ON THE PHYSICAL AND DYNAMIC PROPERTIES OF SOME SOILS DISTRIBUTED IN THE VICINITY OF MT. TSUKUBA
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The landforms originated from slip of slopes (JISUBERI) and rupture of slopes (YAMAKUZURE), termes classified by Yatsu in 1966, can be recognized in many places around Mt. Tsukuba. The present paper is concerned with these mass movements from the point of view of soil mechanics. Sampling point No. 1 is an outcrop composed of weathered granite (Photo. 1) . Sampling point No. 2 is the location where a YAMAKUZURE occurred first in 1976 (Fig. 2 and Photo. 2), and again in 1978 (Photo. 3). Soil sample No. 2 is colluvial detritus composed of mixed debris of weathered granite and gabbro. Sampling point No. 3 is the location where a JISUBERI occurred in 1976 (Fig. 2, identical with "s" in Fig. 2 in previous paper; Matsukura et al., 1979). Soil sample No. 3 consists of weathered gabbro debris, containing swelling chlorite. The results of soil tests for physical properties are summarized in Table 1 and Fig. 3. Sample 1 and 2 are sandy soils and sample 3 is clayey soil (Fig. 3). These grain size distributions have a close connection to the Atterberg's limit, and so plasticity index (PI) is low in case of samples 1 and 2, and is high in sample 3.Shearing strength of soil (τf) is expressed by the cohesion (c) and the angle of shearing resistance (φ) as follows; τf=c+σtanφwhere σ is the normal stress The c and φ are determined by box shear apparatus with a single shear surface of each soil sample under dry, wet (in natural water contents) and saturated (in submergence) conditions (Fig. 4) . Under dry condition, samples 1 and 2 have a low value of c and largevalue of φ, but sample 3 has values of high c and small φ. In particular, as the water content increases, the shearing strength (τf) of sample 2 and 3 reduces notably. This phenomenon seems to be a main cause of JISUBERI and YAMAKUZURE. YAMAKUZURE is caused by the soil having low plasticity (PI) and cohesion (c) and large angle of shearing resistance (φ), such as sample 2, and JISUBERI is caused by the soil having high PI and c and small φ, such as sample 3.
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