Abstracts

A list of abstracts is provided below.

Pillar failure database (A method for estimating surface subsidence due to unplanned coal pillar system failures in the NSW coalfields)

Subsidence events due to unplanned coal pillar system failures are relatively uncommon when compared to other unplanned subsidence mechanisms associated with shallow coal mine workings. Whilst these events are typically infrequent, they have the potential to cause significantly greater economic loss and damage to surface infrastructure due to their much larger areal extent when compared to sinkholes, pitfalls or plug failures.
Due to their infrequent, unplanned nature, the compilation of a useable database of events containing measured subsidence effects from which future predictions are then able to be made, is challenging. This has resulted in potential subsidence impacts being estimated using empirical databases derived from either longwall or pillar extraction subsidence data, generally with some modification to account for remaining coal pillar volume.
This paper introduces a database of unplanned pillar system failures within the Newcastle, Tomago, Greta, Hunter and Lithgow Coal Measures of New South Wales, compiled using historical subsidence data collected by the NSW Department of Mines, NSW State Archives, individual collections retained by historical societies, libraries and private historical record collections, as well as previous publications and case studies within NSW.
An empirical method is proposed in which subsidence effects can be estimated for a variety of mining and geological conditions. This is accompanied by observations on factors that appear to influence the initiation of unplanned pillar system failure events in historical mine workings. The characterisation and behaviour of an unplanned event, in comparison with subsidence due to longwall or other high extraction mining methods, is also discussed.

Subsidence engineering in the early development of the northern coalfield, NSW

In contrast to early recognition in European and British coalfields, subsidence engineering as a distinct discipline involving elements of civil/structural engineering as well as geology and mining engineering is a far more recent concept in Australia, with the first true delineation occurring in the early 1970’s in both industry and government. Whilst not treated as a distinct discipline, the various engineering professions that existed in NSW prior to the 1970’s commonly interacted with each other in the management of subsidence for various purposes, typically with different priorities and usually with very different acceptable levels of risk tolerance.
This paper outlines ways in which risks associated with surface subsidence were managed in the early NSW coalfields. Both proactive and reactive ways in which subsidence was managed through early strata control and mine design are described, as well as methods used to mitigate the risk of damage in the design and planning of surface infrastructure. The results of an analysis of several components of the pillar system used in early coal mines is presented to illustrate how early methods of mine design and strata control were applied.

Successful Implementation of Mine Design to Control Subsidence caused by Strata Softening above Pillars – Airly Mine, NSW

Airly Mine experienced subsidence levels up to 700mm that was found to be caused by softening of the strata above pillars between miniwalls. A new mine design was required to limit subsidence to 100mm in the new mining area under Mt Genowlan. The design approach included numerical modelling to simulate rock failure about various mine geometries. The aim was to design the pillar widths to limit strata softening above the pillars between miniwalls so that the pillar compression subsidence was elastic. This paper presents the design approach and subsidence results after successful extraction of the first 4 miniwall panels in the new mining area.

Mining Induced Hydraulic Conductivity above Longwall Panels – a New Approach of Estimating the Relative Hydraulic Conductivity

This paper presents the outcomes of an ACARP project that provides a methodology of estimating mining induced hydraulic conductivity changes above longwall panels for input into groundwater models. There is currently a gap between geotechnical model fracture conductivity at longwall panel detail and groundwater model bulk conductivity input requirements. Investigations into upscaling methodologies have provided changes to mining induced hydraulic conductivity of only about 1 order of magnitude, not the minimum 3-5 orders of magnitude lower hydraulic conductivity that the groundwater modellers use. The outcome of this research is a proposed hypothesis that desaturation is responsible for the significant reduction in relative conductivity. This paper presents a method for determining the site specific relative conductivity to use in groundwater models, based on the limiting conductivity layer at the top of the desaturated zone, using two mine site examples.

