An assessment of multiple seam mine stress conditions using a numerical modelling approach

Abstract

Underground coal mining in Australia has primarily focussed on mining the single mostneconomical seam though many may exist within a lease area (Gale, 2004). The processnof extracting more than one of these seams is called multiple seam mining. The practicencan cause significant geotechnical hazards including pillar and roof instability, in somencases to the point of collapse. While multiple seam mining has been extensivelynpracticed overseas, particularly in the United States and the United Kingdom, due tonthe differing geological and geotechnical conditions the multiple seam guidelinesndeveloped overseas cannot be applied in Australia (Gale, 2004). At this stage, designnguidelines do not exist to minimise the geotechnical hazards of multiple seam miningnfor Australian conditions. nThe project aimed to begin the process of establishing multiple seam mining guidelinesnby utilising the numerical modelling package, LaModel, to understand the occurrencenof multiple seam interactions in a study of an underground coal mine in the BowennBasin, Queensland. The study aimed to quantify the stress impacts during developmentnand extraction of longwall panels in the Argo seam as a result of the overlying bord andnpillar and longwall workings in the Castor seam at Cook Colliery. An understanding ofnthe nature of multiple seam interactions will allow mines to identify regions where poornground conditions are likely and implement procedures, including primary andnsecondary support plans to mitigate the adverse impacts. nUsing LaModel the total vertical stress, multiple seam stress, seam convergence andnpillar safety factor were mapped for the thirteen longwall panels of the Castor seam.nModelling found that severe interactions would occur below goaf edges and chainnpillars of overlying longwalls, which is in line with the prior research. A region ofnincreased interaction was found below areas of very complex bord and pillar workingsnwhere secondary extraction has taken place. The results were used to conduct a risknanalysis of the proposed Argo seam longwall panel gate roads to identify regions ofnlow, medium and high risk of adverse ground conditions as a result of multiple seamninteractions. It is recommended that in regions of medium and high risk further worknshould be conducted to plan mitigation methods (e.g. primary and secondary supportnmodifications) for increased vertical stress.nWhile LaModel provided adequate results for the purpose of this project, the programnconsiders a frictionless laminated overburden therefore does not account for horizontalnstress impacts acting on the seam. Furthermore, the pillar safety factors are generatednin such a way that they yield unrealistically low results and as such could be relied uponnfor relative comparison only. The validity of the model was only loosely confirmed byncomparison to site experience from only LW201 panel and extensometer or stress datanwas not available. It is recommended that once mining of the LW202 panel isncompleted the model be validated from experience and data in the gate roads of thisnpanel and modified accordingly. nFurther work on this project could include expanding the project to several multiplenseam sites in Australia to establish an Australian database for multiple seam miningninteractions and develop multiple seam mining design guidelines for Australiannconditionsn