Numerical Modelling of Floor Deformation Mode at Longwall Face - Winton Gale - Published 2005High stress concentrations ahead of the longwall face often exceed the floor strength and induce fractures in the floor strata. While concentrations of the vertical stress alone induces fractures in the roof ahead of the longwall face, combinations of the vertical and horizontal stress appear to be the dominant factor in formation of floor fractures. These fractures develop in response to the triaxial stress conditions exceeding rock strength. In the immediate floor, fractures appear to form at frequent intervals dipping under the goaf at a steep angle while more complex bedding shear appears to dominate the floor failure at a greater depth. In a stronger floor the fractures appear to occur less frequently. If weak bedding planes are present in the floor, shear failure along these beddings can occur far ahead of the longwall face. The post failure displacements along the fractures and the formation of new fracture surfaces often occur in response to the stress relief, bending or buckling of thin bedded layers in the floor. The post failure displacements can be
large and may interfere with mining operations.
This paper presents the computational approach using FLAC to model the development of fractures in the floor strata. The model uses programmable “fish routines” that allow simulation of failure modes that may occur in response to the changing stress field ahead of the longwall face. Continuous monitoring of the two dimensional stress field is used to predict the fracture types and the direction at which the fractures may propagate. The fractures are then simulated using FLAC ubiqitous elements that allow to assign the joint direction and the reduction of joint strength in the direction of the calculated fracture. The stress state is tested continuously during the execution of the program and fractures are simulated when the stress exceeds the rock strength. This procedure can simulate the progressive development of fractures during the longwall advance. The method is particularly helpful to estimate the type of fractures and their frequency that depend on the strength of floor strata and stress build up during a longwall advance. The depth of floor failure can have a significant influence on the gas release from the floor strata in gaseous mines. The type of fractures and the fracture orientation that is computed can be presented in the movie files to view the development of fractures in the floor during the longwall advance. Numerical-Modelling-of-Floor-Deformation-Mode-at-Longwall-Face-W.Gale.pdf1.5 MB
Computer Simulation of Ground Behaviour and Rock Bolt Interaction at Emerald Mine - Winton Gale - Published 2004A collaborative project between RAG Emerald Mine, NIOSH, and SCT Operations was conducted to investigate ground behaviour, reinforcement performance, and stress redistribution in a coal mine entry subjected to a severe horizontal stress concentration. Field measurements indicated that the stresses applied to the study site nearly doubled during longwall mining, resulting in roof deformations extending to a height of 4.8 m (16 ft) above the entry.
This paper focuses on the computer simulation that was undertaken to provide more insight into the roof behaviour and rock bolt interaction during mining. The model’s input rock properties were derived from extensive laboratory testing, and the model itself simulated a broad range of failure mechanisms. The effects of different bolt patterns on roadway behaviour were evaluated. Comparison between the model results and the field measurements indicated that that the model effectively simulated the critical elements of the actual roadway’s behaviour. With the confidence gained, the model was used as a baseline for additional simulations that evaluated the expected performance of alternative roof support systems. The study will also provide a benchmark data set for future applications of numerical modelling to U.S. coal underground mining. Computer-Simulation-of-Ground-Behaviour-and-Rock-Bolt-Interaction-at-Emerald-Mine-Winton-Gale.pdf697 KB
Experience in Computer Simulation of Caving Rock Fracture and Fluid Flow in Longwall Panels - Winton Gale - Published 2002Recent advances in computer simulation together with field measurements of caving and microseismic activity about longwall panels, has allowed a much better understanding of the caving process and the variability due to geology. Research between SCT Operations and CSIRO Division of Exploration and Mining has initiated new methods of computational modelling predicting various caving patterns and strata failure far ahead of the longwall face.