Multi-seam subsidence investigation

Anglo American’s Aquila Mine is located near Middlemount in the Bowen Basin of Queensland. The mine is extracting coal by longwall method in the Aquila Seam, which lies approximately 110 to 120 metres above the German Creek Seam, which has previously been extracted by longwalls at Grasstree Mine.
In order to determine the feasibility of extracting future longwalls directly beneath an overland conveyor, Aquila Mine conducted a mine subsidence study to provide detailed information on the nature, magnitude and development of mine subsidence movements. This paper summarises the findings of the subsidence study.

Managing potential subsidence impacts on high voltage transmission line

Anglo American’s Grosvenor Mine successfully mined directly beneath Powerlink’s high voltage transmission line. This paper describes the investigations, selection and implementation of risk controls and experiences observed when mining directly beneath the transmission line.

Post Mining review of Longwall Mining beneath Small to Medium Commercial and Industrial Properties

In 2019 Tahmoor Coal completed the mining of a series of 3 longwalls directly beneath the Picton Industrial Area. The Picton Industrial Area comprises over 100 small to medium businesses from self-storage buildings and commercial offices to heavy industrial fabrication and machining factories containing precision machining equipment and overhead gantry cranes and large storage hoppers in concrete batching plants.
This paper is a follow up to the paper delivered in 2022 and summaries the impacts of subsidence and the processes involved in managing the repair and/or compensation to business owners.

Managing the impacts of LW subsidence on mine and surface infrastructure

In October 2022, Tahmoor Coal began longwall mining directly below the mine site with subsidence of up to 1.0m predicted for surface facilities, including a total of 142 building structures, tanks and dams.
This presentation discusses the monitoring and engineering mitigation strategies for high-risk structures including, overhead and underground coal conveyors associated tunnels and structures, drift portal, winders, overhead gantry cranes and monorails and mine buildings and general structures. Discussion includes;
Comparison between predicted versus actual subsidence in terms of movement of critical infrastructure.
Unpredicted consequences to mine structures and adjacent gas and water pipelines, school buildings and paths and the main southern railway.
Success of mitigation strategies in particular concrete underground tunnels, overhead conveyors and large coal storage bins.
Operational and productivity impacts from mining on Tahmoor Coal Infratructure.
The presentation will also include the management of subsidence impacts to service and utility supply to the mine and significant innovations in managing the adjacent Main Southern Railway (maybe separate paper?).

Destressing a gas main caused by non-conventional mine subsidence using a conventional mitigation

Tahmoor Colliery is located approximately 80km southwest of Sydney in the township of Tahmoor, NSW, and it is managed and operated by Tahmoor Coal which is owned by SIMEC Mining Division of the GFG Alliance group. Since 1980 the mine has produced coking coal for export and domestic use in steel production. Tahmoor Coal have mining development approval to extract coal south of the mine site towards Bargo.
The Tahmoor South longwalls undermine properties and infrastructure such as roads, the Main Southern Railway Line, power, water, communication cable and gas supply pipelines. As part of the requirements in the mine subsidence management plan, Tahmoor Coal engaged Worley to carry out an investigation of the mine subsidence impact on the structural integrity of the Jemena’s DN150 steel high pressure gas main which is buried alongside Remembrance Driveway.
Worley performed a series of stress analyses of the gas main subjected to the predicted conventional subsidence, and non-conventional subsidence due to the presence of a hidden creek. When assessing the impact of non-conventional subsidence, idealised ground movements such as a sharp vertical shear and ground compression were applied in the model. The analysis found that the gas main can withstand the predicted conventional subsidence but the pipe stress could exceed the code allowable stress limit for the abrupt compression ground strain or closure – i.e. the non-conventional subsidence. The trigger level was determined such that the appropriate mitigation can be implemented when the ground movement survey data reached that level.
During mining of the 2nd and 3rd longwall panels, compressive ground strain at three locations were approaching the trigger level. In consultation with and approval from Jemena, Tahmoor Coal successfully decoupled the gas main from the ground by excavating a 50m long trench at the affected locations, allowing the exposed pipeline to deflect laterally to relieve the compressive stress. The position of the exposed pipeline was monitored during the passage of the longwall below and beyond. From the pipe deflected shape and operating condition, the calculated stress was found to be reduced to well below the allowable limit. The trenches were subsequently reburied when there was no further ground movement.
This paper presents the process of managing and mitigating the non-conventional subsidence impact on the gas main. Some interesting results and lessons learned will be highlighted.

