SHEAR STRENGTHS OF SLIP SURFACE OF RAINFALL-TRIGGERED MUDSTONE LANDSLIDES (Invited Presentation)

The shear strength of a slip surface is the principal factor governing landslide occurrence and movement. It is not a uniform characteristic of a particular soil, as it changes from the post-peak to fully softened and residual strength according to the soil condition.

Some earlier studies of shear strength of slip surface of first-time landslides showed that the shear strength along the slip surface was at the fully softened strength, while the others suggested that the shear strength mobilized during the activation of a first-time landslide to be a value between the fully softened and the residual strength. In the case of reactivated landslides, it believes that residual conditions exist along the entire slip surface.

Landslides occurring in the area of the Neogene Shimajiri mudstone formation that dominates the geology of the central and the southern parts of the Okinawa Island, Japan are classified into four types, namely, the first-time activated landslides, the quasi-first-time activated landslides, the re-activated colluvial-soil slides and other slides. The major distinctions of the quasi-first-time activated mudstone slides include the existence of previous slide blocks within the lower part of the slide mass. The possibility of progressive failure being involved in the occurrence of this type of landslides is not only due to the existence of a bedrock with geological discontinuities, but also due to the old landslides that exist in the lower part of slide mass. For such type of slides, shear strength of the slip surface cannot be represented by a single set of values obtained from laboratory shear tests as belonging exclusively to either the residual, fully softened or peak stages of strength owing to the heterogeneity in displacement of different part of slide block along the slip surface.

In this study, we present the shear strength characteristics of the soil/rock of the slip surface of a mudstone landslide measured using ring shear and triaxial compression tests. Additionally, we also discuss the mobilized shear strength along the slip surface at the activation and reactivation of the landslide based on a stability analysis utilizing a combination of these shear strengths.