A Perspective on the Mechanics of Mine Subsidence above Longwall PanelsPublished Jun, 2022This paper presents a perspective on current understanding of the mechanics of overburden caving processes above longwall panels in Australia and the influence of these processes on surface subsidence movements. Experience of monitoring vertical subsidence, horizontal ground movements, sub-surface ground movements, and groundwater interactions is reviewed to explore the mechanics of the underlying processes that contribute to the ground movements observed on the surface as subsidence. Part of this perspective involves reflecting on the journey to gain this understanding, the culture we are fortunate to enjoy in NSW, and the people who have pioneered the transition of subsidence engineering from art to science. A-Perspective-on-the-Mechanics-of-Mine-Subsidence-above-Longwall-Panels-KMills-MSTS-2022-compressed_1.pdf539 KB
Observations of Multi Seam Subsidence at Ashton Underground MinePublished Jun, 2022Ashton Underground Mine (Ashton) is an underground longwall mine located northwest of Singleton in the Hunter Valley of NSW. The mine has extracted longwall panels in three seams, each seam progressively deeper than the last. The mining geometry in each of the seams is regular, parallel and either offset or stacked relative to the panels in the seams above. The high-quality survey monitoring dataset now available from Ashton provides significant insight into the mechanics of ground behaviour in the multi-seam geometry at this site. This paper presents a summary of observations of multi-seam subsidence at Ashton after mining in first two seams and then three seams. Observations-of-Multi-Seam-Subsidence-at-Ashton-Underground-Mine-K-Mills-and-S-Wilson-MSTS-2022-min.pdf1.2 MB
Further insights into the mechanics of multi seam subsidence from Ashton Underground MinePublished Feb, 2021Ashton Underground Mine (Ashton) is an underground longwall mine located northwest of Singleton in the Hunter Valley of NSW. The mine has so far extracted longwall panels in three seams with mining in a fourth seam planned and each seam progressively deeper than the last. The mining geometry in each of the seams is regular, parallel and either offset or stacked relative to the panels in the seams above. A subsidence line crossing all panels in each seam has been regularly surveyed in three dimensions since the commencement of mining. The high quality data set available from this line provides insight into the mechanics of ground behaviour in a multi-seam environment. This paper presents an update of the observations and interpretation presented in Mills and Wilson (2017) for mining in two seams with the inclusion of results from mining in a third seam.
Observations of the characteristics of multi-seam subsidence continue to indicate that although subsidence movements above multi-seam mining are more complex than single seam mining, these movements are nevertheless regular and predictable. In an offset geometry, remote from pillar and goaf edges, tilt and strain levels are similar or lower than single seam levels, despite the greater vertical subsidence, due to the general softening or reduction in shear stiffness of the overburden with each episode of subsidence. At stacked and undercut goaf edges, transient tilts and strains are significantly elevated.
Cumulative vertical subsidence after longwall mining in three seams has now reached 5.8m with incremental vertical subsidence increasing as a percentage of incremental mining height with each episode of subsidence. Latent subsidence from near stacked goaf edges is recovered when mining in the seam below. A site-specific methodology developed to forecast subsidence behaviour is allowing measured subsidence effects to be estimated reliably.
A Review of the Mechanics of Pillar BehaviourPublished Feb, 2019In recent years, the drive to reduce the impacts of surface subsidence has led to mine layout designs in New South Wales and Queensland that rely for their effectiveness on the long-term stability of pillar systems. The University of New South Wales (UNSW) pillar design methodology has become a benchmark for assessing long-term stability of pillars in Australia. The method is being applied in a wide range of geological settings and for a broad range of pillar geometries. Galvin, et al. (1999) warn that the UNSW methodology approach is empirical and only suitable for the conditions in which the methodology was developed; a warning that tends to be ignored.
