RESEARCHING PROCESS TO PRESERVE ENGRAVED ROCK ARTS IN THE FUGOPPE CAVE, HOKKIDO, JAPAN

The Fugoppe Cave is located in Yoichi town, Hokkaido, Japan and is registered as a National Cultural Heritage. The geology of Maruyama Hill including the cave is composed of Miocene tuffaceous sandstones, which are soft and porous. The cave has very unique engraved rock sculptured arts of ca.1600yB.P. The walls with engraved rock arts have been weathered and grown cracks prone to rock falls because of exposing for exhibition for a long time. As a result, the engraves such as human bodies and boats have become obscure. In order to preserve the rock arts and engraved walls, first of all, we have been researching for the possibility of rock fall, wall separation and disappearance of the arts, by detailed observation of rock facies and discontinuities prone to rock falls, by evaluation of the degree of weathering, and by measurement of water content. In addition, using several devices, we were monitoring crack opening, temperature and humidity inside the cave. Through this research, we found out the important factors for preserving the rock arts, namely that the engraved wall can be preserved if appropriate temperature and humidity inside the cave are maintained to be constant, and that increase in weathering speed causes disappearance of the rock arts, and that if appropriate water content is kept, the walls can prevent from separation due to decreasing bondage ability of clay mineral in the rock mass. As mentioned above, the engineering geological ways are important to find out key factors for preserving cultural heritages such as a kind of engrave arts on rock walls. Further research is now processing to estimate better conditions for their preservation