Increases in ground water pressure due to rainfall or thawing are known to give occasion to landslide movement in many cases. However, time series behavior of ground water pressure has not been considered in the studies which aim at quantitatively analysis of landslide movement. The present paper deals with the statistical confirmation of the effect of ground water level as against landslide movement and thus presents a simple landslide model, to lead to a long-term determination of landslide movement due to ground water level fluctuation.

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A heat balance method is applied to estimate melt water in a landslide area of a heavy snow district. Comparisons between calculated fluctuation of heat fluxes and observed melt water show that during the early period of snow cover when the heat flux is lower, melt water can be estimated by a heat balance method. When melt water is generated in the surface layer of snow in a middle period of snow cover, most of it can not reach the bottom of the snow pack because of the low temperatures and ice layers below the snow surface. Throughout the snowmelt season, 40-60mm/day of melt water is generated by the seasonally increasing short wave radiation, sensible and latent heat fluxes. In the snowmelt season, the calculated heat flux value corresponds to the observed melt water. During rain on snow events, melt water cannot be accurately estimated by the heat balance method because of unknown factors such as water holding capcity of snow cover.

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From the central to the southwestern part of Niigata Prefecture is a well-known landslide area, where landslides occur extensively and most frequently in Japan. However, there are some differences in the type of movement depending on the location, that is, in some regions “slide type” tends to occur, whereas “creep type” tends to occur more frequently in other regions.
Although the main triggering cause is snow-melting or rainfall, there is no doubt that the basic primary cause of these remarkable phenomena is due to the geologic condition, that is, the mechanical or physicochemical properties and geohistorical background of the mother rocks.
The author collected the various rock test data which were examined for the bedrocks in the landslide area, and studied the mechanical properties on each geologic formation and the relationship between their properties and the type of movement. Although much of the data is obtained from laboratory tests, some in -situ rock shear tests and deformation tests are included in it. As a result, it was clarified that the strength and the ductility of the bedrocks do not necessarily increase or decrease in the sequence of geologic formation, and such transition of mechanical properties corresponds to the regional difference in the types of landslide movement.
In conclusion, the mechanical properties of landslide-mother-rocks have a strong influence on the type of movement. On the other hand, since their qualitative properties are succeeded to the secondary or tertiary landslide debris, the type of secondary or tertiary landslide movement is also regionally different.
In this paper (I), mechanical properties of intact rocks identified by the laboratory tests are discussed. Those properties of rocks identified by the in-situ tests and the conclusion based on both tests will be discussed in the following paper (II) with the same title.

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The Geto landslide area, being dormant at present, is divided into three groups of the landslide cluster based on their elevation. Five units of landslide topography are recognized at the places of around 1, 050 meters in altitude, six are around 900 meters, and three are around 800 meters, respectively. Of these, those at the lowest place seem to be formed most recently, since they still leave their head scarp and toe ridge. In order to estimate the capability of recurrence of the Geto landslides, mineral contents in the landslide clay found over the area were investigated. As the clay minerals, montmorillonite, chlorite and sericite are abundantly found, and clinoptilolite, heulandite etc. are found as the zeolite minerals. According to Shuzui (1987) chlorite changes into smectite through the mixed mineral of chlorite-smectite. And it is inferred that clinoptilolite changes into smectite (1984). These suggest that smectite and montmorillonite may increase, but cannot decrease, in content with time. The montmorillonite, when it absorbs water, swells to bring decreasing in its shear strength that acts a role of the primary factor of landslide. The Geto landslide site is considered to have high potential of future activity, since there is abundant in montmorillonite. It is feared, therefore, that the landslide may recur when an artificial change of landform is carried out.

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In this paper, the mechanism of pore water fluctuation is discussed through the data obtained from the field observation in the Kuchisakamoto landslide area. From the discussion of hydrogeological structure and the result of seepage flow analysis, the fluctuation of pore water pressure in the lower block of landslide area is caused by the increase of artesan pressure, which can be easily propagated through the aquifer layer and water path exsisting in the sliding mass, originated from the infiltration of rain through the tension cracks.

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A new direct shear apparatus for measuring shear strength of soft sedimentary rocks was designed. The features of this apparatus are: 1) friction force between upper and lower boxes is eliminated by controlling the gap between two boxes, 2) effect of friction force between specimen and side wall of the box on the normal stress acted on the failure plane can be corrected by measuring normal load at the upper and lower ends of the specimen, 3) intact rock specimens are able to be tested and both peak and residual strengths can be obtained from one specimen, 4) cyclic shear test with any cycle and magnitude of shear displacement can be performed automatically.
Two series of test were performed with the samples obtained from a site where a landslide occured. Residual strength parameters obtained from the test results using intact specimen almost coincide with those obtained from the results using the specimen consolidated from the state of slurry. The merit of this test apparatus which gives both peak and residual strengths with one specimen was verified from the experimental results.

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One of the world largest dams is under construction in the Yangtze Three Gorges area, China. This area is prone to landslides and rockfalls, and their occurrence and areal distribution are strongly controlled by the geological structure and materials of the slipped zone. Eighty-five percent of landslides in the basin is of re-sliding type, and the slipped zone is composed of sandy clay intermingled with detritus of mudstone and marl. After the construction of the dam, it is apprehended that many landslides recur due to the reduction of the effective stress caused by the rising of the water level in the reservoir. In order to elucidate the mechanizm of the landslides in the Three Gorges area, it is useful to take the strength-lowering concept of mudstone into consideration. Several ideas were suggested for this purpose.

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