As a clay soil along the slip surface bears the property of strain softening, to evalute the slope stability in consideration under this condition, the finite element method combined with joint element reflecting the discontinuous plane of slip surface is discussed on this paper. The effection of ground water fluctuation, which is mainly moving cause, on the slope stability can be easily discussed by the application of effective stress conception at the time of evaluation of joint element strength. By using this calculative, approach, progressive failure along the slip surface may be accounted quantitatively.

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A number of landslides occur in the northern mountain districts of Nagano Prefecture which consists of the Tertiary layers. Most of landslides in those districts (over 90%) are the type of shallow layer slides, the slide surfaces of which lie within 10 meters in depth. These shallow layer slides have not drawn attention of the public, because the damages done by shallow layer slides are usually small in scale. However, since slide areas are used as cultivated fields and the lives of inhabitants of mountain villages are maintained by cultivating arable land in the slide areas, countermeasures for landslides are essential for protecting the lives of the inhabitants. In the shallow layer slide areas, the mass movement occurs gently and intermittently in groups: in consequence, landforms peculiar to shallow layer slide areas are formed and the landforms are fairly different from those of rock slide areas which are accompanied with fierce slides.
In this report, the occurrence mechanism of shallow layer slides and the landform characteristics of shallow layer slide areas are investigated based on the actual examples. In the shallow layer slides areas, the boundaries, between sliding areas and non-slide areas are ambiguous. The topside slide scarps and the side slide scarps are not, in many cases, formed and ponds and ditch-like depressions are not created in the down side of slide scarps. And the whole landform shows an undulating mountain slide which has several swelled soil mass similar to closed shells. Such a landform expands gradually to do damage to the surrounding villages but does not spread rapidly to damage severely those villages. The shallow layer slides occur in the mountain districts consisting of the Tertiary mudstone basement rock. Since the continued slide movements make mountain sides gentle, the shallow layer slide areas are generally gentler in slope than the non-slide areas. The mountain sides without landslides are generally steep in slope and utilized as woodlands. On the contrary, the mountain sides with landslides are gentle in slope and used as vegitable fields or paddy fields.

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The residual strength of clayey soil is specially concerned with the stability of slopes, and it is very usefull for landslide engineers to recognize on this theme.
It is generally agreed that only residual strength is available if movement is renewed along the pre-existing slip surface.
The residual strength is independent of stress history and decreases with increasing clay fraction, or increasing Ip.
The residual strength depends on effective normal stress. It is known that the residual friction angle was unaffected by temperature changes between 10°C and 60°C. The residual strength is not noticeably rate dependent. Three modes of residual shear behaviour are recognized: a turbulent mode, a transitional mode and a sliding mode, the mode depending on dominant particle shape and on the coefficient of interparticle friction.
Ring shear test, reversal direct shear box test and triaxial test containing a pre-cut surface are used to measure the residual strength. Among these, direct shear test is the simplest method to measure it. The shear strengths measured by this method are in good agreement with the analyses of post-slip movements, although it's strain is restricted, while ring shear tests give lower results. Considerable field data support the adequacy of the direct shear test.

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The present authors gathered the data from various tests made on the landsliding soils in Niigata Prefecture and investigated the characteristics of the soils. According to the statistical analysis, it has become clear that the landsliding soils have a different range of physical value according to each geological formation. On the other hand, we found that the residual strength of the landsliding clay, determined by the count backward method of stability analysis and other methods, has areverse relationship to the plasticity index (I_p) and the clay fraction (under 0.002mm), and that it shows adifferent range of value according to each geological formation. Moreover, on the basis of these results and the cassification of landslides by Uemura, we stated the relationship between landsliding forms and soil mechanics, by the index of the physical value of soil.

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The Hoppo landslide area is underlain by the Miocene “Green tuff layers” partially covered by the Plio-Pleistocene volcanic rocks and the Pleistocene and Recent debris. Those layers and debris are partially and severely altered to characteristic clays in solfataric areas. The clay minerals in this area are mainly composed of Montomorillonite and kaolinte.
Two types of landslides can be identified on the location of slide surfaces in geologic cross sections. One of them is a type of debris slides in which the slide surfaces are observed in the debris or along the altered clay layers between the debris and “Green tuff” or the volcanics. In another type of slides, the slide surfaces or potential surface of slide are considered to locate in the “Green tuff layers”.

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