• 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. K.Mills

    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
  • Insights into the mechanics of multi seam subsidence from Ashton Underground Mine - Ken Mills - Steve Wilson

    Examples 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
  • Validation of a Subsidence Prediction Approach of Combined Modelling and Empirical Methods - Yvette Heritage

    Subsidence prediction is often required outside the limits of empirical databases where we look to other methods to expand our understanding of overburden caving and subsidence effects. Computer modelling, through simulation of rock failure and
    overburden caving, provides a means to extrapolate beyond current experience and to investigate other aspects of caving processes that are becoming increasingly important; aspects such as multi-seam interactions, irregular overburden geologies and groundwater interactions.

    This paper describes examples and a range of useful outcomes from modelling simulations of rock failure and overburden caving to illustrate how modelling is being used to extend understanding of multi-seam mining scenarios, irregular overburden geology, “greenfield” mining areas, increasing overburden depths and the requirement to understand overburden fracture formation and vertical hydraulic connectivity. A case study from the Bowen Basin is used as an example of the value of combining modelling and an empirical approach to improve subsidence prediction and provide validation and calibration of the prediction methodologies for future subsidence prediction. Validation-of-a-Subsidence-Prediction-Approach-of-Combined-Modelling-and-Empirical-Methods-Y.Heritage2017.pdf2.5 MB
  • Experience of monitoring shear movements in the Overburden Strata - Luc Daigle - Ken Mills

    Surface 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 Puller

    The 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
  • Connectivity of Mining Induced Fractures Below Longwall Panels A Modelling Approach - Yvette Heritage - Winton Gale - Adrian Rippon

    Gas make into active longwall panels is an important issue in ventilation and gas drainage design. A method of simulating the mining induced fracture network and associated increase in hydraulic conductivity is a necessity for improved mine design, hazard management planning and gas drainage efficiency. This paper identifies and illustrates the key components in determining the connectivity of lower gas sources to an active goaf. Computer modelling identifies the formation of cyclic fractures that form below the longwall face and extend down back below the goaf. These cyclic fractures form when the stress conditions are high enough and the strata properties allow for shear failure to extend down through the strata.

    The mining induced fracture formation and stress redistribution creates increased hydraulic conductivity of the floor strata below the active goaf. The stress redistribution and fracture volume also reduce the pore pressure below the goaf, allowing gas desorption to occur from lower seams. The combination of gas desorption and increased hydraulic conductivity allows gas connectivity from gas sources below the seam to the active goaf. A monitoring program at a NSW mine as part of ACARP Project C23009 allowed for preliminary validation of the concepts illustrated from the computer modelling. Preliminary field gas flow measurements are within the range of connectivity expectations based on rock failure modelling of longwall extraction. This report presents the first validation results for the modelling approach presented in this paper. Further results from ACARP Project C23009 on optimisation of gas drainage will follow in future publications. Connectivity-of-mining-induced-fractures-below-longwall-panels-A-Modelling-Approach-Y.Heritage-W.Gale-A.Rippon-2017.pdf1.3 MB
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