The Implications of Chain Pillar Geometries for Chinese Coal Mines - Winton GaleThe retreating longwall method using single entry gateroads is the predominant coal mining method in Chinese underground mines. Leaving only a very small pillar between adjacent panels has become the normal longwall mining practice.
It has been found that, rock bolting can improve the roadway conditions, however the general stability against roadway collapse will be determined by the adjacent goaf fracture geometry and chain pillar strength. With the increasing percentage of gateroads supported solely by rock bolting, there is now a key requirement to reexamine chain pillar design issues and the implications for gateroad support practice in China.
This paper describes the current industry practices and experiences with chain pillar design in China and attempts to define some of the chain pillar design implications for current Chinese practice. Considerations for future design of chain pillars with regard to roadway support practices and overall gateroad stability against major roadway collapses induced by re-mobilisation of adjacent goaf are provided. The-Implications-of-Chain-Pillar-Geometries-for-Chinese-Coal-Mines-W.Gale.pdf1 MB
Investigation Into the Extent and Mechanisms of Gloving and Un mixed Resin in Fully Encapsulated Roof Bolts - Stuart MacGregorEffective strata control, utilising fully encapsulated roof bolts is dependent on the installed quality of the reinforcement elements. One mechanism by which roof bolts may become less than fully efficient is by glove fingering (gloving) and un-mixing of the resin. Following a routine installed bolt quality audit and some small roof failures containing gloved bolts, a work programme was initiated to determine the extent of the gloving and un-mixing problem and to develop an understanding of mechanisms involved. Results have shown that gloving and un-mixing is a systematic and widespread pheno mena, occurring across the range of resin and/or bolt manufacturers, and in a variety of roof types. Gloving was found in bolts installed using either hand held pneumatic or continuous miner mounted hydraulic bolting rigs, under run of mine (ROM) condition s by operators, and under controlled manufactures “best practice” conditions.
The mechanisms involved have been confirmed as being the development of a pressure front as the bolt encounters the resin cartridge and is spun up the hole, which in turn, leads to over -pressurisation and radial expansion of the resin cartridge. The result is an increase in the diameter of the plastic cartridge. Allowing the bolt to be spun up inside the cartridge without making sufficient contact to shred the cartridge or the hardener envelope, typically resulting in a portion of the cartridge enveloping the bolt and unmixed resin mastic and catalyst.
Once the mechanisms involved and extent of the problem became clear, further research was undertaken to assess alternative bolt profiles and modifications in an effort to minimise and/or eliminate the gloving and unmixing phenomenon. Research has been undertaken using recovered bolts from various mine sites, as well as test bench trials and the quantification of the loading characteristics of gloved bolts using strain gauge roof bolts.
To understand the impacts of gloved and un-mixed bolts on roof control, failure pathways and reinforcement requirements a FLAC 2D numerical simulation was undertaken, with the results being incorporated into the strata management plan for a particular operation. Laboratory data has been collected and analysed to assess magnitudes of resin pressure as the bolt encounters the cartridge1 and the effects of gloving and un-mixing on the load transfer characteristics of the resin bolt system. Investigation-Into-the-Extent-and-Mechanisms-of-Gloving-and-Un-mixed-Resin-in-Fully-Encapsulated-Roof-Bolts-S.MacGregor.pdf502 KB
Sand Propped Hydraulic Fracture Stimulation of Horizontal In seam Gas Drainage Holes at Dartbrook Coal Mine - Rob JeffreyPublished Jan, 2004Longwalls 107, 108 and 109 at Dartbrook Coal Mine contained coal with a high gas content and low permeability. Horizontal in-seam drain holes were found to have low gas production rates compared with drainage rates in previous panels. Hydraulic fracture stimulations, using water and sand, were therefore carried out in three boreholes in Longwalls 109 and 108 at Dartbrook to assess the effectiveness of sand propped fractures in stimulating gas drainage from in-seam boreholes. Boreholes 108-10-10 and 108 -7-1 were stimulated with 20 and 10 fractures respectively and, on average, 100 kg of sand was placed into each fracture. The fractures placed into LW 109 were to be mined and mapped, but operational constraints precluded mapping of these fractures.
The stimulations produced a significant increase in gas drainage rates from the two boreholes. Hole 108-10-10, which ran perpendicular to the major joint system in the seam, increased its early gas rate by a factor of about 180 while hole 108-7-1, which was drilled parallel to the joint set, increased its rate by about 22 compared to pre-stimulation rates. The stimulated gas rates continuously increased for several weeks and the higher rates were sustained for the entire period the holes were monitored. Based on the higher stimulation effect achieved in hole 108-10-10 (drilled perpendicular to the jointing) compared with hole 108-7-1 (drilled parallel to the jointing), target drainage holes drilled perpendicular (northsouth) to the jointing are better stimulation candidates.
Fracture modeling suggests the sand proppant bank may extend to 15m from the borehole. The unpropped portion of the fracture may extend to more than 40m. A purpose-built fracturing system was developed and used at Dartbrook to stimulate holes that covered most of LW109. This full-scale enhancement of gas drainage was successful and allowed efficient mining of that panel.
