Ground-Geology and Natural Disasters due to Soft Ground Conditions in the Shizuoka-Shimizu Region, Central Japan

Abstract

The occurrence of not only such natural disasters as ground subsidence due to the taking up groundwater, earthquake damage, flood losses, etc. but also such public nuisances as river pollution, air pollution, etc. have recently increased in the urban areas of Japan which are situated on recent alluvial lowlands along the coastal region, and accompany the rapid development of these regions, i. e. the rapid growth of industry and the rapid increase of population in these regions. Among the disasters, almost all of the natural disasters occurred in soft ground areas in connection with poor land and water utilization. Therefore, to examine and forecast the danger from disasters due to soft ground conditions is most important to allow reasonable plans for the regional development and the prevention of disasters to be made.
The Shizuoka-Shimizu region, situated on the Pacific coast of Central Japan, is expected to be developed as a metropolitan area by the combination of two cities, Shizuoka and Shimizu (Fig. 1). The distribution and physical properties of soft ground, and problems of prevention of natural disasters caused by soft ground conditions, particularly earthquake damage, are described and discussed in this paper.
(1) The recent alluvial lowlands in the region investigated, the Shizuoka-Shimizu Lowland, are limited by the Tertiary mountains to the north and Suruga Bay to the south (Fig. 2). The River Abe flowing down from the southern part of the Southern Japan Alps and having a great landslide area on its upper reaches, Ohya-kuzure, passes through in the western part of this region to enter the sea, and the eastern part of this region lies in the catchment area of the River Tomoe, which rises in the Tertiary mountains (Table 2). The Udo-san Hilllands composed mainly of Pleistocene gravel layers and having a plateau on the top, called Nihondaira, is located in the middle part of this region.
Fig. 2 is a landform classification map compiled by aerial photographic interpretation. Micro-landform units shown in this map are classified systematically by origin and composition to inferred engineering soils, based on the methods and techniques of micro-geomorphological analysis for engineering soil surveys from aerial photographs (Kadomura 1965, 1966). From this map, the Shizuoka-Shimizu Lowland is divided into several “landform areas” as shown in Fig. 3 and Table 1, depending upon the distribution pattern of micro-landform units, the geomorphological agents, their topographic locations and other criteria.
The “landform areas” consisting of gravelly layers are divided into two categories fluvial landforms such as alluvial fans and gravelly valley bottom lowlands, and marine landforms such as coastal shingle bars and sand banks. The areas of poorly-drained deltaic lowlands, which originated from former lagoons blocked by shingle bars and sand banks, are distributed along the both sides of the R. Abe Fan and in the downstream area of the R. Tomoe. These deltaic lowlands are formed of thick muddy deposits mentioned later. During the formation of these poorly-drained muddy lowlands as well as the formation of soft ground, coastal shingle bars and sand banks, which formed before the formation of the muddy lowlands, played a important role.
(2) The ground in this region investigated and the surrounding mountain lands is divided into unit layers as in the other coastal lowlands of Japan shown in Table 3, for comparison of their relative foundation bearing strengths. Among these unit layers, the Upper Muddy Layer (Um), the Lower Muddy Layer (Lm), and a part of Upper Sand-and-Gravel Layer (Usg) composed mainly of sandy soils (Us), which are distributed along the margin of the Shirnizu Sand Bank and in abandoned low water channels, should be noted as soft ground layers.