Investigation into Abnormal Surface Subsidence Above Longwall Panel Southern Coalfield - Winton GalePublished Jan, 2011The subsidence over a longwall panel at Tahmoor Mine in the Southern Coalfield of NSW, Australia, was found to be approximately twice the size it had been in previous measurements. An investigation into the potential causes was conducted using computer modeling together with hydrological characterization and detailed geotechnical characterization of the strata.
The abnormal subsidence was found to be consistent with localized weathering of joint and bedding planes above a depressed water table adjacent to an incised gorge. The study showed that other factors such as variation in stress field, joint zones, variation
in rock strength and topographic factors did have sufficient impact to induce the abnormal subsidence.
The results have significant implications to subsidence prediction in areas that may be prone to the phenomenon found at Tahmoor. Key indicators of the potential for this form of abnormal subsidence are presented. Investigation-into-Abnormal-Surface-Subsidence-Above-LW-Panel-Southern-Coalfield-2011.pdf2.6 MB
Using Helium as a Tracer Gas to Measure Vertical Overburden Conductivity Above Extraction Panels - Yvette Heritage - Winton GalePublished Feb, 2009This paper investigates helium injection into the goaf as a tool to measure goaf to surface connectivity. Laboratory studies confirmed a relationship between gas velocity and fracture conductivity through helium injection. Field trials of helium injection into the goaf were successfully conducted to determine whether a connection exists between the surface and the goaf. A repeatable technique of borehole helium injection, with a borehole drilled into the highly permeable caved zone of the goaf, proved to demonstrate more quickly whether a connection to the surface exists. Using-Helium-as-a-Tracer-Gas-to-Measure-Vertical-Overburden-Conductivity-Above-Extraction-Panels-Y.Heritage-W.Gale-2009.pdf281 KB
An Investigation into Underground Mine Interaction with Overlying Aquifers Huntly, East Mine, New Zealand - Winton Gale - Published 2006In recent years, Huntly East Mine has operated at a depth range of approximately 100 m to 220 m below a Quaternary aged clay, sand and silt aquifer that is connected to a nearby large river system (Waikato River). A key issue for mine planning and environmental management has been the development of mine design criteria to allow efficient mining of the reserves and to maintain the integrity of the aquifer.
A case study and back analysis at Huntly East Mine is presented, which investigates the overburden conductivity and the impacts caused by mining-induced caving. The case study includes: i. computer modelling of the mine geometry, caving and overburden fracture networks created; ii. field investigation to develop an engineering geological model of the overburden within the goaf to validate the goaf geometry as defined by the computer generated model; iii. in situ field measurement of overburden conductivity in the pre- and post-mining condition; iv. interference testing across the goaf to determine the level of interconnectivity; and v. measured water pressure profiles above the mine. An-Investigation-into-Underground-Mine-Interaction-with-Overlying-Aquifers-Huntly-East-Mine-New-Zealand-W.Gale.pdf822 KB
Water Inflow Issues above Longwall Panels - Winton Gale - published 2006The aim of this paper is to discuss the issues which relate to surface water inflow through the fractured overburden above longwall panels. The information used is a combination of field experience and computer modeling. Computer models used in this study simulate the fracture process in the geological units throughout the overburden. Analysis of the mining induced fracture patterns and in situ joint patterns allows an estimation of the hydraulic conductivity within the overburden. The cubic flow relationship has been used in examples presented. Water-Inflow-Issues-above-Longwall-Panels-W.Gale.pdf267 KB
Application of Computer Modelling in the Understanding of Caving and Induced Hydraulic Conudctivity About Longwall Panels - Winton Gale - Published 2005Computer modelling is being used to simulate rock fracture, caving and stress redistribution about longwall panels with increasing confidence. The models are being assessed against field monitoring and have significantly increased the understanding of caving mechanics within the overburden. This paper discusses the modelling approach and provides some examples of its application to overburden damage and induced hydraulic conductivity. Computer models used in this study simulate the fracture process in the geological units throughout the overburden. Analysis of the mining induced fracture patterns and in situ joint patterns allows an estimation of the hydraulic conductivity within the overburden. The cubic flow relationship has been used in the examples presented. Application-of-Computer-Modelling-in-the-Understanding-of-Caving-and-Induced-Hydraulic-Conudctivity-About-Longwall-Panels-W.Gale.pdf271 KB
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
Application of Computer Modelling in the Understanding of Subsidence Movements - Winton Gale - Published 2001Computer modelling is being used to simulate rock fracture, caving and stress redistribution about longwall panels with increasing confidence. The models are being assessed against field monitoring and have significantly increased the understanding of caving mechanics within the overburden.
The modelling supports the concept that the ground subsides from the seam and progresses upwards and as such subsidence is the end point in a failure pathway within the overburden. The nature of the strata, in situ stresses and the mining geometry will influence the subsidence. The nature of the fractures created and the enhanced permeability will influence the interaction of mining with aquifers and surface water. Application-of-Computer-Modelling-in-the-Understanding-of-Subsidence-Movements-W.Gale.pdf425 KB