[Coal Operators' Conference, University of Wollongong & the Australiasian Institute of Mining and Metallurgy, 2004] Sand-Propped-Hydraulic-Fracture-Stimulation-of-Horizontal-In-seam-Gas-Drainage-Holes-at-Dartbrook-Coal-Mine-R.Jeffrey-2004.pdf265 KB
Growth Analysis and Fracture Mechanics Based on Measured Stress Change near Full Size Hydraulic Fracture - Rob Jeffrey - Ken MillsThis paper describes the successful measurement of stress changes induced in a crystalline rock mass adjacent to a full size hydraulic fracture. A hydraulic fracture was initially formed using water and subsequently 2-D numerical models. The full three dimensional stress changes were measured using four ANZI stresscells installed and tested in situ prior to the start of hydraulic fracturing. The instruments were installed in pairs in two boreholes located some 7-8mm laterally and 17-20m above the injection point. The in situ stressfield at the site was such that the hydraulic fracture passed within approximately 5m of the instruments allowing the stress changes associated wit the passage of the fracture tip to be monitored as well as the stressed induced in the rock by the hydraulic fracture once the fracture tip was well past. The instruments were logged at 15 second intervals throughout the hydraulic fracture treatments to provide a time history of the complete three dimensional stress changes that occurred as each hydraulic fracture grew toward and then passed close to the instruments.
The monitoring was undertaken as part of a larger project aimed at preconditioning a rock mass with multiple hydraulic fractures. The result of only two of the fractures are considered in this paper. Analysis of the other fractures is ongoing. Analysis of the stress change data provides information about the fracture rate and mode of growth orientation, and about the excess pressure acting inside the fracture to open it. Growth-Analysis-and-Fracture-Mechanics-Based-on-Measured-Stress-Change-near-Full-Size-Hydraulic-Fracture-R.Jeffrey-K.Mills.pdf370 KB
Mapping Techniques for Determining Sandstone Roof Channel Paleodrainage Direction in Coal Mines - Luc DaigleSandstone channels are constant hazards to coal mining operations, the presence of a channel over a seam can result in thinning of the seam, changes in stress orientations, variation in stress intensity, abrasive cutting, roof hangup, faulting, jointing, loading of the face and even wind blasts. Prediction of the trend of a channel is usually difficult as channels tend to meander and follow complex orientations. However, geological mapping of development roadways can provide very good information on where to expect channels.
Mapping paleodrainage indicators at Yancoals’ Ashton Coal Mine in the Hunter valley demonstrates channel trends can be projected through analysis of these indicators. Recognising and recording oriented indicators on hazard plans can assist in forewarning where channels may be anticipated. The experience at the Ashton Coal Mine shows how recording these features provide a means of predicting channel trends. Drainage pattern interpretation of the sandstone channels was possible due to excellent exposure of coarse grain channel sediment, levee, and overbank facies deposition. Comparison of the plotted drainage trends with paleodrainage direction indicated by oriented plant debris demonstrates direct correlation, using paleodrainage indicators in the absence of an exposure of the channel base can be used to predict the trend of the channel at a specific location. Mapping-Techniques-for-Determining-Sandstone-Roof-Channel-Paleodrainage-Direction-in-Coal-Mines-L.Daigle.pdf1.2 MB
Rock Damage Characterisation from Microseismic Monitoring - Winton GaleThis paper outlines the concepts used to correlate rock failure with microseismic events and presents examples of microseismic monitoring together with associated computer modelling of the rock failure. This study is motivated by the need to develop improved ways to reduce ground control hazards in underground mining. Toward this end we present and compare results from numerical modelling and microseismic monitoring studies conducted at several different mine sites. Emphasis is on integrating results obtained with these tools to characterize, and thus increase our understanding of, important mine deformation processes. The ultimate goal is to use this knowledge to design mine structures, and develop mitigation
measures, that minimize specific ground control hazards. Rock-Damage-Characterisation-from-Microseismic-Monitoring-W.Gale.pdf700 KB
Hydraulic Fracturing Applied to Stimulation of Gas Drainage From Coal - Rob JeffreyHydraulic fracturing is routinely applied to stimulation of oil, gas, and coalbed methane wells around the world. The stimulation effect is achieved in coal seams as in other reservoirs, by producing a conductive fracture, connecting the well to the coal reservoir. The conductivity of the fracture is usually maintained by placing a round and sieved sand proppant in the fracture channel. The proppant prevents the fracture faces from closing back completely on one another after the treatment. The design of the fracture treatment, therefore, centers on selecting fluids, injection rates, and slurry concentrations that will produce the desired propped fracture channel. Hydraulic-Fracturing-Applied-to-Stimulation-of-Gas-Drainage-From-Coal-R.Jeffrey.pdf501 KB
Mechanics of Horizontal Movements Associated with Coal Mine Subsidence in Sloping Terrain Deduced From Field Measurements - Ken MillsThe 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 JeffreyIn 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 GaleThe 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 GaleCoal 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 GaleThe 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 GaleThe 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 MillsThis 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 GaleThe 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 GaleThis 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 MillsThis 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 MillsRadar 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