The UNSW approach and most other empirical approaches do not specifically consider the changing characteristics of coal strength or the influence of the roof and floor strata on the ability of pillars to develop confinement. This paper describes how two independent components of coal strength continue to give the strength characteristics of coal pillars observed in prac6tice and the implications for pillar design. A-Review-of-the-Mechanics-of-Pillar-Behaviour-6-KWM-2-1-19.pdf1.5 MB
Experience of Using the ANZI Strain Cell in Exploration Boreholes to Determine the Three Dimensional Stresses at Depth - J.Puller, K.MillsPublished Sep, 2018This paper describes recent use of the ANZI (Australia, New Zealand Inflatable) strain cell and the overcoring method of stress relief in exploration boreholes to determine three dimensional in situ stresses at depths approaching 1km in a one-day operation. The results from each of the various stages of a routine overcoring operation are described to illustrate the information each step can provide. The results from an Australian site is presented to illustrate the opportunities to characterise the three dimensional in situ stress environment when multiple high confidence measurements are achieved. 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 have allowed the system to be deployed routinely in surface exploration boreholes. Recent development of a downhole high-precision data logger, a wireline enabled drilling system and an instrument deployment system has simplified the process of obtaining three dimensional overcore measurements remote from any underground excavation at depths approaching 1km. J.Puller-Experience-of-Using-the-ANZI-Strain-Cell-in-Exploration-Boreholes-to-Determine-the-Three-Dimensional-Stresses-at-Depth.pdf1.3 MB
Monitoring and Measuring Hydraulic Fracturing Growth During Preconditioning of a Roof Rock over a Coal Longwall Panel - Rob Jeffrey - Ken MillsPublished Mar, 2018Narrabri Coal Operations is longwall mining coal directly below a 15 to 20 m thick conglomerate sequence expected to be capable of producing a windblast upon first caving at longwall startup and producing periodic weighting during regular mining. Site characterisation and field trials were undertaken to evaluate hydraulic fracturing as a method to precondition the conglomerate strata sufficiently to promote normal caving behaviour at longwall startup and reduce the severity of periodic weighting. This paper presents the results of the trials and illustrates the effectiveness of hydraulic fracturing as a preconditioning technique.
Initial work was directed at determining if hydraulic fractures were able to be grown with a horizontal orientation, which would allow efficient preconditioning of the rock mass by placing a number of fractures at different depths through the conglomerate from vertical boreholes drilled from the surface. The measurements and trials were designed to determine the in situ principal stresses, the hydraulic fracture orientation and growth rate, and whether the fractures could be extended as essentially parallel fractures to a radius of at least 30 m. Overcore stress measurements were used to determine the orientation and magnitude of the in situ principal stresses, a surface tiltmeter array was used to determine the hydraulic fracture orientation, and stress change monitoring, pressure monitoring and temperature logging in offset boreholes were used to establish the fracture growth rate, lateral extent, and that the fractures maintained their initial spacing to a radial distance of greater than 30 metres. The measurements and trials demonstrated that horizontal fractures could be extended parallel to one another to a distance of 30 to 50 m by injection of 5,000 to 15,000 litres of water at a rate of 400 to 500 L/min. Results from the trial allowed a preconditioning plan to be developed and successfully implemented. Monitoring-and-Measuring-Hydraulic-Fracturing-Growth-During-Preconditioning-of-a-Roof-Rock-over-a-Coal-Longwall-Panel-R.Jeffrey-K.Mills-2018.pdf1.8 MB
Insights into the mechanics of multi seam subsidence from Ashton Underground Mine - Ken Mills - Steve WilsonPublished Feb, 2017Examples of subsidence monitoring of multi-seam mining in Australian conditions are relatively limited compared to the extensive database of monitoring from single seam mining. The subsidence monitoring data now available from the mining of longwall panels in two seams at the Ashton Underground Mine (Ashton) provides an opportunity to significantly advance the understanding of subsidence behaviour in response to multi-seam mining in a regular offset geometry. This paper presents an analysis and interpretation of the multi-seam subsidence monitoring data from the first five panels in the second seam at the Ashton Underground Mine. The methods used to estimate subsidence effects for the planned third seam of mining are also presented.
