A Review of the Accuracy and Reliability of Empirical Subsidence Predictions - Ken MillsPublished May, 2014The 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 MillsPublished Jan, 2012This 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
Building New Bridges on the Hunter Expressway over Abandoned Coal Mines - Ken MillsPublished Jan, 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-2012.pdf5.3 MB
Subsidence and Mitigation Strategies Hunter Expressway - Ken MillsPublished Jan, 2011The 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 GalePublished Jan, 2011The 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 GalePublished Feb, 2009This 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 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
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
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
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
Development and Application of Strata Management in Coal Mines - Stuart MacGregorPublished Jan, 2005The 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 2005Resin 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
Numerical Modelling of Floor Deformation Mode at Longwall Face - Winton Gale - Published 2005High stress concentrations ahead of the longwall face often exceed the floor strength and induce fractures in the floor strata. While concentrations of the vertical stress alone induces fractures in the roof ahead of the longwall face, combinations of the vertical and horizontal stress appear to be the dominant factor in formation of floor fractures. These fractures develop in response to the triaxial stress conditions exceeding rock strength. In the immediate floor, fractures appear to form at frequent intervals dipping under the goaf at a steep angle while more complex bedding shear appears to dominate the floor failure at a greater depth. In a stronger floor the fractures appear to occur less frequently. If weak bedding planes are present in the floor, shear failure along these beddings can occur far ahead of the longwall face. The post failure displacements along the fractures and the formation of new fracture surfaces often occur in response to the stress relief, bending or buckling of thin bedded layers in the floor. The post failure displacements can be
large and may interfere with mining operations.
This paper presents the computational approach using FLAC to model the development of fractures in the floor strata. The model uses programmable “fish routines” that allow simulation of failure modes that may occur in response to the changing stress field ahead of the longwall face. Continuous monitoring of the two dimensional stress field is used to predict the fracture types and the direction at which the fractures may propagate. The fractures are then simulated using FLAC ubiqitous elements that allow to assign the joint direction and the reduction of joint strength in the direction of the calculated fracture. The stress state is tested continuously during the execution of the program and fractures are simulated when the stress exceeds the rock strength. This procedure can simulate the progressive development of fractures during the longwall advance. The method is particularly helpful to estimate the type of fractures and their frequency that depend on the strength of floor strata and stress build up during a longwall advance. The depth of floor failure can have a significant influence on the gas release from the floor strata in gaseous mines. The type of fractures and the fracture orientation that is computed can be presented in the movie files to view the development of fractures in the floor during the longwall advance. Numerical-Modelling-of-Floor-Deformation-Mode-at-Longwall-Face-W.Gale.pdf1.5 MB
Computer Simulation of Ground Behaviour and Rock Bolt Interaction at Emerald Mine - Winton Gale - Published 2004A 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
Applications of Hydraulic Fracturing to Control Caving Events in Coal Mines - The Moonee Experience - Ken MillsPublished Jan, 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-2002.pdf1.1 MB
Experience in Computer Simulation of Caving Rock Fracture and Fluid Flow in Longwall Panels - Winton Gale - Published 2002Recent advances in computer simulation together with field measurements of caving and microseismic activity about longwall panels, has allowed a much better understanding of the caving process and the variability due to geology. Research between SCT Operations and CSIRO Division of Exploration and Mining has initiated new methods of computational modelling predicting various caving patterns and strata failure far ahead of the longwall face.
The rock fracture distribution and the caving characteristics of a range of strata sections have been simulated by computer methods. The computer simulation of strata behaviour includes coupled fluid and mechanical behaviour. Validation studies of the method were addressed together with case studies. The method allows the simulation of longwall support behaviour and fluid pressure distributions about longwall panels under various geological conditions. The system also allows a prediction of the monitoring data, which is best suited to give an early warning of weighting events or signal various key caving characteristics. Experience-in-Computer-Simulation-of-Caving-Rock-Fracture-and-Fluid-Flow-in-Longwall-Panels-W.Gale.pdf4 MB
Three Decades of Measuring In Situ Stresses and Monitoring Stress Changes with the ANZI Strain Cell - Ken MillsPublished Jun, 2012This 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 Method of Determining Longwall Abutment Load Distributions for Roadway and Pillar Design - Ken MillsPublished Nov, 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-2001.pdf1.2 MB
Application of Computer Modelling in the Understanding of Subsidence Movements - Winton Gale - Published 2001Computer 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.
The modelling supports the concept that the ground subsides from the seam and progresses upwards and as such subsidence is the end point in a failure pathway within the overburden. The nature of the strata, in situ stresses and the mining geometry will influence the subsidence. The nature of the fractures created and the enhanced permeability will influence the interaction of mining with aquifers and surface water. Application-of-Computer-Modelling-in-the-Understanding-of-Subsidence-Movements-W.Gale.pdf425 KB
Displacement Monitoring and Visualisation Using Terrestrial Photogrammetry - Ken MillsDigital photogrammetry has been to shown to be an effective and efficient method for visualising and measuring subsidence behaviour of sandstone cliff formations. The system described in this paper provides the capability to measure subsidence movement over areas that are inaccessible to conventional subsidence monitoring techniques. Preliminary comparisons with a more traditional survey technique (total station survey to prisms) has shown mean variations, between the techniques, in vectors of movement of 17 millimetres.
They system described provides visualisation of the sandstone cliff formations through the generation of 3D images which can be rotated to be viewed from any angle. Visualisation provides a powerful tool for measuring, interpreting and understanding the shape, structure and geology. 3D images provide an advantage in tracking vectors of movement of natural features over a large area of inaccessible escarpment.
This paper provides a description of the technique and results obtained to date. The photogrammetry techniques used are based on photogrammetry techniques developed for mine highwall mapping. The results obtained in mapping subsidence exceed expectation for the camera system used. Further development will be undertaken to develop this teqnique for commercial applications in landform stability monitoring.
Through the generation of a large number of vector movements and visualisation a more complete knowledge and understanding of subsidence behaviour of sandstone cliff formations is derived. MSTS-Conference-2001-Displacement-Monitoring-and-Visualisation-Using-Terrestrial-Photogrammetry-K.Mills.pdf2.1 MB