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.
Geotechnical aspects of the Pike River mine drift recoveryPublished Feb, 2021The Pike River mine exploded on the 19 November 2010. Thirty-one (31) men were working underground at the time of the explosion and only two men were able to escape. The Pike River Recovery Agency was established in January 2018 to conduct a safe manned re-entry and recovery of the Pike River mine drift to gather evidence to better understand what happened in 2010.
SCT Operations Pty Ltd (SCT) have been engaged to assist with the management of strata control hazards as part of the planning and implementation phases of the drift recovery. Initial geotechnical assessments comprised review of available historical geological and geotechnical information to develop a geotechnical baseline report and hazard map to assist with future planning and risk management.
A range of controls have been implemented to manage geotechnical risk to acceptable levels and to ensure that adequate levels of inspection, mapping, monitoring, assessment, and review are maintained at all stages of drift recovery. Additionally, 3D FLAC modelling, surface tunnelling simulations and field loading trials have been conducted to support proposed tunnelling through a Rocsil plug located at the top end of the drift to provide access the rock fall area which marks the end of the mandated drift recovery. Given most of the drift had not been physically inspected following the explosion a range of drillhole assessments comprising downhole camera and laser scanning was also conducted to improve understanding of the drift environment both prior to and during re-entry.
As part of operational implementation and continuous improvement processes modifications were made to both ground support systems and bolting equipment which significantly improved support cycle installation times.
SCT also supplied real-time roof monitoring instrumentation to the site which supplies an almost continuous data feed to the mine control room for interpretation and automatic alerting if TARP threshold levels are exceeded.
Dynamic model of fault slip and its effect on coal bursts in deep minesPublished Feb, 2021The success of deep mining operations relies upon controlling the fractured ground. It is a documented knowledge that many coal bursts occur when mining close to the existing faults. Gradual stress relief towards excavations and other mechanisms can unload stress normal to the nearby fault plane causing it to slip. The generated seismic waves impact the mine roadway rib sides and can produce a coal burst. As part of the ACARP project, the FLAC3d dynamic numerical model was used to show how a fault slip at various locations and orientations may initiate a coal burst. This study simulates an artificial fault slip with peak velocity reaching 4m/s in 0.013 seconds and displacing 119mm in total. Seismic induced peak particle velocities in rock and its influence on coal rib stability were investigated. 89 numerical models with various fault locations and orientations at 450m depth indicated that a 4 tonne coal block can be ejected from the mine roadway rib side at speeds of up to 5m/s. The important finding is that irrespective of the fault slip magnitude, the fault geometry and the in-situ stresses enable to predict which side of the mine roadway may experience the coal burst. Instructing the mine personnel to use the other side of the roadway may improve their safety. Overall, this research produced preliminary results to prove that this method can be used to flag the coal burst dangers for certain fault locations and orientations in deeper mines irrespective of the fault slip properties that are typically difficult to predict. Dynamic-model-of-fault-slip-and-its-effect-on-coal-bursts-in-deep-GV.pdf933 KB
Dynamic analysis of fault slips and their influence on coal mine rib stabilityPublished Feb, 2020Historical data indicate that in deep coal mines the presence of faults in close
proximity to excavations affect the frequency of coal bursts. A number of researchers have
attempted to correlate the fault geometries to the frequency and severity of coal bursts but
dynamic numerical modelling has not been used to show how faults can affect coal ejection
from the rib side. The dynamic numerical analysis presented here show how different
orientations of fault slips may affect coal bursts. To prove the concept, 89 cases of slipping fault
geometries were modelled using the FLAC3D software and their effect on rib stability
investigated. The results indicate that there is a simple and logical correlation between the fault
location, its slip velocity and the ejection of the yielded coal rib side. The seismic compressive
wave generates rock/coal mass velocities that directly impact the rib side. If the coal rib is
relatively disturbed and loose, these velocities can cause its ejection into the excavation. The
slip direction typically impacts one side of the mine roadway only. A 1 m thick loose coal block
attached to the 3 m high rib side in mine roadway was ejected at speeds ranging from 2.5 to 5
m/s depending on the fault location, its orientation and the maximum fault slip velocity modelled
at 4 m/s. Dynamic-analysis-of-fault-slips-and-their-influence-on-coal-mine-2020-GV.pdf1.6 MB
Dynamic events at longwall face, CSM Mine, Czech RepublicPublished Feb, 2020Presented here are the details of the seismic events that occurred at longwall 11
located at the CSM mine in the Ostrava coal region, Czech Republic. This longwall was
excavated in a very complex area located within the shaft protective pillar and adjacent to the
50 m wide and steeply inclined fault zone at a depth of 850 m. In addition, 10 longwalls were
extracted below each other over many years in several sloping seams located on the other side
of the large sloping fault zone resulting in complex stress fields and large subsidence. The
immediate roof above longwall 11 was a very strong sandstone and sandy siltstone with a
uniaxial compression strength of 80 – 160 MPa. When the longwall started, continuous seismic
monitoring of the longwall area indicated 470 small seismic events with energy smaller than
<102 J. The first high energy event of 3.3*105 J occurred when the longwall advanced 85m past
the starting line. Some 30 minutes later a rockburst occurred registering energy of 2.2 *106 J,
causing significant rockburst damage at the tailgate located near the large tectonic zone. The
roadway steel arches were significantly deformed and the maximum floor heave reached up to
1.5 m. To investigate the complex strata behavior in that area, a large FLAC3D model 0.27 km3
in volume was constructed and 10 longwalls were extracted in several sloping seams adjacent
to the large fault zone. The model under construction is now ready to study the complex strata
behaviour and the associated stress fields together with the dynamic strata behaviour to match
the modelled seismic events with those measured underground. Dynamic-events-at-longwall-face-2020-GV.pdf1.8 MB
Numerical model of dynamic rock fracture process during coal burstPublished Feb, 2020Coal bursts present one of the most severe hazards challenging the safe
operations in underground coal work environments. In Australia, these events are becoming
increasingly frequent as coal measures are mined progressively deeper. This study is
supported by the Australian Coal Association Research Program (ACARP) which aims to better
understand the phenomena of coal burst. In this paper the dynamic fracture process of coal
bursts was successfully simulated in the coal roadway. This was achieved using dynamic
analysis utilising DRFM2D routine by Venticinque and Nemcik (2017) in FLAC2D (Itasca, 2015)
which complemented previous study observations by Venticinque and Nemcik (2018). This is
significant because until now the evolving dynamic rock fracture process during coal burst
remained unknown. Additionally, coal/rock burst events were shown from simulation as being
largely driven by the propagation of shear fractures from within the rib. This was demonstrated
to produce effect forcing the dynamic conversion and release of potential energy stored as
compressive strain in the rib into kinetic movement of the entire rib section. This entire process
was shown to occur very fast taking approximately 0.2 seconds for a coal burst to fully establish,
with ejection of several meters of rib at a velocity of 1.6 m/s produced in the model of an
underground coal roadway having 550 m depth of cover.