Abstract
For the purpose of disaster prevention caused by excavation of Building stone, many fundamental investigations were carried out.
1) The Oya stones have been excavated as building stones since olden time and as a result many enlarged caves have been remained under ground.
2) Oya stones are divided into three stratum, hard-coarse (Koshitu-arame), soft-coarse (Nanshitu-arame) and fine (Saime) textures, in ascending order. Especially, in the portion of the fine texture stones, rock-falls and depressions were occurred remarkably.
3) Characteristic fracture systems are of NW and NE directions filled by fissure clay, that is, clinoptirolite. The distribution chart of the fracture systems in the fine texture rock are illustrated in Figs.
