Abstract
In Yanahara Mine, Japan, a large pyrite ore body has been mined for the last 25 yr. The natural conditions of the ore body and country rocks are favourable, but surface subsidence must be avoided. It was considered that to achieve a high extraction ratio of mining without causing any surface damage would be difficult because the ore body is massing and large. Towards the end of primary mining, fracture of rock occurred in the walls of excavations at the pillars. Thereafter, to investigate the effects of secondary mining on the stability of the remaining pillars and the ground, a programme of field measurements has been carried out. This Paper describes the progress of pillar recovery and presents details of following measurements: deformation of roof rock; tilt of the floor of a mine level; rock stress states; subsidence of the roof rock; and average horizontal strain in a pillar. By the end of 1984, about 70% of the mineable ore had been successfully mined without any serious trouble.
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