The shear strength parameters of Asato landslide, which is a representation of a quasi-first activated slide, was investigated using triaxial and ring-shear tests. The peak strengths of the tested samples were plotted in the range of the strongly weathered-fractured and the moderately/slightly weathered strength zones of the shear strength diagram of Shimajiri-mudstone, while the fully softened and residual strength found their respective places in the middle and the lower group of fully softened and residual strength zones.


The primary cause of landslides is due to the nature of soil and is directly related to the shear strength of the soil at the slip surface. Soil shear strength is not just a uniform characteristic of a particular soil, but it changes from post-peak to fully softened and residual strength according to the soil condition. Although, the main input for a stability analysis of a landslide is the shear strength of the materials that line up along the slip surface at the time of sliding, shear strengths are not measured under laboratory conditions most of the time when postlandslide repair work are designed in Japan. Therefore, in such instances, calculated slope stability values tend to be overestimated or underestimated. As far as landslides in Okinawa, Japan, are considered, many have occurred in the Shimajiri-mudstone area of Okinawa, Japan in the past. The landslides occurrences in this area are classified into four types, namely, the first activated landslides, the quasi-first activated landslides, the re-activated colluvial-soil slides and the other slides (Chen et al., 2007). The major distinctions of the quasi-fist activated mudstone slides include the existence of previous slide blocks within the lower part of the slide mass and sudden and large movement at the time of sliding. The possibility of progressive failure of this type of landslides at the first-slide occurrence is not only due to the bedrock with geological discontinuities, but also due to the old landslides that exist in the lower part of slide mass (Gibo et al., 2008).

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