• Definitions of Measured Hydraulic Conductivity Zones above a Longwall Panel

    This paper presents the results of a field investigation to determine zones of hydraulic conductivity above an extracted longwall panel at United Colliery in the Hunter Valley of NSW. The hydraulic conductivity zones were determined from targeted boreholes drilled into the subsided strata above the goaf of Longwall 10, an isolated panel with a supercritical width panel geometry. Five boreholes defined the edge of caving and characterised the hydraulic conductivity within and above the caved zone. The deepest hole in the centre of the goaf was drilled to 30m above the extracted seam, providing hydraulic conductivity data through most of the subsided overburden strata. Definitions-of-Measured-Hydraulic-Conductivity-Zones-above-a-Longwall-Panel-YHeritage-MSTS-2022.pdf901 KB
  • Permeation Grouting of a Subsidence Impacted Watercourse in the Southern Coalfields

    Myrtle Creek has been impacted by the subsidence associated with longwall mining at Tahmoor Coking Coal Mine in Picton, NSW. Specific impacts are fracturing of the rock bed resulting in a reduction in surface flow and pool holding capacity behind the rock bars that are a characteristic of waterways in the southern coalfields. A specific pool, Pool 23, within the impacted Myrtle Creek, was selected for remediation using permeation grouting. First, an investigation was undertaken at Pool 23 to characterise the fracture network and advise on remediation grouting design. The investigation identified the fracture and hydraulic conductivity profiles as well as local hydraulic gradients. Permeation-Grouting-of-a-Subsidence-Impacted-Watercourse-in-the-Southern-Coalfields-BBlacka-MSTS-2022.pdf1.9 MB
  • Determination of the Subsidence Mechanism for Subcritical Miniwall Panels

    Airly Mine is a miniwall operation mining the Lithgow Seam in the Western Coalfield, NSW. Airly Mine currently mines beneath the Mt Airly mesa, where the Lithgow Seam outcrops near the base of the mesa. Each miniwall panel is subcritical in its geometry with 60m wide panels and depth of cover ranging 200-270m. The surface subsidence produced vertical displacements significantly greater than the empirical predictions, up to 713mm, prompting the numerical modelling approach discussed in this paper. The numerical rock failure modelling assessment illustrated the mechanism for the magnitude of subsidence experienced at Airly Mine. Determination-of-the-Subsidence-Mechanism-for-Subcritical-Miniwall-Panels-Y-Heritage-MSTS-2022.pdf1.8 MB
  • A Perspective on the Mechanics of Mine Subsidence above Longwall Panels

    This 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 Mine

    Ashton 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
  • Compression and shear wave sonic velocity measurements in hard rock

    Compression wave sonic velocity (Vp) is routinely measured in rock testing laboratories. Shear wave sonic velocity (Vs) measurement for further application to geomechanical studies is not routinely conducted. This paper outlines the establishment of a laboratory testing technique including waveform analysis for the determination of shear wave velocity.
    The paper outlines the measurement of compressional and shear wave sonic velocities using ultrasonic pulse transmission technique, for several hard rock lithologies recovered during routine (NQ/HQ/PQ) exploration core drilling. Shear wave sonic velocities were measured using a pair of shear piezoelectric transducer elements. Measured shear wave sonic velocities are compared with fundamental and empirical formulas used to predict shear wave sonic velocity, in order to verify the method.
    This paper discusses the need for an Australian Standard that includes a provision for the measurement of shear wave sonic velocity. Measured results are used to calculate dynamic moduli of rock samples and are compared with static moduli. The application of dynamic moduli to geotechnical characterisation of the rock mass is explored.
    Compression-and-shear-wave-sonic-velocity-measurements-in-hard-rock.pdf1.8 MB
  • Measurement of the caved zone above a longwall panel United Colliery

