A weak zone composed of cohesive soils of a tectonically generated special structure is sometimes distributed near the boundary between a deposit and an underlying highly weathered bedrock. Then, if the passive earth pressure, which provides the resistance to sliding, is lost at the tip of the weak zone as a result of slope cutting and the weak zone is exposed on the slope, certain shear acts on the weak zone owing to the self weight of soil mass and causes the weak zone to suffer from creep failure. Thus, a landslide occurs.
In this study, a field survey was conducted for an actual landslide and its characteristics were identified from laboratory creep tests. The following results were obtained in the study
1) The weak zone had a gradient of more than ten degrees and formed a dip slope. As a result of the creep tests, it was found that the creep failure occurred where the stress ratio (ratio of shear stress to the upper limit of yield stress) was equal to or higher than 1.0 and that creep took two years at a stress ratio of 1.0.
2) Where there was a certain amount of rainfall, pore pressure in the weak zone lowered the upper limit of yield stress, reducing the stress ratio and encouraging creep failure.
3) The findings described in 1) and 2) above suggest that cut slope stability can be evaluated based on the gradient and condition of the weak zone, time passed after cutting and the change in slope deformation with time, which indicates whether creep progresses or not.
The paper finally notes that preliminary geological surveys are essential to the identification of the distribution of weak zones for preventing landslides as the one discussed in the paper.
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