Abandoned lignite mines in Mitake, Japan, and some issues

It is well known that some sinkholes or subsidence takes place from time to time in areas where abandoned room-and-pillar-type mines exist. The authors have been involved in studies on the stability of abandoned mines beneath urbanised residential areas in the Tokai region, Japan, and there is a great concern about the stability of these abandoned mines during large earthquakes as well as in the long term. The 2003 Miyagi Hokubu and 2011 Great East Japan earthquakes caused great damage to abandoned mines and resulted in many collapses. Therefore, there is great concern about the stability of abandoned lignite mines in the Tokai region, particularly in Mitake Town, as the region could be affected by the anticipated successive Nankai, Tonankai and Tokai earthquake with a magnitude of 9.0. The authors give a brief summary of the geology of the Tokai region, the short-term and long-term properties of rocks surrounding the abandoned mines and their degradation characteristics. Then, some sinkhole and subsidence problems in the town due to long-term loading in abandoned lignite mines as well as earthquakes are described. The local and global stability issues of abandoned mines are described, and some procedures for estimating their stability in the long term are presented. Some discussions are made on the effect of depth and groundwater on the formation of sinkholes or ground subsidence associated with abandoned room-and-pillar lignite mines.