    This paper presents the results of a targeted goaf borehole program to define the edge of the Longwall 10 caved zone at United Colliery, Hunter Valley NSW. Longwall 10 is an isolated panel with a supercritical panel geometry. Three of the five boreholes defined the edge of caving, while two boreholes characterised the centre of the goaf. The location of the caved zone was depicted from a combination of total water loss during drilling that coincided with a subvertical fracture at the location of total water loss. The boreholes showed the caving angle from the pillar ribs to be 21 degrees on the up dip side and 19 degrees on the down dip side of the panel. An additional borehole drilled 10 metres (m) towards the goaf centre on the up dip side showed a caving angle increase to 25 degrees from the adjacent borehole, indicating the arc shape of the caved zone. The caving angle coincided with a high strain fracture zone and connectivity to the goaf. This caving angle information can inform assessments for hydraulic/gas connectivity and geotechnical engineering applications such as multi-seam overmining or opencut/underground interaction. (Note that the caving angle is not the same as the abutment angle, which the latter is a calculated angle
    based on pillar load.)
    Y-Heritage_Measurement-of-the-caved-zone-above-a-longwall-panel-United-Colliery.pdf1.3 MB
  • Further insights into the mechanics of multi seam subsidence from Ashton Underground Mine

    Ashton 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.
    Further-insights-into-the-mechanics-of-multi-seam-subsidence-from-Ashton_low-res.pdf1.2 MB
  • Geotechnical aspects of the Pike River mine drift recovery

    The Pike River mine exploded on the 19 November 2010. Thirty-one (31) men were working underground at the time of the explosion and only two men were able to escape. The Pike River Recovery Agency was established in January 2018 to conduct a safe manned re-entry and recovery of the Pike River mine drift to gather evidence to better understand what happened in 2010.
    SCT Operations Pty Ltd (SCT) have been engaged to assist with the management of strata control hazards as part of the planning and implementation phases of the drift recovery. Initial geotechnical assessments comprised review of available historical geological and geotechnical information to develop a geotechnical baseline report and hazard map to assist with future planning and risk management.
    A range of controls have been implemented to manage geotechnical risk to acceptable levels and to ensure that adequate levels of inspection, mapping, monitoring, assessment, and review are maintained at all stages of drift recovery. Additionally, 3D FLAC modelling, surface tunnelling simulations and field loading trials have been conducted to support proposed tunnelling through a Rocsil plug located at the top end of the drift to provide access the rock fall area which marks the end of the mandated drift recovery. Given most of the drift had not been physically inspected following the explosion a range of drillhole assessments comprising downhole camera and laser scanning was also conducted to improve understanding of the drift environment both prior to and during re-entry.
    As part of operational implementation and continuous improvement processes modifications were made to both ground support systems and bolting equipment which significantly improved support cycle installation times.
    SCT also supplied real-time roof monitoring instrumentation to the site which supplies an almost continuous data feed to the mine control room for interpretation and automatic alerting if TARP threshold levels are exceeded.
    Geotechnical-aspects-of-the-Pike-River-mine-drift-recovery_low-res.pdf1.2 MB
  • Dynamic model of fault slip and its effect on coal bursts in deep mines

    The success of deep mining operations relies upon controlling the fractured ground. It is a documented knowledge that many coal bursts occur when mining close to the existing faults. Gradual stress relief towards excavations and other mechanisms can unload stress normal to the nearby fault plane causing it to slip. The generated seismic waves impact the mine roadway rib sides and can produce a coal burst. As part of the ACARP project, the FLAC3d dynamic numerical model was used to show how a fault slip at various locations and orientations may initiate a coal burst. This study simulates an artificial fault slip with peak velocity reaching 4m/s in 0.013 seconds and displacing 119mm in total. Seismic induced peak particle velocities in rock and its influence on coal rib stability were investigated. 89 numerical models with various fault locations and orientations at 450m depth indicated that a 4 tonne coal block can be ejected from the mine roadway rib side at speeds of up to 5m/s. The important finding is that irrespective of the fault slip magnitude, the fault geometry and the in-situ stresses enable to predict which side of the mine roadway may experience the coal burst. Instructing the mine personnel to use the other side of the roadway may improve their safety. Overall, this research produced preliminary results to prove that this method can be used to flag the coal burst dangers for certain fault locations and orientations in deeper mines irrespective of the fault slip properties that are typically difficult to predict. Dynamic-model-of-fault-slip-and-its-effect-on-coal-bursts-in-deep-GV.pdf933 KB
  • Dynamic analysis of fault slips and their influence on coal mine rib stability

