Mechanics of Horizontal Movements Associated with Coal Mine Subsidence in Sloping Terrain Deduced From Field Measurements - Ken MillsPublished Sept, 2017The ground movements associated with underground coal mining and, in particular, longwall mining, are recognised to include horizontal subsidence movements, but the mechanics of the processes that cause these horizontal movements are not well understood. Over the last two decades, three-dimensional subsidence monitoring has become routine in Australia and has provided a wealth of measurements of horizontal movements caused by mining subsidence. These measurements and other subsurface observations allow the processes that cause mining-induced horizontal movements to be inferred and, subsequently, verified. In this paper, the mechanics of the processes that cause horizontal movements, particularly those in sloping topography, are described and discussed on the basis of field observations.
There are several processes recognised to generate horizontal subsidence movements. In flat terrain, systematic horizontal movements cause the surface to move initially toward the newly created goaf and, subsequently, in the direction of mining. Tectonic energy stored as horizontal stress is released by mining, and, when the horizontal stresses are high, the magnitude of this horizontal stress relief movement is large enough to be perceptible for some kilometres from the panel. In sloping terrain, there is an additional component of horizontal movement that occurs in a downslope direction. This movement, sometimes referred to as valley closure movement, has a magnitude that is typically much greater than systematic or stress relief movements. Mechanics-of-Horizontal-Movements-Associated-with-Coal-Mine-Subsidence-in-Sloping-Terrain-Deduced-From-Field-Measurements-K.Mills.pdf1.8 MB
Propogation of a Penny Shaped Hydraulic Fracture Parallel to a Free Surface, with Application to Inducing Rock Mass Caving for Mining - Rob JeffreyPublished Sept, 2017In this paper, the problem of a penny-shaped hydraulic fracture propagating parallel to the free surface of an elastic half-space is studied. The fracture is driven by an incompressible Newtonian fluid injected at a constant rate. The flow of viscous fluid in the fracture is governed by the lubrication equation, while the crack opening and the fluid pressure are related by singular integral equations. We construct two asymptotic solutions based on the assumption that the energy expended in the creation of new fracture surfaces is either small or large compared to the energy dissipated in viscous flow. One important outcome of the analysis is to show that the asymptotic solutions, when properly scaled, depend only on the dimensionless parameter R, the ratio of the fracture radius over the distance from the fracture to the free-surface. The scaled solutions can thus be tabulated and the dependence of the solution on time can be retrieved for specific parameters, through simple scaling and by solving an implicit equation. Propogation-of-a-Penny-Shaped-Hydraulic-Fracture-Parallel-to-a-Free-Surface-with-Application-to-Inducing-Rock-mass-Caving-for-Mining-R.Jeffrey.pdf376 KB
Stress Conditions and Failure Mechanics Related to Coal Pillar Strength - Winton GalePublished Sept, 2017The aim of this paper is to discuss the rock mechanics issues which can influence the strength of pillars in coal mines. The paper utilises stress change monitoring results, micro seismic monitoring results and computer modelling to assess the stress history about a chain pillar. The implications and fracture modes developed are discussed, with the outcome being that chain pillar strength can be significantly reduced by the stress path and changes in boundary conditions to the pillar when longwall extraction occurs. It is envisaged that this effect is contained in measured and empirical data bases, however it is important to recognise the stress path process when applying results to various site conditions and mine layouts. Stress-Conditions-and-Failure-Mechanics-Related-to-Coal-Pillar-Strength-W.Gale.pdf524 KB
Estimation of the Hydraulic Conductivity of the Overburden above Longwall Panels in Coal Mines - Winton GalePublished Feb, 2012The aim of this paper is to summarise and update the results of Australian Coal Association Research Project (ACARP) Report C13013 which relate to water inflows into a mine which occur through the overburden above and adjacent to longwall panels. The study assessed available data of inflows into underground coal mines and utilised computer simulation of water flow through fracture networks. The study concluded that flow into mines is typically via an interconnected network of pre-existing and mining induced fractures. The height above the coal seam that mining induced fractures extend is typically related to the width of the panel and the thickness of the coal extracted. However the potential for those fractures to form a connected network which can facilitate flow, is related to the amount of subsidence and the depth of mining. The study compares model simulations with measured data and provides guidelines to estimate the average hydraulic conductivity of the overburden above extracted longwall panels in Australia. Estimation-of-the-Hydraulic-Conductivity-of-the-Overburden-above-Longwall-Panels-in-Coal-Mines-W.Gale.pdf979 KB
Experience of Field Measurement and Computer Simulation Methods for Pillar Design - Winton GalePublished Sept, 2017Coal pillar design has been based on generalised formulae of the strength of the coal in a pillar and experience in localised situations. Stress measurements above and in coal pillars indicate that the actual strength and deformation of pillars varies much more than predicted by formulae. This variation is due to failure of strata surrounding coal. The pillar strength and deformation of the adjacent roadways is a function of failure in the coal and the strata about the coal. When the pillar is viewed as a system in which failure also occurs in the strata, rather than the coal only, the wide range of pillar strength characteristics found in the UK, USA, South Africa, Australia, China, Japan and other countries are simply variation due to different strata-coal combinations and not different coal strengths.
