Development of empirical model for strength estimation of irregular-shaped-heightened-rib/snook for mechanised depillaring

Issues with conventional depillaring techniques resulted in unscientific development of a number of coal seams into rectangular/square shaped pillars, mainly, located at shallow depths in different coalfields of India. Continuous miner based mechanised depillaring (MD) has been introduced to extract these coal pillars which is the main stage of production. The practise of leaving a proper sized rib/snook is a legal requirement as it decides the efficiency and safety of the MD operation. However, MD of square/rectangular shaped pillars created irregular-shaped-heightened-rib/snook. Conventional empirical formula is not capable to estimate strength of such ribs/snooks. These ribs/snooks should be of sufficient size to protect the adjacent slicing operation and junction as well as it should fail in a controlled manner when shifted inside the goaf. Field studies conducted at 39 panels of 7 different mines found a number of factors affecting the design of rib/snook. Proper assessment of performance of different sizes and shapes of ribs/snooks in field is difficult due to the complex and challenging underground mining environment of depillaring. These challenges prohibit a detailed study to be carried out in field. Therefore, parametric study on calibrated numerical model is conducted to estimate the strength of rib/snook with variation in sizes and heights of rib/snook and depth of cover. Further, an empirical relationship is developed to estimate the strength of rib/snook based on multivariate regression analysis. The values of strength estimated by the developed relationship is found to be least when compared with the popular pillar strength formulations due to effect of pillar/rib/snook spalling observed during field investigation with increase in depth of working. Results of field and simulation studies are presented and discussed in this paper for the design of irregular-shaped-heightened-rib/snook.