    Historical data indicate that in deep coal mines the presence of faults in close proximity to excavations affect the frequency of coal bursts. A number of researchers have attempted to correlate the fault geometries to the frequency and severity of coal bursts but dynamic numerical modelling has not been used to show how faults can affect coal ejection from the rib side. The dynamic numerical analysis presented here show how different orientations of fault slips may affect coal bursts. To prove the concept, 89 cases of slipping fault geometries were modelled using the FLAC3D software and their effect on rib stability investigated. The results indicate that there is a simple and logical correlation between the fault location, its slip velocity and the ejection of the yielded coal rib side. The seismic compressive wave generates rock/coal mass velocities that directly impact the rib side. If the coal rib is relatively disturbed and loose, these velocities can cause its ejection into the excavation. The slip direction typically impacts one side of the mine roadway only. A 1 m thick loose coal block attached to the 3 m high rib side in mine roadway was ejected at speeds ranging from 2.5 to 5 m/s depending on the fault location, its orientation and the maximum fault slip velocity modelled at 4 m/s. Dynamic-analysis-of-fault-slips-and-their-influence-on-coal-mine-2020-GV.pdf1.6 MB
  • Dynamic events at longwall face, CSM Mine, Czech Republic

    Presented here are the details of the seismic events that occurred at longwall 11 located at the CSM mine in the Ostrava coal region, Czech Republic. This longwall was excavated in a very complex area located within the shaft protective pillar and adjacent to the 50 m wide and steeply inclined fault zone at a depth of 850 m. In addition, 10 longwalls were extracted below each other over many years in several sloping seams located on the other side of the large sloping fault zone resulting in complex stress fields and large subsidence. The immediate roof above longwall 11 was a very strong sandstone and sandy siltstone with a uniaxial compression strength of 80 – 160 MPa. When the longwall started, continuous seismic monitoring of the longwall area indicated 470 small seismic events with energy smaller than <102 J. The first high energy event of 3.3*105 J occurred when the longwall advanced 85m past the starting line. Some 30 minutes later a rockburst occurred registering energy of 2.2 *106 J, causing significant rockburst damage at the tailgate located near the large tectonic zone. The roadway steel arches were significantly deformed and the maximum floor heave reached up to 1.5 m. To investigate the complex strata behavior in that area, a large FLAC3D model 0.27 km3 in volume was constructed and 10 longwalls were extracted in several sloping seams adjacent to the large fault zone. The model under construction is now ready to study the complex strata behaviour and the associated stress fields together with the dynamic strata behaviour to match the modelled seismic events with those measured underground. Dynamic-events-at-longwall-face-2020-GV.pdf1.8 MB
  • Numerical model of dynamic rock fracture process during coal burst

    Coal bursts present one of the most severe hazards challenging the safe operations in underground coal work environments. In Australia, these events are becoming increasingly frequent as coal measures are mined progressively deeper. This study is supported by the Australian Coal Association Research Program (ACARP) which aims to better understand the phenomena of coal burst. In this paper the dynamic fracture process of coal bursts was successfully simulated in the coal roadway. This was achieved using dynamic analysis utilising DRFM2D routine by Venticinque and Nemcik (2017) in FLAC2D (Itasca, 2015) which complemented previous study observations by Venticinque and Nemcik (2018). This is significant because until now the evolving dynamic rock fracture process during coal burst remained unknown. Additionally, coal/rock burst events were shown from simulation as being
    largely driven by the propagation of shear fractures from within the rib. This was demonstrated to produce effect forcing the dynamic conversion and release of potential energy stored as compressive strain in the rib into kinetic movement of the entire rib section. This entire process was shown to occur very fast taking approximately 0.2 seconds for a coal burst to fully establish, with ejection of several meters of rib at a velocity of 1.6 m/s produced in the model of an underground coal roadway having 550 m depth of cover.
    Numerical-model-of-dynamic-rock-fracture-process-during-coal-burst-2020-GV.pdf960 KB
  • A Review of the Mechanics of Pillar Behaviour

    In 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.Mills

    This 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
  • Mechanics of Rib Deformation Observations and Monitoring in Australian Coal Mines - Yvette Heritage