This paper presents the measured range of pillar strength characteristics and explains the reasons. Methods to design pillar layouts with regard to the potential strength variations due to the strata strength characteristics surrounding the seam are presented.
Geotechnical Issues for Multi Seam Longwall Panels - Winton GalePublished Sept, 2017The design of longwall panel layout for multi-seam mining is a very important issue for mining districts where economic seams are in close proximity. Layout options for longwall panels relate to variations of vertical superposition maintaining a constant chain pillar location or offsetting the panels and undermining overlying chain pillars. A design process to assess the various layout options is discussed, together with the relevant issues related to chain pillar strength, subsidence and induced permeability within the overburden. The design process has utilised computer modelling of the caving process together with international experience to asses the various layouts.
The paper will discuss these issues with regard to a site study undertaken in the Hunter Valley under ACARP funding. Geotechnical-Issues-for-Multi-Seam-Longwall-Panels-W.Gale.pdf6.9 MB
Geological Issues Relating to Coal Pillar Design - Winton GalePublished Sept, 2017The strength characteristics of coal pillars have been studied by many workers and the subject is well discussed in the literature (for example. Salamon and Monro, 1967; Wilson, 1972: Hustrulid, 1976). A range of strength relationships have been derived from four main sources:
- Laboratory Strength measurements on different-sized coal block specimens;
- Empirical relationships from observations of failed and unfailed pillars;
- A theoretical fit of statistical data and observations; and
- Theoretical extrapolation of the vertical stress buildup from the ribside toward the pillar centre, to define the load capacity of a pillar.
Hydraulic Fracturing Applied to Inducing Longwall Coal Mine Goaf Falls - Rob Jeffrey - Ken MillsPublished Sept, 2017This paper describes the first successful use of hydraulic fracturing to induce a goaf event and control the timing of caving events in Australia. Hydraulic fractures are initiated at 7 to 10m above the bottom of a thick conglomerate roof and, because of the low vertical stress magnitude relative to the other two principal stressed, grow as horizontal fractures. The fractures extend radially outward from the injection borehole into the rock until a goaf fall occurs. Hydraulic fracturing has provided a means to control the timing of windblast events and thereby significantly improved safety. The successful implementation of hydraulic fracturing at Moonee Colliery to control the timing of goaf events has enabled the mine to continue operating. Hydraulic-Fracturing-Applied-to-Induce-Longwall-Coal-Mine-Goaf-Falls-R.Jeffrey-K.Mills.pdf240 KB
Performance of Roof Support Under High Stress in a US Coal Mine - Winton GalePublished Sept, 2017The National Institute for Occupational Safety and Health's (NIOSH) Pittsburgh Research Laboratory (PRL), RAG Pennsylvania and Strata Control Technologies of Australia have collaborated to conduct an extensive study of roof bolt strata interaction at the Emerald mine in Southwestern Pennsylvania. The primary goal of the project was to obtain detailed data on the interaction between the mine roof and the support elements for use in modeling studies. The study site was a longwall tailgate subjected to high horizontal stress. Three arrays of instruments were installed at the site, one in the tailgate entry and two in an adjacent crosscut. Pumpable concrete cribs were present in the tailgate array, and cable bolts were installed in one of the crosscut arrays. The instruments included mechanical and sonic extensometers for measuring roof movement, instrumented roof bolts, and three-dimensional roof stress cells. The study was ultimately successful in determining the magnitude of the horizontal stress concentration, the height of roof failure and the roof failure sequence, and the loading history of the primary roof supports. Performance-of-Roof-Support-Under-High-Stress-in-a-US-Coal-Mine-W.Gale.pdf175 KB
