2004年新潟県中越地震によって地すべりが多数発生した。本論文は, 地すべりダムを形成した東竹沢地すべり及び寺野地すべりを対象に, 現地調査及びリングせん断試験を実施し, 再活動地すべりにおける地震時高速地すべりの発生・運動機構を研究したものである。東竹沢地すべりと寺野地すべりとも過去に地すべりを引き起こした移動土塊内部の再活動地すべりである。斜面土層は, 海成の砂層とシルト層から形成されていたことから, 両者からサンプルを採取して地震時地すべり再現試験を行った結果, 砂層ではすべり面液状化が発生し, 高速地すべりが生じること, 一方シルト層はほとんど移動しないことが見いだされた。また, 同時に降雨・融雪により地すべりが発生する場合は, シルト層ですべることも推定された。

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In order to study the physical and chemical weathering and their impact on the shear strength of the soil mass, soil samples were collected from different depths ranging from the ground surface to a depth of 15m at two different drainage wells in a Mukohidehara landslide area. These specimens represent the residual soil near the surface, specimen at a depth range of ground water level fluctuation, less weathered specimen from the depth below the ground water level fluctuation, the remolded soil from sliding zone and the specimen below sliding zone. Measurement of Atterberg's limits, particle size analysis, X-ray diffraction analysis, slake durability test and X-ray fluorescence tests were carried out. Drained simple shear tests, and drained ring shear tests were also conducted on those specimens to measure their strength characteristics.
Test results showed that fully softened shear strength of the less weathered specimen was very close to the peak shear strength of the specimen from the sliding zone. Test results showed no significant variations of index properties (liquid limit, plasticity index and clay fraction) , mineral compositions and chemical compositions with depth. Residual shear strengths of the specimen also did not vary much with depth. In order to compare these findings with rocks from the other types of formation, specimens were collected from the ground surface, ground water fluctuation zone, less weathered zone and sliding zone of three other landslides sites. The test results on the index properties, shear strength properties, mineralogical analysis, and bulk chemistry, of those landslides were consistent with the findings in Mukohidehara landslide.
The test results of this study suggested that physical weathering plays a major role in the reduction of shear strength of mudstone, in comparison to the chemical weathering in the form of mineralogical and chemical alteration. Test results also suggested that physical weathering by slaking and disintegration is significant at ground water fluctuation zone in those mudstone formations.

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