大谷石採掘場跡地における岩盤崩落 : 宇都宮市大谷町坂本地区における1989年陥没の安定解析  [in Japanese] Stability Analysis on a Rock Collapse over an Abandoned Mine Cave in Miocene Tuff  [in Japanese]

A large sinkhole due to rock collapse of a roof of an abandoned subsurface quarry composed of Miocene tuff (Oya Tuff) occurred on 10th February 1989 at Sakamoto, Oya-chou, Utsunomiya City. The plan shape of fallen block shows an almost perfect circle, having a mean diameter of 65m and a thickness of about 30m. Compressive strength, tensile strength and P-wave velocity of Oya Tuff were tested in laboratory. Equotip hardness test was conducted in associated with changing in rock strength due to variation of water content. Shear strength of rock mass was expressed as a function of compressive and tensile strengths as well as water content of rock specimen. Rock mass strength considering the scale effect was estimated using Ikeda's index, i.e., the ratio of P-wave velocity in situ and that in a laboratory for specimens. The stability analysis for the mine roof using the rock mass strength shows that the roof reaches a critical condition when the water content of rock mass increases to about 20%.