Further insights into the mechanics of multi seam subsidence from Ashton Underground MinePublished Feb, 2021Ashton Underground Mine (Ashton) is an underground longwall mine located northwest of Singleton in the Hunter Valley of NSW. The mine has so far extracted longwall panels in three seams with mining in a fourth seam planned and each seam progressively deeper than the last. The mining geometry in each of the seams is regular, parallel and either offset or stacked relative to the panels in the seams above. A subsidence line crossing all panels in each seam has been regularly surveyed in three dimensions since the commencement of mining. The high quality data set available from this line provides insight into the mechanics of ground behaviour in a multi-seam environment. This paper presents an update of the observations and interpretation presented in Mills and Wilson (2017) for mining in two seams with the inclusion of results from mining in a third seam.
Observations of the characteristics of multi-seam subsidence continue to indicate that although subsidence movements above multi-seam mining are more complex than single seam mining, these movements are nevertheless regular and predictable. In an offset geometry, remote from pillar and goaf edges, tilt and strain levels are similar or lower than single seam levels, despite the greater vertical subsidence, due to the general softening or reduction in shear stiffness of the overburden with each episode of subsidence. At stacked and undercut goaf edges, transient tilts and strains are significantly elevated.
Cumulative vertical subsidence after longwall mining in three seams has now reached 5.8m with incremental vertical subsidence increasing as a percentage of incremental mining height with each episode of subsidence. Latent subsidence from near stacked goaf edges is recovered when mining in the seam below. A site-specific methodology developed to forecast subsidence behaviour is allowing measured subsidence effects to be estimated reliably.
A Review of the Mechanics of Pillar Behaviour. K.MillsPublished Feb, 2019In recent years, the drive to reduce the impacts of surface subsidence has led to mine layout
designs in New South Wales and Queensland that rely for their effectiveness on the long-term
stability of pillar systems. The University of New South Wales (UNSW) pillar design methodology
has become a benchmark for assessing long-term stability of pillars in Australia. The method is
being applied in a wide range of geological settings and for a broad range of pillar geometries.
Galvin, et al. (1999) warn that the UNSW methodology approach is empirical and only suitable for
the conditions in which the methodology was developed; a warning that tends to be ignored.
The UNSW approach and most other empirical approaches do not specifically consider the
changing characteristics of coal strength or the influence of the roof and floor strata on the ability of
pillars to develop confinement. This paper describes how two independent components of coal
strength continue to give the strength characteristics of coal pillars observed in prac6tice and the
implications for pillar design. A-Review-of-the-Mechanics-of-Pillar-Behaviour-6-KWM-2-1-19.pdf1.5 MB
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 the Interaction between Ground Deformations and Groundwater above an Extracted Longwall Panel - Ken Mills - Ben BlackaPublished Nov, 2017This paper presents the results of a field measurement program aimed to measure the interaction of groundwater and mining induced ground deformations above a sub-critical width longwall panel in a series of panels, two decades after mining. Three cored holes were drilled from the surface above the centre of a longwall panel down towards the highly fractured zone known to exist just about seam level. Observations including lithology, jointing, mining induced fracturing, groundwater flows and measurements of various hydrogeological parameters were made while the boreholes were open. The holes were then fully grouted and vibrating wire piezometers installed to measure the equilibrium piezometric profile.
The results of this program provide correlation between the experience of ground deformation monitoring and the experience of groundwater monitoring. These results provide a basis to develop groundwater models to faithfully represent the interactions between groundwater and mining induced ground deformations. Experience-of-Monitoring-the-Interaction-between-Ground-Deformations-and-Groundwater-above-an-Extracted-Longwall-Panel-K.Mills-B.Blacka-2017.pdf1.9 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
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
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
Displacement Monitoring and Visualisation Using Terrestrial Photogrammetry - Ken MillsPublished Feb, 2018Digital photogrammetry has been to shown to be an effective and efficient method for visualising and measuring subsidence behaviour of sandstone cliff formations. The system described in this paper provides the capability to measure subsidence movement over areas that are inaccessible to conventional subsidence monitoring techniques. Preliminary comparisons with a more traditional survey technique (total station survey to prisms) has shown mean variations, between the techniques, in vectors of movement of 17 millimetres.
They system described provides visualisation of the sandstone cliff formations through the generation of 3D images which can be rotated to be viewed from any angle. Visualisation provides a powerful tool for measuring, interpreting and understanding the shape, structure and geology. 3D images provide an advantage in tracking vectors of movement of natural features over a large area of inaccessible escarpment.
This paper provides a description of the technique and results obtained to date. The photogrammetry techniques used are based on photogrammetry techniques developed for mine highwall mapping. The results obtained in mapping subsidence exceed expectation for the camera system used. Further development will be undertaken to develop this teqnique for commercial applications in landform stability monitoring.