Picton Weir – Should it stay or should it go

Picton Weir (aka Barton Dam or Barton Weir) is a heritage-listed mass concrete gravity arch structure initially constructed in 1899. The crest of the weir was raised in 1910 and 1947 resulting in a final crest height of 13m at the deepest section. The weir is located in Barton River just downstream of the confluence with Hornes Creek, and is about 3.7km northwest of Bargo, NSW. It is jointly owned by Wollondilly Shire Council and Wingecarribee Shire Council. The weir is no longer used for water supply or recreation. According to Dam Safety NSW, it is not a declared dam.
Tahmoor Coal, owns and operates Tahmoor Mine, received approval for an extraction plan for longwalls in the Tahmoor South domain. These longwalls are located between Tahmoor’s surface facilities to the north and the township of Bargo to the south. They are not directly below Picton Weir and the closest distance to the weir is about 605m. As the weir is located outside the predicted limit of subsidence, it is predicted there will be negligible conventional subsidence movements. However, there could be non-conventional valley closure and upsidence movements that could potentially impact the weir.
Tahmoor Coal engaged Worley to investigate the performance of the weir subjected to different modes of idealised valley movement in a 3D structure model. The extend of concrete damage caused by increasing valley movement was simulated, and a trigger level before partial or complete failure of the weir was estimated. Some mitigation concepts were suggested to stabilise the weir.
A risk assessment was conducted to identify the hazards, the likelihood of occurring and the consequence due to mine subsidence impacts on the weir. One outcome of the assessment was to perform a dam break study in the event the weir collapsed as a result of the non-conventional valley movement. The study informed stakeholders the population at risk (PAR) and potential loss of life (PLL) downstream of the failed weir. It found that the total and incremental PAR and PLL are very low for the flood scenarios considered.
This paper highlights the structural analysis of the weir subjected to the complex valley movement, and the flood inundation analysis should the weir failed during a sunny day event and an extreme flood event. The analyses findings helped to decide whether the weir be strengthened or demolished partially or completely.

Tailoring monitoring to get optimum outcomes

It is easy when planning a project to predefine what you think you require out of your standard box of tricks. Often when you get near the end, with hindsight, you wish you had done something different and had more information from the beginning. This happens to varying extents with every project and by reviewing different projects and keeping an open mind, it will minimise your hindsight “I wish I had done that” moments.
Victoria Bridge over Stone Quarry Creek in Picton is a historic timber bridge on the edge of the mining impact zone, that was exposed to a large closure beyond predictions. The pre-mining investigation showed the bridge could handle a large amount of closure with minimal predicted, so survey marks and a GNSS were installed. When the movement started the monitoring had to be expanded with what could be done quickly. Once the rate of closure reduced and the focus shifted to remediation, we needed to understand how the bridge had moved so the optimum remediation plan could be developed. Using a combination of monitoring data, photographs and forensic investigation the behaviour could be established. In hindsight additional monitoring would have been beneficial. In contrast mining beneath the Tahmoor Collery stockpile had lots of planning, but access restrictions of a working stockpile limited the monitoring that could be installed. The stockpile is a continually varying environment which loads the trestle legs close to the limits potentially everyday. Mining added to the stresses in the stockpile legs, which had to be continuously managed to prevent them being overloaded. Whilst additional monitoring would have been beneficial some of the extras that were installed at the last minute where crucial in understand and managing the stockpile during mining.
Working on these and associated projects a number of challengers had to be overcome, ranging from sensor installation to data analysis. A brief summary of some of the concepts that can be easily adapted to other projects will also be presented.

