1996 Fiscal Year Final Research Report Summary
Gradual distribution of deteriorated Shirasu (dacitic pyroclastic flow deposits) composing steep slopes in southern Kyushu, Japan
| Project/Area Number |
07640608
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Geology
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| Research Institution | Shimane University (1996)
Kagoshima University (1995) |
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Principal Investigator |
YOKOTA Shuichiro Shimane University Interdisciplinary Faculty of Science and Engineering, Professor., 総合理工学部, 教授 (60211653)
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| Co-Investigator(Kenkyū-buntansha) |
IWAMATSU Akira Kagoshima University, Faculty of Sciecne, Professor., 理学部, 教授 (80018663)
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| Project Period (FY) |
1995 – 1996
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| Keywords | slope failures / Shirasu / pyroclastic flow deposit / deterioration / weathering / hardness / Kyushu / hazard map |
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| Research Abstract |
Many slope failures have occurred on steep slopes composed of the Quaternary pyroclastic flow deposits in southern Kyushu. The deposits called as "Shirasu" in local name, are homogeneous non-welded dacitic tuff, and are soft and permeable. Based on the recurrence failure in this type, the occurrence can be understood to have been brought about due to temporal reducing in strength of rocks. To understand the style of reducing in strength within the slopes, the authors tried to measure the deterioration distribution from the slope surface to inward portion in several slopes in there. Penetrative hardness by hardness meter is used as the index of grade of deterioration. Sieving test for grain-size distribution and measuring of dry densities have also made with the X-ray diffraction for clay mineralization.
As a result, distribution of hardness shows gradual changes from the surface to inward portion in the slopes. While deteriorated zone from the slope surface attains to 7 to 8 meters in top region, it is only 1 to 2 meter in foot of the slope. This can be understood as the combination between weathering and erosion in each locations. The process of deteriorating in this deposits may include not only disintegration of grains but also chemical changes such as clay mineralizarion. These distribution in slope surface may control the amount of volume for a slope failure and its recurrence time. The temporal process of deteriorating estimated from these distribution may also support a mathematical model on deterioration proposed by one of the authors.
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