Rock Fracture Caving and Interaction of Face Supports Under Different Geological Environments. Experience from Australian Coal Mines - Winton GalePublished Oct, 2017This paper is presents a summary of recent investigations into fracture and caving about longwall panels. The results of these investigations indicate that rock failure initiates well ahead of the longwall face. Rock fracture typically forms in response failure through the material and bedding planes. Tensile fractures also form in massive units. These fracture patterns typically create a fracture network which determines the caving characteristics encountered at the faceline. The action of longwall face supports under such conditions is to maintain confinement to the fractured ground and develop a consistent caving line. The confinement developed above the canopy under these conditions can be variable on a shear by shear basis and the operational face support procedures play an important role in stability about the face area. Rock-Fracture-Caving-and-Interaction-of-Face-Supports-Under-Different-Geological-Environments.-Experience-from-Australian-Coal-Mines-W-Gale.pdf2 MB
Impact of Vertical Stress on Roadway Conditions at Dartbrook Mine - Ken MillsPublished Aug, 2000A program of stress change and roadway deformation monitoring was undertaken to measure the forward abutment load distribution about a retreating longwall panel at Dartbrook Mine. The results of this monitoring allow roadway conditions observed at various stages of mining to be ranked according to the estimated vertical stress they have experienced . This ranking also provides a means to predict and visualise future roadway conditions at various stages of mining. This paper presents the results of the monitoring and describes the approach developed to predict future roadway conditions.
The monitoring results show the vertical stress distribution associated with the front abutment from a longwall goaf decreases exponentially away from the goaf edge. The vertical stress distribution peaks at the goaf edge at 5 to 6 MPa stress increase and decreases to 1 MPa within 50m becoming imperceptible beyond 100m. By scaling this measured distribution to reflect the loading environment, the vertical stress can be estimated for different overburden depths and different stages of mining. Photograph of roadway conditions at particular vertical stress levels provide a way to visualise roadway conditions at various stages of mining in the future. Impact-of-Vertical-Stress-on-Roadway-Conditions-at-Dartbrook-Mine-K.Mills.pdf2.6 MB
Successful Application of Hydraulic Fracturing to Control Windblasts at Moonee Colliery - Ken MillsPublished Oct, 2017This paper describes the first successful use of hydraulic fracturing to induce caving events “on demand” in Australia. Moonee Colliery operate a longwall immediately below a thick conglomerate strata. This strata temporarily bridges across the extracted longwall panel to create a large area of standing goaf. When this standing goaf eventually collapses, the windblast generated presents a significant hazard to men working on and around the longwall face.
Hydraulic fracturing has been successfully introduced to take control of the timing of these caving events so as to eliminate the risk of windblast injury. The longwall face area is completely evacuated during the treatment. Water is pumped into an injection point located in the conglomerate strata above the standing goaf.
A horizontal fracture is generated and grows outward from the injection point, separating the conglomerate strata below the fracture horizon. At some point the strata can no longer span and a goaf fall is initiated. After a treatment, mining can be recommenced with the windblast hazard eliminated. Successful-Application-of-Hydraulic-Fracturing-to-Control-Windblasts-at-Moonee-Colliery-K.Mills.pdf221 KB
Remote Monitoring of Mine Subsidence Using Radar Interferometry - Ken MillsPublished Oct, 2017Radar interferometry has been increasingly used to generate digital terrain models and to map ground surface displacement. The technology has the potential to monitor subsidence movements in three dimensions over entire coalfields every few weeks. This paper describes some of the experience and challenges associated with using radar interferometry for three dimensional subsidence monitoring. In conventional differential radar interferometry (DInSAR), the ground surface displacement can be measured along the looking direction of the radar system.
