Importance of Fracturing Hard and Massive Overlying Strata for Complete Extraction of Thick Coal Seam – Case Studies

Abstract

Engineering efforts need to be organized as per the rock mechanic conditions of the site for efficient underground extraction of a thick coal seam. Geo-mining conditions of Indian coalfields have favoured development of thick coal seams on Bord and Pillar (B&P) method, where single lift depillaring of total thickness (SLDTT) is vital for their final extraction. Competency of heightened pillar is an important factor for uncontrolled roof-pillar interaction during caving of the hard and massive overlying strata. An increase in extraction height by underwinning of roof coal band of the thick coal seam during retreat, developed along its floor horizon, is found to be the main reason for the strength deterioration. Conducted field studies also showed that the presence of competent overlying strata caused caving after a large overhang inside the goaf varying from 6000 m² to 12500 m². An attempt was made to control the roof caving by a proportional increase in width of the heightened pillars and it worked well but caused the issue of resource conservation. However, another field study with SLDTT working below laminated and weak overlying strata experienced a smooth strata control condition. This fortifies the idea of strength reduction of the competent overlying strata by fracturing for a controlled strata behaviour. This paper presents the result of the field investigations to demonstrate the role of strength reduction of overlying strata for underground extraction of total thickness of a thick coal seam in single lift.