The rock fracture distribution and the caving characteristics of a range of strata sections have been simulated by computer methods. The computer simulation of strata behaviour includes coupled fluid and mechanical behaviour. Validation studies of the method were addressed together with case studies. The method allows the simulation of longwall support behaviour and fluid pressure distributions about longwall panels under various geological conditions. The system also allows a prediction of the monitoring data, which is best suited to give an early warning of weighting events or signal various key caving characteristics. Experience-in-Computer-Simulation-of-Caving-Rock-Fracture-and-Fluid-Flow-in-Longwall-Panels-W.Gale.pdf4 MB
Combining Modern Assessment Methods to Improve Understanding of Longwall Geomechanics - Winton Gale - Published 1998Ongoing, collaborative research between CSIRO's Exploration and Mining and Strata Control Technology has resulted in a better understanding of rock failure mechanisms around longwall extraction. Failure has occurred further ahead of the retreating face than predicted by conventional longwall geomechanics theory. In some cases significant failure has been detected several hundred metres ahead of the face position with demonstrated influences of minor geological discontinuities. Shear, rather than tensile failure has been the predominant failure mechanism in the environments monitored. Validating technologies of microseismic monitoring and new face monitoring techniques have assisted the development of predictive 2D computational modelling tools. The demonstrated 3D consequences of failure has assisted in the ongoing direction of the project to further investigate these effects. Combining-modern-assessment-methods-to-improve-understanding-of-longwall-geomechanics-W.Gale.pdf2.9 MB
Coal Pillar Design Issues in Longwall Mining - Winton Gale - Published 1998Coal 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 variations 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. Coal-Pillar-Design-Issues-in-Longwall-Mining-Winton-Gale.pdf3.3 MB
Methods of Interpreting Ground Stress Based on Underground Stress Measurements and Numerical Modelling - Winton GaleThis paper presents several new methods to help interpretation and understanding of ground stress. The methods are based on data from 239 stress measurements conducted in the virgin ground in NSW and Queensland mines and computational models simulating large scale faulted ground behaviour. The underground stress regime plays an important role in mining profitability and safety however, understanding of the stress tensor is often difficult due to its mathematical complexities and non-intuitive behaviour.
The aim of this study is to explain stress distribution in faulted ground, its origin and propose several methods of stress interpretation. Major findings presented in this study include: increase of maximum horizontal stress with depth based on underground measurements and numerical simulation of faulted ground, affect of faults on ground stress, normalisation technique that allows comparison of lateral stress magnitudes in rock of different stiffness, ‘Strain Tectonic Factor’ concept and its value in understanding stress components and its affect on rock strength. Methods-of-Interpreting-Ground-Stress-Based-on-Underground-Stress-Measurements-and-Numerical-Modelling-W.Gale.pdf429 KB
Successful Use of a Stress Relief Roadway at Appin Colliery - Winton GaleHigh horizontal stress levels can lead to extensive roadway deformation requiring expensive secondary support to ensure stability; this is particularly the case with longwall installation faces. Longwall installation roadways are a critical construction within coal mines. The use of a purpose built ‘Stress Relief Roadway’ to minimise roof deformation in the nearby longwall installation roadway, by reducing stress impacts has been undertaken at Appin Colliery – BHP Billiton Illawarra Coal. Its use led to significant cost and operational benefits. This paper outlines the process used; from identifying horizontal stress as an issue, as well as generating computer models through the various options and culminating in ground monitoring of the constructed roadways to the successful start of the longwall panel. Successful-Use-of-a-Stress-Relief-Roadway-at-Appin-Colliery-W.Gale.pdf581 KB
The Application of Field and Computer Methods for Pillar Design in Weak Ground - Winton GaleThis paper describes the use of strain gauge based borehole instruments to monitor stress changes associated with the creation and extension of hydraulic fractures in massive rock strata at Northparkes Mine in Australia and Salvador Mine in Chile.
This work was conducted as part of the International Caving Study ICSII. These instruments proved very sensitive to the stress changes induced by the hydraulic fractures close to the fracture plane. Analysis of the stress changes observed allowed the fracture orientation and non-symmetric fracture growth to be constrained sufficiently that a clearer insight into fracture behaviour could be obtained at both sites, particularly when combined with other observations. Recognition of the elastic stress reorientation about an opening mode hydraulic fracture has proved to be an important element in the interpretation of stress change monitoring data.