Statistical analysis of ground monitoring data

Extensive ground monitoring data has been collected from underground mining in the NSW coalfields. This data includes traditional survey lines and more recent monitoring methods including Global Navigation Satellite System (GNSS) and tiltmeters.
Much of this ground monitoring data and corresponding mining information have been compiled into a relational database. Statistical analyses of the raw monitoring data allow better understanding of mine subsidence movements and the refinement of predictions. Confidence intervals can be established for the predictions allowing a better understanding of potential risk to surface features.

Subsidence modelling of unplanned pillar systems failure in Newcastle

Investigation and Remediation of Historical Multi-Seam Coal Mine Workings During an Active Subsidence Event, Lake Road Elermore Vale

Historical underground coal mining has been undertaken in the Newcastle, Lake Macquarie and Hunter Valley Regions since the early 1800s with a large proportion of these areas now developed on the surface above the abandoned coal mine workings. This paper describes a trough subsidence event at a site in Lake Road, Elermore Vale, which is undermined by workings in the Dudley Seam and Borehole Seam at depths ranging from 54 m to 80 m and 84 m to 110 m, respectively.
Surface deformations due to mine subsidence were first observed in July 2022, and measurements over the following weeks confirmed that an active event was in progress. The maximum surface subsidence ultimately estimated to be in the order of 0.55 m, with the land area subjected to more than 100 mm of subsidence being approximately 200 m across.
This paper presents the methods and results of geotechnical investigations to assess the likely mine subsidence mechanism responsible for the event. It also describes the grout remediation strategy and methodology that was undertaken concurrently with the investigation as an intervention to arrest the active subsidence event.

Subsidence due to groundwater induced goaf consolidation

In NSW, primary and residual subsidence associated with longwall or pillar extraction operations is both well-defined and able to be predicted with a reasonable level of confidence, with a substantial empirical database available on which to base predictions on.
Whilst generally uncommon, experience in European coalfields suggests a sharp rise in the number of unplanned subsidence events occur following the cessation of active dewatering, with the associated reestablishment of the groundwater regime impacting pillar system stability as well as increasing the likelihood of shallow subsidence mechanisms such as sinkholes, matching observations made in NSW. Additional subsidence associated with the reestablishment of the groundwater regime has also been observed in areas of full extraction, but as yet is not described in Australian subsidence literature.
This paper summarises additional subsidence associated with the reestablishment of groundwater table in areas of Western Newcastle, derived using remote sensing methods and confirmed using ground survey. Four examples of this phenomenon are described, occurring in areas where previously extracted longwalls are present as well as in areas of previous pillar extraction. A simple relationship between primary subsidence measured during active longwall/pillar extraction and the measured subsidence occurring due to reactivation of the goaf area by the reestablishment of the groundwater regime is proposed.

Case Study: How technology can assist the investigation of coal mine subsidence in historically mined areas

A number of challenges exist when investigating unplanned mine subsidence in areas where mining was completed over a century ago. Obtaining information on critical factors used in risk assessment and in the design of potential stabilisation/remediation works generally includes surface survey and site mapping as well as an investigative drilling program. Drilling investigations aim to obtain the general mine layout, mined height, present condition of the remnant void space and the estimation of potential closure between the roof and the floor. Whilst on the surface, mapping of subsidence features as well as the estimation of the magnitude of subsidence (both horizontal and vertical displacements) is completed to define the potential area affected and to optimise the staging of the drilling program.
The number of assumptions made about these critical factors can be reduced with the assistance of technology, increasing confidence in initial assessment on the likelihood of additional subsidence occurring in the short term.
This paper discusses remote sensing technologies that were used during the investigation and stabilisation of three recent mine subsidence events in NSW. Historical LiDAR records were used to compare with post subsidence ground survey to determine the magnitude and extent of the subsidence events, whilst 3-Dimensional laser scanning and traditional downhole sonar was used to map the layout of the former coal mine and obtain information on the present condition of the workings. InSAR data was used to confirm a progressive reduction in subsidence velocities surrounding an area that was rapidly stabilised by mine grouting, with the resulting data used in the design of additional stages of the stabilisation program.

