The time of failure of a slope is forecasted using the basic equation regarding the time process in tertiary creep. First, the equation to forecast the time of slide (t_r) is derived. This equation is the same type as Saito's equation with respect to exp (at). Here, a is the constant, and t is time. Then, if the value of coefficient a is assumed, the time of failure (t_r) can be forecasted using time (t) with respect to three equal intervals Δl in continuous displacement (l). This method of slide forecasting is called the a-t_r method and slide forecasting is made for some field v-t data. The error for this method is not much better than Saito's graphical analysis.

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The behavior of sediment accompanying the collapse of mountainsides was examined as part of a study of the mass flow of sediment and water, which is represented by debris flow. In connection with the behaviour of collapsing sediment, the following equation expresses the applicable resistance law in the case of dry sand.
τ=τ_y+τ_g
where, τ_y: Yield stress, τ_g: Force resulting from the non-elastic collision between grains.
The authors derived the equations for the velocity distribution in the vertical direction and the mean velocity of the mass flow of dry sand from the resistance law mentioned above by making use of Bagnold's grain arrangement model. The velocity distribution thus obtained was evaluated through flow tests of dry sand. Moreover, a numerical simulation model of the flow and sedimentation of collapsing sediment was created by making use of the resistance law mentioned above. This simulation model was compared with flow and deposition tests of dry sand carried out in two-dimensional waterways with the object of verifying its appropriateness.

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The strength parameters of slip layers clay of landslide have been investigated by shear tests. The strength parameters differ from the drainage condition in shear test. It was shown that the movement of landslide was generally dominated by the effective stress. The effective angle of shear resistance was not changed by the remolding. The characteristics of residual strength by the method of shear test were investigated. The minimum residual strength was obtained by the ring shear test. The shearing characteristics between clay and base rock were experimented with the slice shear in triaxial cell. It was clarified that the angle of shear resistance between clay and base rock depend on the roughness of the base rock.

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The residual strength of clay is taken into consideration in the stability analysis of landslids, but the present technology is in such a state that the method of measuring residual strength is not established as yet. The residual strength varies with the type of measuring apparatus used such as ring shear apparatus or reversal direct shear apparatus and also it varies with clay sample condition such as disturbed or undisturbed. The present research is focussed on the condition of clay sample and its relation with residual strength. As a common practice the residual strength of clay is measured with the sample in disturbed condition and with the basic conception that it is not influenced by the stress history. In this research residual strengths of the same clay in both disturbed and undisturbed conditions are measured and obviously the results are different. Thus it is to be suggested that the residual strength should be measured with the clay sample in undisturbed condition.

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This paper describes slope forms and landslide activities in the area underlain by Tertiary mudstone of the Teradomari and Shiiya Formations of Niigata Prefecture, Frequency distribution of angle for randomly sampled slopes in the area showed that the angle of 15 degrees was defined as a criterion for classifying of these slopes. Two types of landslide have been identified in the area: (1) Rapid-type landslides, characterized by their higher speed and larger morphological change, occur in the weathered zone of the mudstone, and (2) Slow-type landslides having a characteristics of their slower speed and smaller morphological change occur in the colluvial deposits derived from previous landslides. The investigation for a large number of slope profiles of these landslides indicated that the rapid-type landslides originally arised on a steeper slope segment than 15 degrees and the slow-type landslides were restricted to a gentler slope segment than 15 degrees.

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