DInSAR results of the same area are required from at least three different looking directions to measure vertical and horizontal displacements in three dimensions. DInSAR results generated from data acquired by the European satellite ENVISAT at three different look angles have been used to develop displacement vectors of mining deformation in three dimensions. Interpretation of the ENVISAT results has been complicated by what is called phase unwrapping errors caused by the high displacement gradients at the edge of the subsidence zone. Results derived from data acquired by the new Japanese satellite ALOS is also used here to demonstrate how the high phase gradient problem can be eased by having the interferometric data with longer wavelength and finer imaging resolution. Remote-Monitoring-of-Mine-Subsidence-Using-Radar-Interferometry-K.Mills.pdf344 KB
Review of Mechanisms Resulting in Observed Upsidence and Closure Movements - Ken MillsPublished Oct, 2017Descriptions of natural valley bulging movements as well as mining induced valley related upsidence and closure movements have been well documented by various authors. It is clear from this research that there are numerous factors which affect these movements and it is likely that the movements comprise a number of mechanisms. Several models have been forwarded to describe valley related movements resulting from longwall mining. A review of some of these models is presented in this paper. This review indicates that several mechanisms may be in operation at any given site depending on the actual conditions at that site. An empirical method to predict these movements was developed as part of two ACARP research projects during 2000 and 2002. This method has proven to provide conservative upper bound predictions in the majority of cases. As the mechanisms and the factors which influence valley related movements are better defined, refinements in the method of prediction can be made so that the level of conservatism in the current method of prediction can be reduced. Review-of-Mechanisms-Resulting-in-Observed-Upsidence-and-Closure-Movements-K.Mills.pdf636 KB
Subsidence Impacts on River Channels and Opportunities for Control - Ken MillsPublished Oct, 2017Subsidence associated with longwall mining is recognised to cause ground movements that generate horizontal compression at topographic low points; a process referred to as valley closure. River channels in the Southern Coalfield of N.S.W. are typically located directly on rock strata that has the potential to be impacted by valley closure. This paper describes the nature of the subsidence impacts that have been observed in river channels in the Southern Coalfield, techniques for monitoring these impacts, and a range of strategies for prevention, and control of the impacts.
The Australian coal industry has supported ACARP research Project C12016 - Damage Criteria and Practical Solutions for Protecting Undermined River Channels. The results presented in this paper are based on the outcomes of this project. Subsidence-Impacts-on-River-Channels-and-Opportunities-for-Control-K.Mills.pdf893 KB
Observations of Ground Movements within the Overburden Strata above Longwall Panels and Implications for Groundwater Impacts - Ken MillsPublished Oct, 2017Longwall mining is recognised to cause disturbance to the overburden strata as the overburden strata moves downward into the void created by mining. These ground movements have been observed as surface subsidence over many decades and by numerous researchers through numerous surface and sub-surface monitoring programs, in a wide variety of different geological settings, using a wide variety of monitoring techniques. This monitoring provides an excellent database of experience from which to characterise the nature and extent of disturbance within the overburden strata above longwall panels. This characterisation is intended to provide a basis for better understanding the effects of longwall mining on the surrounding strata and, particularly in the context of groundwater interactions, the formulation of hydrogeological models used to predict groundwater impacts about longwall panels.
The extend and nature of zones within the overburden are characterised in this paper on the basis of the level of disturbance and the nature of this disturbance. Zones characterised by tensile changes or stretching behaviour are found to be located directly above each panel with the level of disturbance above the mining horizon graduated as a function of panel width from the mining horizon through to about three times panel width above each individual longwall panel. These stretching zones and their influence on the hydraulic conductivity of the overburden strata contrast with zones of increased compression located directly above the chain pillars that separate individual longwall panels. Observations-of-Ground-Movements-within-the-Overburden-Strata-above-Longwall-Panels-and-Implications-for-Groundwater-Impacts-K.Mills.pdf1.5 MB
Mitigation of Subsidence Impacts at Marhynes Hole Rock Bar - Ken MillsPublished Oct, 2017Marhnyes Hole is a local swimming hole located on the Georges River about one kilometre to the north and east of Appin township. To allow Longwall 5A4 at West Cliff Colliery to mine under Marhnyes Hole in September 2002, BHP Illawarra COal undertook a range of mitigation and remediation activities aimed to preserve the integrity of the rock bar and the amenity of the adjacent rock pools. This paper describes the main component of mitigation strategy, a stress-relief slot designed to limit subsidence impacts on the structural integrity of the rock bar.
