斜面崩壊の地質学的研究

抄録

The authors thought that a breakdown of a slope is basically attributable to the augumentation of the slope's instability potential, and that the time-variation of causative characters related to geological and topographical conditions must be taken into account in the discussion of the mechanism of landslide development.
The Chichibu basin was chosen as the area of the present study, and the mechanism of landslide development was considered mainly from the geological stand-point. The major object was the landslides caused by the action of weathering.
Bedrocks
The physical properties of rocks are correlated to one another, and it has become clear that they can be represented by the uniaxial compressive strength and the dry shrinkrage ratio.
Among the Tertiary rocks of the Chichibu Basin, mudstone shows that its physical properties vary with geological horizons, and the ralation between its physical properties and the geological horizons is represented by two cycles which correspond to the Oganomachi group and the Chichibumachi group respectively.
Weathering zone
The weatherability of fresh rocks can be expressed by the uniaxial compressive strength and the dry shrinkage ratio. A weathering zone is easily developed in a area where the basement is composed of rocks with small uniaxial compressive, strength and large dry shrinkage ratio.
A weathering zone can be divided into four subzones, I-IV, from the ground surface to the fresh bedrock down below. The physical properties of each subzone change according to the degree of weathering, but their changes from one sabzone to another are gradational.
Moreover, it has become clear that the quantity of cracks in the weathering zones of the study area is far from negligible.
Development of weathering landslides
On the basis of the factors which augument the slope's instability potential with the lapse of time, the authors classified the landslides into three groups, namely, the weathering landslide, the stream erosion landslide, and the head erosion landslide. In the present paper the authors focussed the discussion on the mechanism of development of the weathering landslide.
In order to infer the state of development of the weathering zone before the breakdown of the slope, the authors carried out seismic prospecting along the margin of the destroyed slope. There are two types of landslides, one occurs mostly in the first velocity layer with less than 0.4 km/s, and the other extends down to the second velocity layer with 0.4-0.8 km/s. It is evident that the character related to the cracks of the weathering zone and to the softening of rock fragments is contributing to the development of landslides.
The depth of the weathering zone has its limit in accordance with the slope's gradient. The depth of the weathering zone increases with time, and in this process the part that overstepped the critical stability would be broken down.