ICGCM Papers:
Operator Case Histories
 
 
UK Experience and Numerical Modelling for Planning and Supporting Deep Longwall Over-Mining Operations
35th International Conference on Ground Control in Mining
UK Experience and Numerical Modelling for Planning and Supporting Deep Longwall Over-Mining Operations
by
Lorraine KentNicholas LightfootBowler Jonathan, Golder Associates (UK) Ltd., Nottingham, United KingdomTereza Crisu, Golder Associates (UK) Ltd, Nottingham, United KingdomStuart Walker, Thorseby Colliery Ltd., Mansfield, United Kingdom
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[Conference] 35th International Conference on Ground Control in Mining
[Price] Free  [Comments] 0
[Topical Area] Operator Case Histories
[Author] Lorraine KentNicholas LightfootBowler Jonathan, Golder Associates (UK) Ltd., Nottingham, United KingdomTereza Crisu, Golder Associates (UK) Ltd, Nottingham, United KingdomStuart Walker, Thorseby Colliery Ltd., Mansfield, United Kingdom
[Abstract] 
Key Conclusions:
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It has been concluded from this work that existing guidelines regarding the layout of over-mining panels are conservative and that more flexible layouts can be achieved with appropriate support management strategies. Continuum modelling of collapse above longwalls can produce inaccurate stress redistributions above and below the boundaries of mined ground that can underestimate the extent of the associated stress shadow. The discontinuum code UDEC is capable of producing more accurate representations of goafing in coal measure strata. It is possible to implement over-mining of previously mined coal seams of more complex extraction geometries if sufficient flexibility is built in to the initial support strategy to be adopted in the over-mining seam. This allows mining in more challenging conditions thus contributing to optimum economic recovery of overlying reserves.
Key Findings:
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Case histories were examined for a mine operating at a depth of approximately 700 m where 5 longwall panels were extracted above workings 35-40 m below. The over-mining of various extraction geometries below led to challenging conditions in areas of high vertical stress concentration. It was found that results from one longwall panel could be suitably used to predict behaviour in a subsequent panel. The experience was used to update previously existing guidelines on the positioning of roadways in relation to subsidence troughs. Existing guidelines recommended that only as a last resort should longwalls be planned to retreat over narrow pillars or from over solid ground into a subsidence trough. This new work showed that such multi-seam geometries are manageable with appropriate support management strategies. Numerical modelling provides an invaluable tool for both mine layout and ground support design. For mine layout design it is important to simulate as accurately as possible the modified in-situ stress regime into which the over-mining will take place. Previously, the numerical codes FLAC and Map3D have been used, this new work illustrates how these continuum modelling codes can produce inaccurate stress redistributions above and below the boundaries of mined ground that can underestimate the extent of the associated stress shadow. This paper reports results from work where the numerical codes MIDAS and UDEC have been considered. Using UDEC it was found that more realistic goaf caving could be simulated. The paper describes a specific case history where longwall mining was taking place in a seam at 700m depth where there were known plans to over-mine the seam 60 m above. It was found that interaction effects when over-mining mined ground can be kept at manageable levels by maintaining consistent panel orientations, driving longwalls as long as practicable, practicing sequential extraction and maintaining adequate dimensions of pillars in the underlying seam. However the case history concludes that although there may be an ideal extraction sequence in the underlying seam this may be changed as a result of geological variation and, as a result, changing layout design in one seam can affect conditions in the other. With sufficient flexibility in the support management strategies it is possible to accommodate such geotechnical changes. This, in return, allows mining in more challenging conditions thus contributing to optimum economic recovery of overlying reserves.
Objective of the Paper:
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Within this paper the current challenges and mitigation strategies of over-mining in the UK are described following the completion of a 3 year European Commission funded project in 2015. The challenges and mitigation strategies are related to: • Over-mining older workings of variable geometry; • Numerical modelling for overworking layout and support design; and • Planning mine layout when considering future over-working.
Problem Statement:
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Generally, it has been the practice in Europe to extract the most economic seams first: taking the seams sequentially, top down, to avoid operating within the fractured strata formed by the subsidence trough above previous worked coal seams. In the 1990’s, the UK coal industry found itself, at a number of sites, having to operate over one or more previously worked seams. This led to specific geotechnical challenges with respect to maintaining economic, safe and stable rectangular roadways with rockbolted support.