Recent Advances in Testing and Field Investigations for Understanding Stability of Flyash as Back Filling Material in Opencast Mines – A Case Study

Abstract

This paper presents the results of laboratory investigation, field geotechnical investigations and numerical analysis for understanding the stability of overburden Material including Fly ash in Jindal power opencast coal Mine, India. Numerical models were used for estimation of stability of the back filling material with varying geometry of layers of fly ash and overburden. It is observed that two types of dumps with alternative layer of only fly ash and overburden material, and overburden mixed with fly ash (25%) are found to be stable with safety factor more than 1.2. Geometry of the stable dump has total height of 120 m, 4 number of decks, individual deck height of 30 m, and slope of each deck as 32°. The dump formed with alternate layer of overburden and mixture of fly ash is found stable with a safety factor of 1.78 for the above geometry. Therefore, the above geometry of the back filling material was recommended for the above mine and field trials were successfully conducted for the first time in India. As a part of reclamation with plantation, top soil of nearly 2 m thick was spread over the dump material in the field. Displacement pattern of the monitoring stations during 2012 to 2014 indicated no significant displacement in the Overburden dumps with fly ash ensuring stability of the dump.