Remediation of Historical Mine Subsidence in Lambton, NSW: A Case Study

Post-mining subsidence poses a risk to urban areas constructed above abandoned coal mine workings. Subsidence can cause significant impacts to surface improvements. In this paper, the occurrence of historical mine subsidence within an urban area over the duration of a decade, its confinement (over the study area), investigation techniques and remediation at Lambton Gardens, Lambton, NSW, has been investigated as a case study.
As per the mine record tracing, the site is underlain by abandoned workings in the Borehole Seam from which coal was extracted using the bord and pillar method. Pillar extraction has occurred in some areas. The area surrounding the site is known to have experienced surface depressions or pothole formation prior to urban development. Three subsidence events were triggered within the study area in 2012, 2014 and 2024. During this period, a number of investigation techniques were employed which included ground survey, drilling of about 25 geotechnical investigation boreholes, about 80 grout production bores, grout verification bores, geophysical logging and downhole laser measurement and CCTV camera inspection. Results of the study indicate that both the maximum subsidence and the rate of subsidence varied between events. The post remediation survey data has been used to evaluate the remediation of mine subsidence. This study highlights the need for using proper investigation techniques and timely response to prevent unwanted consequences due to mine subsidence.

Advances in Autonomous 3D Position Monitoring and Multi-sensor Integration

At the 2022 Mine Subsidence Technical Society (MSTS) Geomatix presented a paper documenting the development of a simple, affordable, robust GNSS based three-dimensional (3D) position monitoring system and it’s applications for mine subsidence monitoring.
Building on our Legacy – three years is a long time in the ‘tech world’ and during this time the Geomatix GNSS based monitoring system has undergone considerable further development. 3D position accuracy has been improved to the point where it rivals manual Total Station based monitoring methodologies (± 1 – 2mm), satellite based telemetry systems have meant connectivity can be maintained outside of the 4G / 5G cellular network coverage and, system reliability for years of completely autonomous use has improved significantly.
This paper details the ongoing development of the system, with an emphasis on accuracy improvements as well as form factor refinements for structural / localised applications. Further to this, the paper describes how additional sensors, such as tiltmeters / accelerometers, potentiometers, weather stations, etcetera can be integrated to continuously augment the 3D position data. To highlight how the use case for an integrated system a case study will be presented where the near real-time 3D position data is augmented with bi-directional tiltmeters on a network of power poles that are being undermined.

Assessment of blast impacts on natural geological features

The damage to Junkan Gorge in the Pilbara in 2020 sent shockwaves through the community, miners and regulators. Since that event there has been a renewed focus on blasting impacts on natural features such as caves, rock overhangs and steep slopes.
Currently there are no widely recognised guidelines for assessing what level of blasting impacts are acceptable for these type of features. Consequently, there are a number of approaches that are being adopted to assess what levels of blasting impact may be acceptable. This includes case studies, guidelines on vibration impacts on anthropogenic structures, and theoretical limits of geological capacity.
This paper examines these approaches and suggests an approach based on case studies, theory and other guidelines. Both vibration and air blast impacts are assessed as well as the sensitivity of the natural feature type and geological conditions to such impacts.

Duration of mine subsidence events

This proposed paper examines the timeframe of several mine subsidence events to assist with the engineering design of structures. Engineers are being required to design for high ‘safe’ subsidence parameters based on a worst- case event, although this occurs over time.

Groundwater system response to mining at NSW Western District mines.

This paper compares effects on groundwater systems of different underground coal mining methods and extraction void dimensions in the Western Coalfield of New South Wales. Case studies from Clarence and Springvale mines are presented to explain variability in groundwater response in similar hydrogeological environments.
The paper presents case studies based on up to 19 years field monitoring data from Clarence and Springvale mines, including:
groundwater behaviour in the context of mine subsidence interaction with geological faults and aquitards.
review of measured groundwater behaviour against theoretical models for subsidence geotechnical zones and groundwater depressurisation
measured limits of influence of mine subsidence on groundwater systems
near surface aquifer water level response to mining
mine water inflow patterns

