シラスと砂のセン断特性の相違
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概要
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The aim of this study is to clarify the difference in shearing-characteristices between Shirasu and sand by means of triaxial shear tests. Shirasu, a sort of vast volcanic product from Aira and Ata volcanoes, is widely distributed in southern Kyushu, Japan. It becomes granular soil and has high compressibility when disturbed. Toyoura sand, Nejime beach sand, and Shirasu were used in the test and their physical properties are shown in Table 1. The shape and surface-texture of the grains are shown in Table 2 and Photo 1. Shirasu was obtained at Tose in Kagoshima City, and was geologically in the category of the unwelded part of pumice flow. Only the part of Shirasu having the same gradation as sand was used in this study. The grainsize-distribution-curves are shown in Fig. 1. Toyoura sand and Shirasu-A are in the class of fine sand, and Nejime beach sand and Shirasu-B, coarse sand. Toyoura sand and Nejime beach sand consist mainly of quartz and feldspar. Almost all the fine grains of Shirasu are made of the glass fragments, the coarse grains consist mainly of pumice and of a small amount of derived rock fragments, such as andesite. Drained triaxial compression-tests were performed on compacted specimens varying their void-ratios from loose to dense packing. The size of specimen is initially 5.0 cm in diameter and 12.5 cm in height. Loose specimens were prepared by pouring the material slowly into a cylindrical mold. A dense state of packing was achieved by vibrating and tamping the mold. The specimens were consolidated fully in triaxial cell under the confining-pressure ranging from 0.3 to 2.0 kg/cm^2,and then they were subjected to deviator stress at a constant rate of strain of 0.8 per cent per minute. The axial load, the axial strain, and the volume-change during the shearing were measured. Test results are shown in Figures 2 through 19 and Tables 3 through 8. Basing on the tests the following conclusions might reasonably be made : (1) Both the initial tangent-modulus equaling the slope of the deviator stress versus axial strain plot, at the start of the test, and the secant-modulus at failure equaling the slope of the line between the origin and the point of failure, on the plot of deviator stress versus axial strain, are larger in sand than in Shirasu. (2) Under shearing-process Shirasu has a high compressibility, compared with sand. This property is due to the singularities of the shape and the surface of Shirasu grains and to the effect of grain-crushings during the shearing. Shirasu grains are angular, flat, or elongate, but sand grains subangular or round. In the range of confining-pressture used in the tests, for Shirasu the degradation occurs, for sand, however, any degradation is not clearly observed.(3) The dilatancy-rate at failure for sand is independent of the confining-pressure, that for Shirasu is, however, dependent of the confining-pressure, and shows a tendency to decrease with the increase in it. (4) Shearing-strength of the fine grain-fraction of Shirasu is larger than in case of sand, which occurs owing to the peculiar shape of grains, but that of the coarse grain-fraction of Shirasu is smaller than in sand, which happens owing to the grain-crushings during the shearing. (5) Both sand and Shirasu have the linear relationship between frictional parameter of shearing-strength, φ_d, and dilatancy-rate. (6) The cohesional and frictional contributions to the shearing-resistance are mobilized to be mechanically independent. The chohesion component of strength reaches the peack value at early stage of the strain, while the frictional component requires several times the strain before its full mobilization to have been realized. The following notations are used in the study : c : general expression of cohesion c_d : apparent cohesion for drained test d_<10> and D_<10> : effective grain size after and before test, respectively D_r : relative density e : initial void-ratio m^^- : Mackey's mean shape fa
- 鹿児島大学の論文
- 1976-03-20