Through the generation of a large number of vector movements and visualisation a more complete knowledge and understanding of subsidence behaviour of sandstone cliff formations is derived. MSTS-Conference-2001-Displacement-Monitoring-and-Visualisation-Using-Terrestrial-Photogrammetry-K.Mills.pdf2.1 MB
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
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
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
Successful Management Strategy for Mining Adjacent to a Sensitive Natural Feature - Ken MillsPublished Oct, 2017BHP Billiton Illawarra Coal operates Dendrobium Mine in an area 10-20 km west-northwest of Wollongong in NSW, Australia. The mine recently completed mining in Area 3A adjacent to an overhanging natural rock feature known as Sandy Creek Waterfall. Illawarra Coal undertook measures to protect the waterfall and the section of Sandy Creek immediately upstream of the waterfall from the effects of longwall mining using an innovative management process and an array of high resolution monitoring systems. This paper describes the management options that were considered and the processes that were adopted to successfully protect the waterfall from the valley closure effects of mining four adjacent longwall panels in close proximity to the waterfall while continuing to maximise recovery of the coal resource in the area.
The management structure adopted involved a Technical Committee, a Steering Committee, and an external independent reviewer. The Technical Committee comprised senior representatives from Illawarra Coal, three external specialists in rock mechanics and subsidence, and a government observer. The Technical Committee was responsible for design of the monitoring systems, interpretation of the monitoring results, and the provision of recommendations to the Steering Committee suitable to guide decisions on when to cease mining each adjacent panel. The Steering Committee comprised Illawarra Coal management and technical personnel. Although the Steering Committee took advice from the Technical Committee, all decisions relating to mining were made by the Steering Committee. An external reviewer was engaged by the Steering Committee at the end of each longwall panel to review the results, interpretation and management decisions. This management structure was effective in successfully protecting the very sensitive structure of Sandy Creek Waterfall from potential impacts of nearby mining in the midst of active ongoing natural erosion processes. Successful-Management-Strategy-for-Mining-Adjacent-to-a-Sensitive-Natural-Feature-K.Mills.pdf2.2 MB
Developments in Understanding Subsidence with Improved Monitoring - Ken MillsPublished Nov, 2017Ground movements associated with coal mining have been occurring since coal mining was first practiced but the ability to interpret these movements and develop and understanding of the mechanics involved was initially limited by irregular mining geometries and the vagaries of pillar behaviour. The introduction of longwall mining to Australia with its regular geometries, full extraction, and single seam extraction has provided opportunities to eliminate many of the mining related variables that are present in pillar extraction operations and so provide a much more controlled environment in which to conduct measurements and develop understanding of the mechanics of overburden caving and subsidence processes. The understanding of these processes has developed as a result of improvements in surveying and monitoring techniques and the application of these techniques to satisfy the requirements of regulatory authorities in response to changing community expectations. This paper presents as overview of the developments in monitoring technique for characterising subsidence and sub-surface ground movements and the developments in understanding of subsidence related ground behaviour that have been possible as a result. Developments-in-Understanding-Subsidence-with-Improved-Monitoring-K.Mills.pdf690 KB
Experience of Monitoring Subsidence at Ulan Coal Mine - Ken MillsPublished Nov, 2017The 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. Experience-of-Monitoring-Subsidence-at-Ulan-Coal-Mine-K.Mills.pdf225 KB
The Effects of Mining Subsidence on Rockbars in the Waratah Rivulet at Metropolitan Colliery - Ken MillsPublished Nov, 2017Metropolitan Colliery is currently longwall mining below a section of Waratah Rivulet in the Woronora water supply catchment. The effects of mining subsidence on the quantity and quality of water and the ecological integrity of the rivulet are of interest to the Sydney Catchment Authority. The Colliery has undertaken a significant program of work aimed to assess the likely impact of mining on the rivulet. The program includes subsidence monitoring, instrumentation to monitor the effects of mining subsidence on rockbars, flow and water quality monitoring and monitoring of vegetation, aquatic ecology and archaeological sites.
This paper describes the results of subsidence and rockbar monitoring up to the completion of Longwalls 9 and 10. The-Effects-of-Mining-Subsidence-on-Rockbars-in-the-Waratah-Rivulet-at-Metropolitan-Colliery-K.Mills.pdf3.9 MB
Pillar Design to Control Subsidence at Moonee Colliery - Ken MillsPublished Jul, 1998Moonee 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. MSTS-Conference-1998-Pillar-Design-to-Control-Subsidence-at-Moonee-Colliery-K.Mills-1998.pdf271 KB