SYSTEMATIC AERIAL PHOTO ANALYSIS OF SOFT GROUND CONDITIONS

Abstract

In such a mountainous country as Japan, alluvial lowlands are basically important for human activities, and are most densely populated. As the occupance of the earth's surface by man-made structures is essential for human activities, ground conditions of lowland areas should be considered the significant natural environment. Ground conditions are also closely related with human activities as causative factors which result in natural disasters such as ground subsidenc and earthquake damage. Almost all problems due to ground conditions which affect land use, land development and disaster prevention have resulted in the areas with soft ground conditions, as evidenced in such industrial cities situated on coastal deltaic lowlands as Tokyo, Osaka, Nagoya and Niigata, and in many other poorly-drained lowlying lowlands throughout Japan where rapid regional development is now in progress. Ground conditions have mainly been studied by soil engineers and engineering seismologists to the present in Japan. Considering the significance of ground conditions as environment, their formations, areal differences and influences to human activities must be the subjects of physical geographical study. Numerous studies on photo analysis and interpretation in geology and soils have been conducted by many specialists, but no systematic study on ground conditions has been made in Japan or in other countries. The first purpose of this paper is the presentation of the methods and techniques used to analyze ground conditions, above all soft ground conditions by aerial photographs. The second purpose is the contribution of the methods and techniques to engineering soil survey and regional planning from the viewpoint of disaster prevention. The main tool of aerial photo analysis and interpretation used in this study is a systematic analysis of micro-landforms supported by the know-ledge of geomorphological development. Geology and soils comprising ground conditions cannot be seen directly on aerial photographs. The main keys for interpreting geology and soils are micro-features of the earth's surface and tonal characteristics. But, the analysis of tonality is handicapped in alluvial lowlands of Japan because of intensive land utilization, and the systematic analysis of micro-landforms plays the most important role. Micro-landforms seen on lowlands can be regarded as a record of a recent episode of geomorphological development and the surface expression of sedimentary materials deposited by various agents. In other words, it may be said that micro-landforms of lowlands are the most significant indicator showing the distribution of geologic materials. Hence, the delineation of the terrain units composed of a single kind of geologic materials depending on the similarlity and homogeneity of micro-features, and the correlation of those units with engineering soils are the essential processes for determining ground conditions from the air. The first step to infer ground conditions is the delineation of “micro-landform units”, which are the smallest land units and are composed of similar geologic materials having similar physical properties. The standard micro-landform units which are used to interpret engineering soils in alluvial lowlands of Japan are presented in Table 1. Classification of micro-landform units is based on genetic classification of landforms. These units are suited for delineating the areal extent of soft ground areas. However, geologic materials or soils inferred directly from such units are generally limited to those forming near the surface of the ground.