A PRELIMINARY STUDY OF THE LANDSLIDE IN THE SOUTHERN PART OF BOSO PENINSULA, CHIBA PREFECTURE

Abstract

This paper is a preliminary report of the landslide in the southern part of BOSO Peninsula at Soro Village. The landslide of Soro Village, situated 6 km., west of the town of Kamo-gawa, occurred on May 23, 1952 near the top of Mineokn Mountain and gradually enlarged and extended itself down the slope. The mechanics of the landslide may be explained as follows 1) The serpentine was distributed in the vicinity of the divide of Mineoka Mountain and was weathered in depth. When a great amount of ground-water percolated into weathered materials, they were generally liable to slip. Landslips of the serpentine have occurred on the upper part of the slope so that a great deal of debris was accumulated there. 2) The materials of the landslide consisted of rock debris and clayey soils derived from serpentine. This debris was weathered by percolating water and became a clayey soil colored white-blue. This clay had a very low liquid saturation point (Tab. 1.), hence, it was liable to slide. On the other hand, stationary earth materials generally were found on gentle slopes composed of sandstone, greywacke and shale (Mineoka group); their soils had a relatively high liquid saturation point (Tab. 1) and have resisted slides. 3) There are paddy fields on the upper part of the slope and corn fields below. Hench, groundwater from the paddy fields also percolated into the debris. 4) The temporary increase of pore water pressure, which lead to a disa-strous reduction of internal frictional resistance, may be considered as another of the causes of the landslide. The determinations of the depth of the slip plane, the shearing strength of creeping materials, and of the chemical changes of serpentine will be necessary for the continuance of studies on the mechanics of the landslide.