Observations of the characteristics of multi-seam subsidence indicate that although more complex than single seam mining, the subsidence movements are regular and reasonably predictable. Movements are constrained within the general footprint of the active panel. They are however sensitive to the relative panel geometries in each seam and to the direction of mining. In an offset geometry, tilt and strain levels are observed to remain at single seam levels despite the greater vertical displacement. At stacked goaf edges tilt and strain levels are up to four times greater. Latent subsidence recovered from the overlying seam has been identified as a key contributor to the subsidence outcomes. Some conventional single seam concepts such as angle of draw and subcritical/supercritical behaviour are less meaningful in a multi-seam environment. Insights-into-the-mechanics-of-multi-seam-subsidence-from-Ashton-Underground-Mine-K.Mills-S.Wilson-2017.pdf2.5 MB
Experience of monitoring shear movements in the Overburden Strata - Luc Daigle - Ken MillsPublished Feb, 2017Surface subsidence monitoring shows horizontal movements occur around longwall panels for a considerable distance outside the footprint of a longwall panel; typically several hundred metres to several kilometres. Less is known about how these movements are distributed between the surface and the mining horizon. A range of systems have been developed to measure how horizontal movements are distributed within the overburden strata generally and sometimes around specific geological structures. This paper describes the experience of using a range of these systems at various sites and some of the insights that these measurements bring with particular focus on the use of deep inclinometers.
The capability to measure induced displacements has developed over time from surface observations to use of borehole systems such as multi-arm callipers, downhole camera imaging and specially installed inclinometers placed to depths up to 300 m. Some techniques such as open boreholes and the multi-arm, oriented calliper have mainly been used at shallow depths where breakout and squeezing ground do not compromise the measurements. Others such as the borehole camera provide context but are not so suitable for quantitative measurement. The inclinometer installed in a large diameter borehole backfilled with pea-gravel has been found to provide high resolution measurements up to a horizontal displacement on any one horizon of about 60-80 mm. Inclinometers have been used at multiple sites around Australia to measure shear displacements to depths of up to about 300m. Shaped array accelerometers are an alternative that provide temporal resolution of a few minutes and provide continuous monitoring over a limited interval but tend to be most useful for monitoring the onset of low magnitude shear displacements. Experience-of-monitoring-shear-movements-in-the-overburden-strata-L.Daigle-K.Mills-2017.pdf1.7 MB
Development of the ANZI strain cell for three dimensional in situ stress determinations in deep exploration boreholes - Ken Mills - Jesse PullerPublished Feb, 2017The Australia, New Zealand Inflatable (ANZI) strain cell is an instrument used to determine the three dimensional in situ stresses with a high level of confidence, through the overcoring method of stress relief. The ANZI cell has been used for over three decades at numerous sites around the world, typically in short inclined boreholes drilled from underground mines. Technical advances during the last decade have seen the ANZI cell deployed and overcored in increasingly deeper surface exploration boreholes. Recent development of a downhole electronic data logger, a wireline enabled drilling system and an instrument deployment system has greatly simplified the process of obtaining three dimensional overcore measurements at depth. This paper describes the ANZI strain cell, its operation and recent development for overcoring in exploration boreholes. The capability to deploy ANZI strain cells in exploration boreholes represents a significant breakthrough for the design of underground mines and underground excavations generally. Being able to obtain high confidence measurements of the in situ stresses at the planning stage of any underground construction activity provides the opportunity to take advantage of these stresses. Not only does it become possible to protect key infrastructure by locating it away from areas of stress concentration, advantage can be taken of the major stresses to promote caving through appropriate design. Development-of-the-ANZI-strain-cell-for-three-dimensional-in-situ-stress-determinations-in-deep-exploration-boreholes-K.Mills-J.Puller-2017.pdf887 KB
In-Situ Stress Measurements and Stress Change Monitoring to Monitor Overburden Caving Behaviour and Hydraulic Fracture Pre-Conditioning - Jesse Puller, Ken Mills, Rob JeffreyPublished Jul, 2015A 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 monitoringPublished Jan, 2015This 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
Monitoring of Ground Movements at Sandy Creek Waterfall and Implications for Understanding the Mechanics of Valley Closure Movements - Ken MillsPublished May, 2014BHP 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 PullerPublished Feb, 2014This 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 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
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
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
Developing Methods for Placing Sand Propped Hydraulic Fractures for Gas Drainage in the Bulli Seam - Ken Mills - Published 2006Published Feb, 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
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