The stress-relief slot was some 28.5m long, 18-20m deep and up to 150mm wide. It was constructed adjacent to Marhnyes Hole rock bar to limit the impact of subsidence compression on the structural and aesthetic integrity of the rock bar to a level that would allow it to be restored to its original condition once mining was complete.
The slot was successful in achieving its intended aims. The structural integrity of Marhnyes Hole rock bar was protected sufficiently to allow it to be subsequently remediated. Most importantly for the BHP Illawarra Coal, West Cliff Colliery was able to mine Longwall 5A4 without interruption and adverse publicity was controlled. The cost of the slot construction was less than 3% of the estimated cost of steeping the longwall panel around the site with consequential interruptions to longwall continuity. Mitigation-of-Subsidence-Impacts-at-Marhynes-Hole-Rock-Bar-K.Mills.pdf1002 KB
Acoustic Scanner Analysis of Borehole Breakout to Define Stressfield Across Mine Sites in Sydney and Bowen Basins - Stuart MacGregor - Published 2002Published Feb, 2002The role of horizontal stress, its orientation and magnitude, in defining the behaviour of strata in underground coal mines has been well established. Poor panel layouts have led to gate end stress concentrations, roof falls and lost production. The ability to define the horizontal stress regime over a mine site has historically been limited to point measurements, in part due to technology and cost. Recent advances in the application of geophysical tools, notably the acoustic scanner (borehole televiewer) have resulted in a new technique to conduct stress measurements. By quantifying the nature of borehole breakout and the mechanical properties of rocks in which they occur, this technique provides the ability to:
• obtain a vastly greater number of measurements, both at different depths and spatial distribution, than other techniques such as overcoring or hydraulic fracturing
• readily obtain depth versus stress relationships
• define geotechnical domains on the basis of stress direction and in-situ stress magnitude for mine planning purposes
This paper presents an overview of the technique and presents case histories in its application at a mine site in the Sydney Basin, Australia. Acoustic-Scanner-Analysis-of-Borehole-Breakout-to-Define-Stressfield-Across-Mine-Sites-in-Sydney-and-Bowen-Basins-S.MacGregor.pdf666 KB
Definition of Stress Regimes at Borehole Mine and Regional Scale in the Sydney Basin through Breakout Analysis - Stuart MacGregor - Published 2003Published Feb, 2003The role of horizontal stress in affecting strata behaviour in underground coal mines has been well documented (Siddal and Gale1, Hebblewhite2, Mark3). In Australia, the nature and depth of the underground coal resources has resulted in high levels of horizontal stress, typically 2-3 times the vertical stress, and up to 9 times that expected by lithostatic burial. Horizontal stress impacts on all facets of strata behaviour, and is a fundamental input into the geotechnical design process.
Borehole breakout analysis, particularly using high resolution acoustic scanner images, provides the ability to collect large data sets that have significant depth and spatial coverage. In real terms this provides the ability to investigate a range of stress phenomena at different scales, and assess the factors controlling in situ and mining induced stress regimes.
This paper highlights a range of stress phenomena that have been observed through breakout analysis in the Sydney Basin and outlines the impact these have on underground mining operations. Definition-of-Stress-Regimes-at-Borehole-Mine-and-Regional-Scale-in-the-Sydney-Basin-through-Breakout-Analysis-S.MacGregor.pdf1.1 MB
Investigation into Abnormal Increased Subsidence above Longwall Panels at Tahmoor Colliery NSW - Winton GalePublished Oct, 2017Tahmoor Colliery, located in the Southern Coalfield of NSW, has been in operation for over 30 years. Longwall extraction of 23 successive longwall panels has occurred with subsidence occurring within predictions and resultant impacts to natural and built environments occurring to expectations.
Subsidence over a recent longwall panel of approximately twice that previously measured occurred at Tahmoor Colliery. An investigation of the potential causes was conducted using computer modelling together with hydrological characterisation and detailed geotechnical characterisation of the strata.
The abnormal subsidence was found to be consistent with localised 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 not sufficiently induce the abnormal subsidence.
The results have significant implications to subsidence prediction in areas which 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-Increased-Subsidence-above-Longwall-Panels-at-Tahmoor-Colliery-NSW-W.Gale.pdf2.4 MB