    The risk of fatalities from rib failure is still prevalent in the coal mining industry. This risk has prompted further research to be conducted on rib deformation in order to understand the mechanisms of rib failure, with the long-term objective being to improve rib support design. This paper presents the results of ACARP research project C25057, which investigated the mechanics and drivers of rib failure. The results of rib deformation monitoring at three different mines in Australia provides rib deformation characteristics for overburden depths ranging from 160 m to 530 m. Monitoring includes deformation during development drivage conditions and during the longwall retreat abutment stress environment. The rib deformation monitoring covered three different seams: the Goonyella Middle Seam, Ulan Seam, and Bulli Seam in the Bowen Basin, Western Coalfield, and Southern Coalfield, respectively. The observed mechanisms driving the rib deformation ranged from bedding shear failure along weak claystone bands to vertical shear fractures to kinematic failures driven by shear failure dilation. The variation in mechanisms of rib failure, together with the seemingly consistent method of rib support design, prompts the question: What exactly is the role of rib support? Mechanics-of-Rib-Deformation-Observations-and-Monitoring-in-Australian-Coal-Mines-Yvette-Heritage-2018.pdf6.1 MB
  • Mechanics of Rib Deformation at Moranbah North Mine A Case Study - Yvette Heritage

    Moonee Colliery are longwall mining in the Great Northern seam at depths ranging from 90m to 170m. Surface infrastructure above the first four longwall panels includes the Pacific Highway and several residential and commercial properties.

    This paper describes the pillar design approach used to manage surface subsidence in the area. The approach is based on previous detailed subsidence and pillar monitoring in nearby Wallarah Colliery and measurements of subsidence throughout the Lake Macquarie area for a wide range of pillar sizes and overburden depths. Undermining the Pacific Highway requires consideration of not only the amount of subsidence but also the timing and nature of subsidence. Various options were considered and a design developed to control surface subsidence to acceptable levels. This paper summarises the results of previous monitoring and outlines the issues considered in the longwall panel design for subsidence control at Moonee Colliery. COAL-2018-Mechanics-of-Rib-Deformation-at-Moranbah-North-Mine-A-Case-Study-Y.Heritage-2018.pdf2.6 MB
  • The Role of Gas Pressure in Coal Bursts Winton Gale 2018

    Rock and coal fractures and micro seismic vibration are common occurrences during development mining. It is very uncommon for coal and rock to be propelled into the roadway during normal mining operations. However, such occurrences do occur and appear to require significantly more energy than is available from strain energy release during coal cutting. The sources of energy, which can contribute to the propulsion of coal from the face or ribs, are typically strain energy from the surrounding ground, seismic energy from a rapid rupture of the ground in the vicinity, or rapid expansion of gas from within the burst source area.

    The aim of this paper is to briefly review the bursts that could be related to strain energy or seismic energy. However, the greatest emphasis is placed on the effect that gas within the coal could play in moderate to gassy mines.

    It has been found that the bursts related to the expansion of gas can occur in coal and stone. The volume of gas involved in coal bursts is typically lower than in gas outbursts; however, the process is generally similar. The-Role-of-Gas-Pressure-in-Coal-Bursts-Winton-Gale-2018.pdf4.2 MB
  • Monitoring and Measuring Hydraulic Fracturing Growth During Preconditioning of a Roof Rock over a Coal Longwall Panel - Rob Jeffrey - Ken Mills

    Narrabri 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 Energy Sources of Bursts in Coal Mines and the Effective of Prevention and Control Measures - Mahdi Zoorabadi - Winton Gale

    Coalburst is a general term, which is commonly used in the coal mining industry for the violent failures of coal in the ribs and face of roadways and panels in underground coalmines. Due to lack of interest in the industry to reveal the causing source of the event, or due to uncertainty about the source, they happily use this term. The term by its own does not reveal the source of the energy, which causes the event. There are three sources of energy that can cause a burst event in underground coalmines: 1) store elastic strain energy, 2) seismic events and 3) gas expansion energy. This paper presents the fundamentals about these sources of energies and discusses our known and unknown facts about the mechanisms. Additionally, it discusses the reliability and effectiveness of stress relief holes and gas exhaust holes as controlling measures to prevent burst events. Insights-into-the-Energy-Sources-of-Bursts-in-Coal-Mines-and-the-Effective-of-Prevention-and-Control-Measures-M.Zoorabadi-2018.pdf1.4 MB
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