Hideo Kasai

On the Collapses of Drag-Roads (Yamanashi Pref.) owing to Typhoon 15 (1981)

In August, 1981, many collapses occurred in Yamanashi Pref. owing to Typhoon 15. In this report, the types and features of collapses are described from a geological point of view, based on the field investigationtions at the stricken areas (South Alps Super Drag Road, Norogawa Drag Road, Sakeishi Drag Road, and Magi Drag Road). The results obtained from these investigations are as follows: (1) The types and features of collapses are decided by the lithic characters. (2) In case of Masa (weathered Granite) and Talus deposit, collapses are generally occurred at the head of 1st order valleys or within 0-order valleys, and the debris frequently run down valleys in the form of mud-flow. In the former case the types of the collapse are mostly circular slide, and in the latter, most collapses are of the type that slide along the surface of the bedrocks. (3) In case of lutaceous rocks, the collapses are generally occurred at the fracture zones, and concequently, their breaking points have nothing to do with valley order.