Monitoring the process of ground failure in repeated landslides and associated stability assessments

Field monitoring of two repeated landslide areas, namely the Sodechi and the Yonaihata landslide areas on the island of Honshu, Japan, has revealed that the spread of ground failure occurred as in normal landslide, even though these landslide events were repeated slides in clayey material. A rapid increase in surface displacement (the beginning of third-stage creep) occurred simultaneously with an increase in strain in the ground (the cause of local failure along the sliding surface), resulting in large, continuous landslide movements. Thus, an entire slope began to collapse when the full length of the failure surface became connected. Ring-shear tests of landslide clay were conducted to determine three kinds of shear strength parameters: (1) fully softened strength parameter φ′ (parameter for peak strength of remolded clay); (2) “specimen-separated strength” parameter φ′ss (newly proposed in this study); and (3) residual strength parameter φ′r. Stability analysis revealed that φ′ss was the most appropriate shear-strength parameter to express slope stability in the ground failure transmission stage. Thus, this result provides a first step in applying the mechanism of ground failure transmission to the problem of slope stability by defining the specimen-separated strength parameter φ′ss.