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• W161721131 abstract "Recent 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 charact(:ristics. BACKGROUNJ) The authors have been undertaking research into strata fracture and caving mechanisms about longwall panels to better understand the extent and geometry of rock fractures about longwall fa(:es. Recent studies of microseismic activity (Kelly, et a1 1998) and abutment stress measurements about longwa1l panels have demonstrated that previous assumptions of caving mechanisms and stress redistributions were either too simplistic or not suited to certain geologies. This work is being undertaken in conjunction with CSIRO Division 01' Exploration and Mining in Brisbane. The general scope of the project is that CSIRO undertakes microseismic monitoring to determine fracture location during mining and the authors undertake computer simulation of longwall extraction to pre.dict fracture geometries, stresses, caving mechanics and fluid flow characteristics about longwall panels. Monitoring of 10ngwalI support pressures and convergence is undertaken in association with these investigations to assess the interaction of caving and supports under various geologies. This project was initiated to; . predict rock fracture patterns about longwall panels; . understand caving mechanics in differing geologies; . optimise gas drainage drilling to intersect gas sources and floJ;' networks; . predict abutment stresses occurring under various geologies; and . assess 1ongwal1 support requirements. Managing Director, Strata Control Technology Senior Geotechnical Engineer, Strata Control Technology COAL98 Conference Wollongong 18 -20 February 1998 156 Key sites for the study presented here were from Gordonstone and South Bulga Mines. Recent microseismic monitoring has been conducted at Gordonstone Mine, and computer simulations have been undertaken of these and other sites. COMPUTER MODELLING APPROACH The aim of the study is to understand the ground caving mechanics under the geological and mining conditions present and the influence of longwall supports in this process. To achieve this, the model must simulate the dynamic caving process as the longwall retreats. The progressive mining mechanism is achieved by assuming a two-dimensionallongitudinal slice down the central zone of the panel and sequentially excavating Imshears in the model. The advancing longwall supports are used to provide roof support at the longwall face. The stress redistributions, rock failure and ground movement then occur in response to an incrementally changing geometry thus simulating a reallongwall face. The finite difference code FLAC (Itasca, 1995) is used to simulate the incremental excavation. A numerical model has been formulated to simulate development of fractures in the becjded strata using the FLAC routines. The programmable fish routines allow interrogation of the stress state at any point of the model and the determination of the type of fracture that may develop. Various failure models are use.d to predict the type of fracture, orientation and its properties. The rock failure routines calculate the likelihood of shear and tensile fractures through intact rock and shear and tensile failure along the bedding. The fractures are simulated by changing the rock and joint properties to model the strain softening behaviour of rock derived from the triaxial rock testing. The rock failure and permeability routines used in the code have m~en developed by the authors to realistically simulate actual strata behaviour. In this study emphasis is on the rock failure, caving characteristics of the ground and longwall support behaviour. The computer simulation capability is being refined and validated as an ongoing process in association with CSIRO Division of Exploration and Mining researching longwall caving mechanics and other research projects undertaken by the authors. Although the computational model is two-dimensional, the third dimension can be approximated by mining the distance equal to one half of the longwall face width from the reflective boundary located on the goaf side. This method allows longwall simulations of subcritical width with the front abutment stresses similar to those measured underground. The properties of strata used in the model are based on triaxial testing of overburden material. A typical section of the modelled strata is shown in Fig. 1. An enlarged portion of the longwall face is presented in Fig. 2. The model of the longwall supports is constructed using the grid and support elements. The stiffness of the canopy and base parts is chosen to approximate the properties of the actuallongwall supports. The modelled supports have the ability to advance forward and reset each time the coal is cut. The set loads are gradually increased to the yield value in response to the support convergence. The support loads are monitored and can be compared with the leg pressures measured underground. The goaf behind the supports is allowed to free fall a nominated distance to reach the zone where a convergence induced vertical load is applied to the goaf roof. The vertical load is gradually increased until the full goaf load is experienced at a nominated convergence above floor level. The progressive excavation of the longwall panel and associated ground response can be captured in a movie file, which allows visualisation of caving cycles and stress changes as the longwall retreats. MODELLING OF STRATA OF V ARIABLE STRENGTH Vastly different caving styles have been defined in this project. Two examples are presented to demonstrate the variability in caving as a result of rock strength properties and stressfi~ld. 157 COAL98 Conference Wollongong 1820 February 1998 Fig. 1 Typical section of the mo{[eUed strata" @default.
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• W161721131 date "1998-01-01" @default.
• W161721131 modified "2023-09-27" @default.
• W161721131 title "Prediction of stata caving characteristics and its impact on longwall operation" @default.
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