Mine Subsidence Sinkhole Repair, Blue Mountains, Australia

Mine subsidence sinkholes are observed to occur in the Lithgow area, west of the Blue Mountains, NSW, due to extensive underground bord and pillar coal mining undertaken throughout the area from the mid-19th Century.
In some areas, the underground coal mining is at shallow depth of cover and extends under Lithgow City Council assets, such as roads and road reserves. During late 2022, after a period of protracted rainfall, a relatively large mine subsidence sinkhole was identified on the downslope side of Browns Gap Road. The sinkhole was 15m diameter and 5m deep, with the upper edge extending into the road shoulder, and associated pavement cracking observed in the trafficked, northbound lane of the road. At this time the road was closed due to unacceptable risk associated with landslide and rockfall hazards present at the site, also attributed to the period of extended rainfall. Repair of the sinkhole was required as a priority to safely allow Lithgow Council to proceed with urgent work aimed at reducing the risk of these hazards. Subsidence Advisory, NSW requested that WSP provide advice to them on sinkhole repair.
Repair of the sinkhole to restore support to the road required consideration of a number of safety concerns during construction whilst still achieving an acceptable technical outcome. The sinkhole was located on a heavily vegetated, steeply sloping embankment with no vehicular access other than from above. At the road level above the sinkhole, review of the mine record tracings indicated uncertainty in the exact location and depth of cover of the mined seams under the road reserve. Access to the site and work methods were also restricted due to the risk posed by rockfall hazard from the upslope cutting and slopes yet to be remediated. These combined stability concerns restricted the size and position of construction vehicles and work sites, and the remoteness of the site limited the availability of backfill materials.
The paper presents WSP’s practical and collaborative approach and lessons learned from working with Subsidence Advisory and local contractors to safely and cost-effectively backfill the mine void, which allowed Lithgow City Council to proceed with repair and management of the remaining slope hazards affecting Browns Gap Road, and ultimately re-open the road to the public under a Council managed access plan. For other sites with similar geological conditions and depth of cover above historic mine workings, this case study highlights the need for consideration of site-specific constraints in construction methodology, with recommendations for similar projects in the future.

Grouting for subsided rock mass permeability reduction

Grouting remediation works have been carried out as part of a rehabilitation management plan to target permeability reduction associated with near-surface deformation resultant from mining-induced valley closure movements. Shallow pattern and curtain grouting techniques utilising the injection of polyurethane (PUR) products was undertaken to reduce the permeability of the subsided rock mass by filling voids. The technique resulted in the reduction of subsurface flow pathways promoting surface flow and pool holding capacity.
Ground characterisation works were undertaken to inform the remediation program including drilling, geotechnical logging and geophysical logging of boreholes. Packer testing was carried out to provide a general indication of hydraulic conductivity of the subsurface strata. Remediation grouting involved construction of low permeability barriers below the surface by injection of PUR under pressure. Boreholes were drilled along predetermined alignments across targeted rockbar/pool locations. Post-grouting performance monitoring was carried out either via packer testing within the targeted areas or through installation of water level monitoring bores.
At the conclusion of each grouting campaign, post-assessment works were undertaken to evaluate the change in permeability within grouting alignment and aesthetic surface works were completed through remedial concreting and sparging of borehole collars and adjacent cracks within the target areas. This paper summarises the results of the investigation, assessment and remedial grouting approach undertaken at a number of rockbar locations.

Other Titles:

Thick seam mining

Grouting of a steeply dipping Greta Coal Seam

Designing for mine subsidence

Updated Estimates of Grout Volumes for Subsidence Mitigation for Various Conditions.

Height of fracturing

Dendrobium heritage assessment

Experience of Using a Slot to Protect a High Value Aboriginal Heritage Site at Moolarben – need to confirm with Alex or Liam if OK to present.

Investigations and monitoring of sandstone features to manage protection consistent with approval conditions – need to confirm with Ulan OK to present

Using subsidence monitoring to estimate frictional properties of bedding plane shears.