The nature of the stress reorientation is useful in discriminating between opening and shearing mode fracture growth. A technique of identifying a range of possible solutions of fracture orientation and non-symmetric fracture growth consistent with the stress changes observed on multiple instruments has been developed. Unique definition of fracture orientation from the stress change instruments is possible if the instruments are sufficiently distributed relative to the hydraulic fracture plane. The-Application-of-Field-and-Computer-Methods-for-Pillar-Design-in-Weak-Ground-W.Gale.pdf2.4 MB
Experience in the Application of Computer Modelling to Coal Mine Roadway Design in Weak Rock - Winton GaleA summary of the weak rock failure process is presented to demonstrate the application of computer modelling to coal mine roadway design. The weak rock failure mechanism was chosen because its discovery required a design tool (modelling) that was not bound by a preconception of the results. Modelling was used to decode the relative influence of the geological and geotechnical factors.
It is emphasised that computer simulation techniques are best applied in a practical sense if accompanied by field measurement and observation. The field measurements are used as both a means of validating the initial model and to confirm that actual events are within design expectation. Experience-in-the-Application-of-Computer-Modelling-to-Coal-Mine-Roadway-Design-in-Weak-Rock-W.Gale.pdf1.2 MB
Experience in Modelling Longwall Support Behaviour - Winton GaleRecent advances in computer simulations of strata caving mechanisms and the response of longwall supports to strata behaviour has allowed much better understanding of longwall support requirements. The computational method allows the simulation of longwall support behaviour under a wide range of geological conditions with emphasis on comparing different support geometries and support loading conditions. This paper presents results of the computational trials to simulate various longwall support geometries including the comparison of the two leg and the four leg support options, the premature caving of strata at the canopy rear and its influence on roof falls at the longwall face.
The rock fracture distribution and caving characteristics of a wide range of strata geologies has a significant influence on the longwall support behaviour. Underground measurements and computer simulations were undertaken to investigate the caving characteristics of strata and some of the common problems typically encountered at the longwall face. The computer simulations highlight the importance of the longwall support geometry and location of the applied roof loads to minimise potential problems leading to major roof falls at the longwall face. Experience-in-Modelling-Longwall-Support-Behaviour-W.Gale.pdf1.9 MB
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
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.
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
Reinforcement Design of Cataract Water Tunnel to withstand Longwall Subsidence - Winton GalePublished Nov, 2017This paper presents the procedures followed in the design of the reinforcement system for Cataract Tunnel. The Tunnel is a 19th century unlined tunnel under which a series of longwalls will be extracted at Appin Colliery. Based on the prediction of subsidence
levels, three dimensional stress analyses were conducted to investigate the range of additional stresses which were likely to be induced around the tunnel. The impact of the induced stresses on the tunnel were then analysed in detail and a reinforcement system designed.
It should be noted that, as in most geotechnical situations it was not possible to define all the relevant variables. Stress variations outside those used in this analysis are possible. In recognition of this and the uncertainty of the actual stress redistributions, the design has been used as the basis for a minimum support pattern. It is envisaged that this may be supplemented by additional reinforcement such as additional bolts or cable tendons as determined by a planned program of monitoring and observation. Reinforcement-Design-of-Cataract-Water-Tunnel-to-withstand-Longwall-Subsidence-W.Gale.pdf446 KB
Prediction of strata caving characteristics and its impact on longwall operation - Winton GalePublished Feb, 1998Recent advances in computer simulation together with field measurements of caving and microseismic activity about longwall panels, has allowed a much better understanding of the caving process and the variability due to geology. The joint research between SCT Operations and CSIRO Division of Exploration and Mining has initiated new methods of computational modelling predicting various caving patterns and strata failure far ahead of the longwall face. This work was validated by field measurements of caving and microseismic activity at the longwall face.
The rock fracture distribution and the caving characteristics of a range of strata sections have been simulated by computer methods. Validation studies of the method were addressed together with case studies. The interaction of caving with support convergence and face control is presented. The method allows the simulation of longwall support behaviour under various geological conditions. The system also allows a prediction of the monitoring data, which is best suited to give an early warning of weighting events or signal various key caving characteristics. Prediction-of-strata-caving-characteristics-and-its-impact-on-longwall-operation-Winton-Gale-1998.pdf2.9 MB