Welcome to SCT's own publications library which contains a collection of recent publications and other resources with reliable research about our technology.
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
Validation of a Subsidence Prediction Approach of Combined Modelling and Empirical Methods - Yvette HeritagePublished Nov, 2017Subsidence 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 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
Connectivity of Mining Induced Fractures Below Longwall Panels A Modelling Approach - Yvette Heritage - Winton Gale - Adrian RipponPublished Feb, 2017Gas 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
Impact of bedding plane and laminations on softening zone around the roadways - 3D Numerical Assessment - Mahdi ZoorabadiPublished Feb, 2017When the distributed rock stress around the roadways exceeds the strength of the rock, the rock is failed and a softening zone is formed. Roof deformation developed in the roof and ribs of the roadways are highly controlled by the depth of softening zones. The rock failure process starts from a point ahead of the face and grows into the roof, floor and ribs by advancing roadway. The
maximum stress that can be transferred through the failed rocks would be equal to its residual confined strength. Therefore, rock stress is moved above failed zone and will create new failure zone if it is higher than the confined strength of rock at that depth. This process continues until the confined strength of the rock becomes higher than stress components. Bedding and lamination planes play a big role into the failure pathway of rocks around roadways. The thickness of softening zone is significantly influenced by the shear and tensile strength of bedding planes and laminations. This paper presents a 3D numerical assessment of the bedding and lamination planes impacts to the forming and extension of the softening zones. It highlights the requirements for better characterisation of bedding and lamination planes for reliable simulation of roadways. Impact-of-bedding-plane-and-laminations-on-softening-zone-around-the-roadways-3D-Numerical-Assessment-M.Zoorabadi-2017.pdf1.5 MB
Deformability Modulus of Jointed Rocks, Limitation of Empirical Methods and Introducing a New Analytical Approach-Mahdi-ZoorabadiPublished Feb, 2016Deformability modulus of jointed rocks is a key parameter for stability analysis of underground structures by numerical modelling techniques. Intact rock strength, rock mass blockiness (shape and size of rock blocks), surface condition of discontinuities (shear strength of discontinuities) and confining stress level are the key parameters controlling deformability of jointed rocks. Considering cost and limitation of field measurements to determine deformability modulus, empirical equations which were mostly developed based on rock mass classifications are too common in practice. All well-known empirical formulations dismissed the impact of stress on deformability modulus. Therefore, these equations result in the same value for a rock at different stress fields.
This paper discusses this issue in more detail and highlights shortcomings of existing formulations. Finally it presents an extension to analytical techniques to determine the deformability modulus of jointed rocks by a combination of the geometrical properties of discontinuities and elastic modulus of intact rock. In this extension, the effect of confining stress was incorporated in the formulation to improve its reliability. Deformability-Modulus-of-Jointed-Rocks-Limitation-of-Empirical-Methods-and-Introducing-a-New-Analytical-Approach-M.Zoorabadi-2016.pdf943 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
Analytical Procedure to Estimate the Horizontal Anisotropy of Hydraulic Conductivity in Coal Seams - Winton Gale - Mahdi ZoorabadiPublished Feb, 2015The horizontal hydraulic conductivity anisotropy of coal seams is a controlling parameter for designing gas drainage boreholes. The ratio between the maximum and minimum horizontal hydraulic conductivity (RkH-kh) and the orientation of maximum horizontal conductivity defines this anisotropy in horizontal plane.
This paper presents a new analytical procedure based on the field stress data and geometrical properties of coal cleats to calculate these two parameters. The application of this procedure for a real case in Eastern of Australia resulted in an average ratio of 20.9 for RkH-kh and orientation of NE for maximum horizontal conductivity. The comparison between these results with the measured values validated the accuracy of proposed procedure to estimate the anisotropy of horizontal hydraulic conductivity of coal seams. Analytical-Procedure-to-Estimate-the-Horizontal-Anisotropy-of-Hydraulic-Conductivity-in-Coal-Seams-W.Gale-M.Zoorabadi-2015.pdf377 KB
A Combined 2D and 3D Modelling Approach to Provide Adequate Roof Support in Complex 3D Excavations - Yvette HeritagePublished Jul, 2015Traditional methods for assessing effective roof support can be difficult to apply to complex 3D excavations. Through worked examples, this paper illustrates the successful approach of combined 2D and 3D numerical modelling to understand the mechanisms of rock failure for unique excavation geometries. The modelling approach provides adequate roof support recommendations for complex 3D excavations in Australian coal mines. A-Combined-2D-and-3D-Modelling-Approach-to-Provide-Adequate-Roof-Support-in-Complex-3D-Excavations-Y.Heritage-2015.pdf2 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
Investigation into Abnormal Surface Subsidence Above Longwall Panel Southern Coalfield - Winton GalePublished Jan, 2011The subsidence over a longwall panel at Tahmoor Mine in the Southern Coalfield of NSW, Australia, was found to be approximately twice the size it had been in previous measurements. An investigation into the potential causes was conducted using computer modeling together with hydrological characterization and detailed geotechnical characterization of the strata.
The abnormal subsidence was found to be consistent with localized weathering of joint and bedding planes above a depressed water table adjacent to an incised gorge. The study showed that other factors such as variation in stress field, joint zones, variation
in rock strength and topographic factors did have sufficient impact to induce the abnormal subsidence.
The results have significant implications to subsidence prediction in areas that may be prone to the phenomenon found at Tahmoor. Key indicators of the potential for this form of abnormal subsidence are presented. Investigation-into-Abnormal-Surface-Subsidence-Above-LW-Panel-Southern-Coalfield-2011.pdf2.6 MB
Using Helium as a Tracer Gas to Measure Vertical Overburden Conductivity Above Extraction Panels - Yvette Heritage - Winton GalePublished Feb, 2009This paper investigates helium injection into the goaf as a tool to measure goaf to surface connectivity. Laboratory studies confirmed a relationship between gas velocity and fracture conductivity through helium injection. Field trials of helium injection into the goaf were successfully conducted to determine whether a connection exists between the surface and the goaf. A repeatable technique of borehole helium injection, with a borehole drilled into the highly permeable caved zone of the goaf, proved to demonstrate more quickly whether a connection to the surface exists. Using-Helium-as-a-Tracer-Gas-to-Measure-Vertical-Overburden-Conductivity-Above-Extraction-Panels-Y.Heritage-W.Gale-2009.pdf281 KB