Evaluation of induced vertical stress during depillaring in blasting gallery panel workings

Abstract

Selection of a suitable mining method for extraction of a thick coal seam with optimal recovery and safety is an extremely delicate process. In fact, single lift working of full thickness of a thick coal seam has always a verge over the multi-slice working due to favorable economics and high production and productivity. Blasting Gallery (BG) method is suitable for extraction of virgin thick seams as well as developed pillars in thick seams in single lift. The method was very successful resulting in 85% of extraction with high productivity. But, this method experienced strata control problems during final extraction. The presence of strong and massive overlying roof strata caused high values of the mining induced stress over the pillars facing the goaf line during the depillaring. High values of mining-induced stresses in and around a depillaring face of thick coal seam generally create a threat of pillar overriding. This paper describes the assessment of mining induced vertical stress in a BG panel during depillaring in specific geomining conditions, together with the results of the laboratory and field investigations. The attempt was based on field monitoring data of depillaring faces in BG panel of Godavari Khani (GDK) No 10 Incline, Singareni Collieries Company Ltd. (SCCL). Based on the results of laboratory investigations on simulated (FEM) models using ANSYS software, the numerical simulation (FEM) results are validated with field investigation results. The safety factor of the pillar is evaluated using Hoek and Brown failure criterion at different advances of goaf edge. The research revealed that the width of abutment zone is estimated to be about 35 m to 40 m from the diagonal line of extraction in the BG panels and the induced vertical stress and roof deformation decreases as one goes away from the goaf edge. It was observed that the safety factor of the pillar decreases when line of extraction moves towards center of the pillar. Results of finite element analysis using ANSYS are validated with the strata monitoring instrumentation data in the BG 2B panel of GDK No. 10 Incline, SCCL.