Web-pillar design for highwall coal mining – a case study

Highwall mining is a mechanized and effective way to recover coal which is locked-up in the barrier of open cast mines. In a multi-seam environment, the design of the highwall ribs or web-pillars pose a great challenge as most of the areas either have human population or have rural/industrial infrastructure. West Bokaro open cast coal project is one of the captive mines of Tata Steel Limited, India. Here, high wall mining is now a well-established process and is being operated since last six years. All the seams are gently dipping, with 3°-6° inclination and occur at a depth of 75 – 110 m within the proposed highwall site. In this paper, design of the web pillars is done through numerical modelling using FLAC 3D even for thick seams considering multi-pass extraction condition. The paper explains the procedure adopted to design web pillars based on the depth of penetration of galleries, gallery width, height of extraction, deviation of the highwall miner and the physico-mechanical properties of rocks/coal in PCP3 area (Primary Crushing Plant 3) of Quarry AB in West Bokaro open cast coal project. Simulation through numerical modelling is done by two methods viz. long pillars and equivalent square pillars and the results are compared. The coal seams designed for extraction through highwall mining in this PCP3 area are seams V, VI & VII. The width of web pillar is designed to optimize the extraction percentage of coal without compromising the risk of subsidence by maintaining a Factor of Safety (FoS) more than 2.0.