Causes of Subsidence Potentiality above old abandoned underground coal mines workings at Karharbari formation, Giridih, Jharkhand

Subsidence is the motion of a surface as it shifts downward relative to a datum caused by various manmade and natural activities. Most of the coalfield of Indian are facing significant subsidence problem due to underground coal mining. Extraction of thick seams at shallow depths has damaged the ground surface in the form of subsidence and formation of cracks reaching up to the surface, enhancing the chances of spontaneous heating of coal seams leading to mine fires. A proposed alignment of Giridih-Koderma broad gauge (BG) railway track near Giridih Railway station lies above old, abandoned underground coal mine workings. This site lies at the southern fringe of the Giridih coalfield (also known as Karharbari formation). The mining was commenced in 1871, first by the Bengal Coal Co., then by the National Coal Development Corporation (NCDC) and lastly by the East India railway and it was closed in 1936. Bord and pillar mining method was practiced in this mine and it was closed without any de-pillaring and backfilling. The present undulating topography depicts the existence of subsidence at different parts of the coalfield. Total 117 boreholes were drilled for investigating the ground condition and to evaluate ground stability for laying railway tracks to run MBG (1987) train load along the proposed layout above the basin. The paper deals with the causes and subsidence potentiality over old abandoned underground coal mines near Giridih town of Karharbari formation