Abstract
Kushiro Coal Mine in Kushiro, Japan is now excavating coal seams at a shallower level than before. Approximately 100 old workings, which were excavated about 50 years ago, were observed at either longwall coal faces or sidewalls of roadways. Most of them were completely closed with a few exceptions, which were in open condition without severe deformation. Two thirds of the observed old workings were closed mainly by roof sagging. The sagged roofs were either deflected or plugged in. Development of the tension cracks were observed in the deflected roof. One third of the old workings were closed mainly by floor heaving, mostly accompanied by the severe deflection. Complete closure of the old workings can be attributed to the following two reasons: One is rock stress of considerable magnitude composed from the initial one and the induced one due to mining. Another is weakening of the sedimentary rocks surrounding the opening due to increase of the water content brought mainly by the supply of water detached from the injected filling material with high water content. It is certain that the two mechanisms work simultaneously to the closure of the old workings, but the latter is considered to be prior to the former in its effect.
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