Web pillar design approach for Highwall Mining extraction

• Hybrid empirical and numerical technique devised for web pillar design in India.
• Estimation of web pillar load variation over seam horizon using numerical tool.
• Elasto-plastic analysis for design in complex multi-seam extractions.
• A case example of the first Highwall Mining extractions in India.

“Highwall Mining” is practised when the open pit coal mines reach their pit limits due to the presence of surface dwellings or uneconomical stripping ratios. It involves driving a series of parallel entries separated by web pillars using a remotely operated continuous miner into a coal seam exposed at the highwall. The design of web pillar is crucial to the overall stability and safety of the Highwall Mining operations. In multiple seam extractions it is also necessary to protect the overlying workable seams and minimise surface subsidence. Hybrid empirical and numerical modelling techniques adopted for web pillar design in Indian conditions are described in this paper. Web pillar vertical stress was estimated for both up dipping and down dipping coal seams using numerical modelling tool, which accounts for highwall slope and abutment stresses caused by the open-pit excavation and the “end-effect” of the highwall excavations facing the solid boundary. Using numerical modelling tools, a correction factor is suggested in the empirical pillar strength equation for slender pillars having width to height ratio less than unity. Numerical strain-softening models, representing various possible configurations for multiple seams in close proximity and multi-pass extraction within a thick coal seam, are run and the peak stress is calibrated against pillar strength determined by the empirical equation. Finally a case example of successful design of web pillars for Highwall Mining extractions is presented.