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
Experience of Using the ANZI Strain Cell in Exploration Boreholes to Determine the Three Dimensional Stresses at Depth - J.Puller, K.MillsPublished Sep, 2018This paper describes recent use of the ANZI (Australia, New Zealand Inflatable) strain cell and the
overcoring method of stress relief in exploration boreholes to determine three dimensional in situ
stresses at depths approaching 1km in a one-day operation. The results from each of the various stages
of a routine overcoring operation are described to illustrate the information each step can provide. The
results from an Australian site is presented to illustrate the opportunities to characterise the three
dimensional in situ stress environment when multiple high confidence measurements are achieved.
The ANZI strain cell is an instrument system that uses the overcoring method of stress relief to determine
the three dimensional in situ stresses in rock. The instrument has been used successfully for over three
decades in numerous underground mining and civil projects, but technical advances over the last
decade have allowed the system to be deployed routinely in surface exploration boreholes. Recent
development of a downhole high-precision data logger, a wireline enabled drilling system and an
instrument deployment system has simplified the process of obtaining three dimensional overcore
measurements remote from any underground excavation at depths approaching 1km. J.Puller-Experience-of-Using-the-ANZI-Strain-Cell-in-Exploration-Boreholes-to-Determine-the-Three-Dimensional-Stresses-at-Depth.pdf1.3 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
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
In-Situ Stress Measurements and Stress Change Monitoring to Monitor Overburden Caving Behaviour and Hydraulic Fracture Pre-Conditioning - Jesse Puller, Ken Mills, Rob JeffreyPublished Jul, 2015A coal mine in New South Wales is longwall mining 300 m wide panels at a depth of 160–180 m directly below a 16–20 m thick conglomerate strata. As part of a strategy to use hydraulic fracturing to manage
potential windblast and periodic caving hazards associated with these conglomerate strata, the in-situ stresses in the conglomerate were measured using ANZI strain cells and the overcoring method of stress relief. Changes in stress associated with abutment loading and placement of hydraulic fractures were also measured using ANZI strain cells installed from the surface and from underground. Overcore stress measurements have indicated that the vertical stress is the lowest principal stress so that hydraulic fractures
placed ahead of mining form horizontally and so provide effective pre-conditioning to promote caving of the conglomerate strata. Monitoring of stress changes in the overburden strata during longwall retreat was undertaken at two different locations at the mine. The monitoring indicated stress changes were evident 150 m ahead of the longwall face and abutment loading reached a maximum increase of about 7.5 MPa. The stresses ahead of mining change gradually with distance to the approaching longwall and in a direction consistent with the horizontal in-situ stresses. There was no evidence in the stress change monitoring results to indicate significant cyclical forward abutment loading ahead of the face. The forward abutment load determined from the stress change monitoring is consistent with the weight of overburden
strata overhanging the goaf indicated by subsidence monitoring. In-Situ-Stress-Measurements-and-Stress-Change-Monitoring-to-Monitor-Overburden-Caving-Behaviour-and-Hydraulic-Fracture-Pre-Conditioning-Jesse-Puller-Ken-Mills-Rob-Jeffrey-2015.pdf1.8 MB
Experience of Using the ANZI Strain Cell for Three Dimensional In Situ Stress Determinations in Deep Exploration Boreholes - Ken Mills - Jesse PullerPublished Feb, 2014This paper describes the Australia, New Zealand Inflatable (ANZI) strain cell, its operation, and recent development for overcoring in exploration boreholes. The ANZI strain cell is an instrument system that uses the overcoring method of stress relief to determine the three-dimensional, in-situ stresses in rock. The instrument has been used successfully for over three decades in numerous underground mining and civil projects, but technical advances over the last decade or so have allowed the system to be deployed in surface exploration boreholes to greater depths than was previously possible. Recent development of a downhole electronic data logger, a wireline-enabled drilling system, and an instrument deployment system has simplified the process of obtaining three-dimensional overcore measurements at depths approaching 1km to a single shift operation.
The capability to deploy ANZI strain cells in surface exploration boreholes represents a significant breakthrough for the design of underground mines and underground excavations generally. Highconfidence characterisation of the in-situ stresses at the design stage provides the opportunity to design key infrastructure and mining systems to take advantage of the in-situ stress field from the outset before mining begins. Understanding the three-dimensional, insitu stress field not only provides a measure of the magnitude and direction of loads acting within the rock mass, it also provides insight into the mechanics of all the various processes driving ground deformations, including which geological fault structures are at limiting equilibrium. Experience-of-Using-the-ANZI-Strain-Cell-for-Three-Dimensional-In-Situ-Stress-Determinations-in-Deep-Exploration-Boreholes-2014.pdf2.6 MB