strata - Paul Cartwright - Published 1997

    The 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
  • In Situ Stress Measurement Using the ANZI Stress Cell - Ken Mills

    This paper describes the operation of the ANZI (Australia, New Zealand Inflatable) Stress cell. Laboratory and field measurements are used to illustrate the instrument's operation. The ANZI stress cell has a pressuremeter design that enables 18 electrical resistance strain gauges to be pressure bonded directly to the rock of a borehole wall. The strain gauges are monitored during overcoring to obtain stress relief strains.

    An up hole pressure test is undertaken prior to overcoring to obtain the elastic properties of the rock in situ and to confirm the correct operation of all the strain gauges. The elastic properties of the rock are also obtained after overcoring in a biaxial test. The ANZI stress cell is widely used for routine three dimensional stress measurement in underground coal operations in Australia. It is being increasingly used in the United Kingdom, China, Japan and Vietnam in coal mining, civil and hard rock applications. In-Situ-Stress-Measurment-using-the-ANZI-Stress-Cell-K.Mills.pdf1.2 MB
  • Statistical Analysis of Undeground Stress Measurements in Australian Coal Mines - Winton Gale - Ken Mills

    This paper presents a summary of 235 underground stress measurements conducted in the virgin ground of NSW and Queensland mines. The main objective of this study is to analyse the statistical information from the measurements that are relevant to strata control and mine planning with a view to estimate the risk involved with strata failure.

    Major findings include the statistical increase of maximum horizontal stress with depth in Queensland and NSW mines, a comparison of normalised lateral stress magnitudes and measurements in rock of a different stiffness, ‘Tectonic Factor’ concept, and maximum lateral stresses and their directions in NSW and Queensland coalfields. These findings can provide a valuable benchmark for mine planning and strata control with potential savings in mine operating costs. Statistical-Analysis-of-Underground-Stress-Measurements-in-Australian-Coal-Mines-W.Gale-K.Mills.pdf2 MB
  • Growth Analysis and Fracture Mechanics Based on Measured Stress Change near Full Size Hydraulic Fracture - Rob Jeffrey - Ken Mills

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

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

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

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

    The aim of this paper is to discuss the rock mechanics issues which can influence the strength of pillars in coal mines. The paper utilises stress change monitoring results, micro seismic monitoring results and computer modelling to assess the stress history about a chain pillar. The implications and fracture modes developed are discussed, with the outcome being that chain pillar strength can be significantly reduced by the stress path and changes in boundary conditions to the pillar when longwall extraction occurs. It is envisaged that this effect is contained in measured and empirical data bases, however it is important to recognise the stress path process when applying results to various site conditions and mine layouts. Stress-Conditions-and-Failure-Mechanics-Related-to-Coal-Pillar-Strength-W.Gale.pdf524 KB
  • Experience of Field Measurement and Computer Simulation Methods for Pillar Design - Winton Gale

    Coal 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 variation 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.

    Experience-of-Field-Measurement-and-Computer-Simulation-Methods-for-Pillar-Design-W.Gale.pdf513 KB
  • Geological Issues Relating to Coal Pillar Design - Winton Gale

    The 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.

    Geological-Issues-Relating-to-Coal-Pillar-Design-W.Gale.pdf240 KB
  • Performance of Roof Support Under High Stress in a US Coal Mine - Winton Gale

    The 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
  • Impact of Vertical Stress on Roadway Conditions at Dartbrook Mine - Ken Mills

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

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

    Longwall mining is recognised to cause disturbance to the overburden strata as the overburden strata moves downward into the void created by mining. These ground movements have been observed as surface subsidence over many decades and by numerous researchers through numerous surface and sub-surface monitoring programs, in a wide variety of different geological settings, using a wide variety of monitoring techniques. This monitoring provides an excellent database of experience from which to characterise the nature and extent of disturbance within the overburden strata above longwall panels. This characterisation is intended to provide a basis for better understanding the effects of longwall mining on the surrounding strata and, particularly in the context of groundwater interactions, the formulation of hydrogeological models used to predict groundwater impacts about longwall panels.

    The extend and nature of zones within the overburden are characterised in this paper on the basis of the level of disturbance and the nature of this disturbance. Zones characterised by tensile changes or stretching behaviour are found to be located directly above each panel with the level of disturbance above the mining horizon graduated as a function of panel width from the mining horizon through to about three times panel width above each individual longwall panel. These stretching zones and their influence on the hydraulic conductivity of the overburden strata contrast with zones of increased compression located directly above the chain pillars that separate individual longwall panels. Observations-of-Ground-Movements-within-the-Overburden-Strata-above-Longwall-Panels-and-Implications-for-Groundwater-Impacts-K.Mills.pdf1.5 MB
  • Acoustic Scanner Analysis of Borehole Breakout to Define Stressfield Across Mine Sites in Sydney and Bowen Basins - Stuart MacGregor - Published 2002

    The 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 2003

    The 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
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