NaCl solution has significant effect on the residual shear strength of soil from mudstone areas. Soil samples were collected from the intact rock (unsheared zone) and sliding surface (sheared zone) of five mudstone landslide areas. Mixtures of distilled water and the intact rock possessed higher electric conductivity than those of distilled water and the sliding surface soil. When these samples were leached with distilled water for five times, they showed a substantial decrease in the electric conductivity. The residual shear strength varies according to the NaCl concentration in the pore water. Ring shear tests were also carried out in the commercial standard clays as bentonite and kaoline to understand the shear strength characteristics of the mudstone soils. When bentonite was treated with NaCl solution as pore water, it showed about three times increase in residual shear strength and decrease to one quarter in liquid limit. The effect of NaCl solution as pore water in kaoline was comparatively low. The residual shear strength of soil from mudstone area also increased when it was treated with NaCl solution as pore water. The result of the X-ray diffraction analysis showed that the proportion of smectite is higher than that of kaolinite in the residual soil from mudstone. These smectite minerals were found to be highly sensitive to NaCl solution. Because of the sensitive changing character of smectite minerals to NaCl solution, soil samples with NaCl rich water showed lower value in consistency limits and higher value in the residual shear strength compared to soil samples with distilled water. The findings suggest that the residual shear strength and index properties should be considered as variable properties as a result of the change in the pore water. Such properties are useful to understand frequent re-activation of the landslide occurring in the mudstone areas.

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This paper first shows that in a uniform slope with a given pore water pressure distribution the location of the critical slip surface for a particular method of slices is dependent only upon the c'/ tan φ' ratio of that slope. Then, two representative types of slope profiles with a rotational sliding mode are presented to model actual slope failures, and two dimensionless parameters are introduced for each of the two slope models. Charts showing the relationship between these two parameters are obtained for typical slope inclinations and three different values of the pore pressure ratio. Using the charts and the safety factor equation of a method of slices, a unique pair of (c', φ') values can be rapidly determined based on the failure surface in a uniform slope. The proposed method is demonstrated on the results back calculated from two well-documented failed slopes.

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Some sediments of the head of landslide blocks were observed, which appeared by road construction. Some sediments of this area were described geologically and classified into three facies according to the forming mechanism. The facies have a sequence of sedimentation as one cycle of landslide erosion. Namely, (1) Occurred of landslide event as a mass movement on the slope (forming of the “event facies”). In this time, sliding area is turned into a low land with a rough surface, which becomes a sedimentary field.(2) The rough surface of the lowland changes into a flat surface by mainly of an inflow water action caused rapid sedimentation and erosion (forming of the “repairing facies”).(3) After weak supply of eroded material, eolian deposits become dominant sedimentation, which cover slowly on the surface (forming of the “covering facies”).
By the clarification of the sequence or multiple sequence of a landslide area, it is possible to recognize a detail history of landslide.

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The number of sliding units in a landslide has greatly affected on the generation mechanism of the landslide. Bottle neck structure is one of the controlling factors of the landslide behavior, however, roles of the bottle neck structure are still not well known due to the limited number of published researches. The bottle neck structure is considered to provide controlling effects the development and direction of movement of the sliding unit. The mutual interaction between multiple bottle neck structures can develop to a single sliding unit which can ultimately cause a large-scale landslide.

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This study shows the posibility to quantitatively observe the discharge of groundwater from collapsed slope by remote sensing. In order to remotely observe the discharge situations of groundwater, the temperature of collapsed slope was observed by means of infrared thermal imaging. A slope had been artificially failed by artificial rainfall then a series of thermal images was taken from a distance. Since the collapse occurred at time when the underground temperature was higher than that of ground surface and air, the temperature of collapsed surface immediately after the failure was relatively higher than that of the original ground surface. The collapsed surface then began to be cooled by convection of the air but the temperatures were found to fall down at different rate. Slow rate was observed in some areas and it is considered to be due to the heat transfer from the deep ground to the surface in such areas. According to the observation in this experiment, temperature around one of such areas formed an onemeter-width peak after failure and, the width became to be about 0.5 m at 36 minutes after the failure.

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Mountain slopes of landslide origin are distributed under the 25km² in the Higashinaruse regeon, Akita Prefecture, northeast Japan, where the Neogene siliceous mudstone are dominant. We have studied the formative process of such landslides through investigating geological structures and rock fractures in the siliceous mudstone in the field. As a result, it has been clarified that such large-scale landslide topography was not formed by a single block movement but has been formed in multi blocks. The topography of these landslide blocks have been formed under control of the fractures of siliceous mudstone and the inclination direction of the stratum. These results are expected to clarify the mechanisms of landslide formation in the Neogene siliceous mudstone beds that are widely distributed in northeast Japan.

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The authors have analized comprehensively the mechanism of the groundwater flow in large scale landslide areas based on the results obtained from groundwater quality analyses in two large scale landslide areas in Tertiary system in Tohoku region. A comprehensive analysis of groundwater flow is effective and credible by executing comparisons of the results of groundwater quality analyses, groundwater level observation, and groundwater tracing in addition to analyses of surveying of geological structure.
In Takisaka landslide the flowing behavior of groundwater was comprehended by the analyses combined with groundwater quality and groundwater tracing with due consideration of variations of groundwater levels in each geological formation and sliding block. Furthermore it is suggested by the analysis of the distribution of Cl^- ion concentration in groundwater that the deep groundwater through granite rock mass has a certain influence for raise up of groundwater level during snow melting period.
In Hirane landslide the flowing behavior of groundwater was comprehended by the interpretation of mixing of groundwater from Tertiary system and groundwater from deep zone. The flowing behavior of the groundwater estimated by the groundwater quality analyses is coincident with the results of groundwater tracing.
As a result a holistic method to analyze behavior of groundwater flow in large scale landslide areas is newly proposed. It is appropriate and effective to carry out the following steps, namely groundwater quality analysis, comparison with groundwater tracing, identification of groundwater level variation, estimation of groundwater flow pass and lastly holistic analysis of characteristics groundwater flow in whole landslide area.

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Three-dimensional safety factors of idealized slopes composed of plane sliding surfaces are calculated and compared based on the exact solution method, Hovland method and the 3D simplified Janbu method. Conclusions are as follows; (1) Hovland method gives smaller safety factor values compared to the exact solutions, especially in cases of narrow failure width and high water pressure along sliding surfaces.(2) the 3D simplified Janbu method gives the same safety factor values as the exact solutions.

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Green rocks that have originated from basalt and tuff are distributed in the Nishinotani landslide area . Chlorite is the major constituent mineral of green rocks, and it is formed in the phenocrysts found in these rocks. The chlorite in these rocks exhibits a little swelling property because it includes several smectite components. Thus, it is proposed that the landslide activity is induced not only by smectite, but also by chlorite, which exhibits a little swelling property.

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