Surface subsidence induced by underground coal mining in Indonesia

The following are summarised by the analysis of subsidence at line 1 and line 2: 1) The time for reaching the complete subsidence of longwall mining in the area in Indonesia meets the cases dependent on the depth of cover in Japan. It is considered that this match is caused by similar longwall width and weak rocks on the roof which falls soon after longwall passes. 2) Limit angle: the measured data show 34° at line1 and 55° degree at line 2. It is known that in general subsidence having weak rocks overburden has bigger limit angle. It is hard to be acceptable that subsidence at line 1 has 34°, considering very small rock strength, e g UCS 1-4 Mpa. Thus, it is considered that subsidence at line 1 was affected by the gob of room and pillar mining which was made before starting longwall mining. It is expected that the limit angle of the subsidence in the area has around 55° like line 2, similar with it in Japan and UK. 3) Subsidence profile: since the subsidence at the centre of the panel at both line 1 and line 2 could not be measured from the beginning of the longwall mining, the predictions of the profile were made by several methods. However, the predicted profile could not be verified due to a lack of the most important subsidence data of the centre of the panel. Whether the NCB model can be applied to the subsidence in Indonesia should be examined by accumulating the subsidence data through mining practices in future.