Every year many sediment disasters have occurred on decomposed granite slopes which are distributed in western Japan. We carried out field survey and simultaneous natural radioactivity and electrical prospecting in order to understand the characteristic of collapses on decomposed granite slopes. From the results, following matters proved to be clear.
1. The results of the investigation of the devastation in Nakatsuoka river basin, the western part of Hiroshima city are as follows.
(1) The average length of collapse is 12.9m and below 20m is 87%. The average width of collapse is 7.6m and below 10m is 83%. The average volume of collapse is 120m³ and below 200m³ is 86%.
(2) The average angle of inclination is 41.3° and 30° -50° is 92% (291 out of 317 spots).
(3) The persentage of collapse in this basin is 80 spots/km².
(4) The volume of collapses is about 24, 000m³/km².
Those are considered the typical examples of decomposed granite slopes wthich are distributed in and around Hiroshima city.
2. As a result of these investigations, it was proved that checkdam works are effective for disaster prevention and mitigation systems.
3. We suggest that it is better to make investigation of the natural radioactivity and the electrical prosrecting at the same time in order to find deradational slopes. It is considered that the area of high radioactivity and low specific resistivity is degradational ground.
As an actual proof we can show the index of slope stability and the predictability of collapse at Ohgusa area and Hitotsukojima district in Hiroshima Prefecture.

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The objective of this report is to state the topography, underground structure as well as the motion properties of largescale deep rock slides in the TERTIARY GREEN TUFF region, using the Takisaka landslide as a model testing area. The results areas follows;
1) Based upon the analysis of the moving quantity obtained by aerial photogrammetry, the Takisaka landslide can be roughly separated into a northern region and a southern region with respect to Sekizawa, and the slope motion type in the southern region corresponding to a planner slide type landslide.
2) The landslide boundary portion and block group are limited by faults.
3) The sliding surface shapes in the main measurement line are not limited by the ascent slope structure, and have a very low angle through the fracture bed of GREEN TUFF existing as a dip slope condition, which look like as boat shaped. In the course of the investigation of the sliding surface, effective use was made of the detectors of sliding surface and sinker method.
4) The movement was mainly made in the snowelt season, and sliding distance strongly correlates with the maximum snow cover depth of snowmelt season. Furthermore, changes in the moving quantity correspond sensiblely with those of the groundwater table.
5) The movement is similar to a creep type and is in accordance with the results of the examination of the soil properties of the fracture bed of GREEN TUFF in Sodenosawa which has a sliding surface.

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In order to increase the accuracy of stability analysis of the first time slide in the bedrock, it is important to clearly perceive the characteristics of geological structure, the residual strength acting on the slickenside surface of discontinuities, and the peak strength of the fractured mudstone in the slip surface zone.
Concerning the landslide of the Neogene mudstone discussed in the present report, it was found that the slip surface has been formed by being strongly controlled by the strikes of bedding and fault planes, cutting through the fractured mudstone. The residual and peak strength parameters obtained from the ring shear test and the triaxial compression test were as follows: c_r=7kPa and φ_r=16° for the slickenside surfaces; c_f′=75kPa and φ_f′=36° for the specimens of mudstones having opening cracks; c_f′=95kPa and φ_f=42° for the mudstones having internal cracks and microcracks. For the stability analysis a new method was introduced to assess the parameters of average shear strength along the slip surface, and applying the strength parameters mentioned above, c′=11kPa and φ′=17.5° were obtained.

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In order to investigate the behaviour of the groundwater level, pore water pressure and underground temperature during snow-melting season in landslide areas, the field investigations were carried out.
Our main conclusions from the field investigations are: i) The decrease of snow depth during snow-melting season has been occurred at uniform rate, irrespective of the snow depth. In addition, the snow-melting which is equivalent to the rainfall about 30mm in a day was observed. ii) The time of the seepage to the grounds of snow-melting due to thawing may be predicted by measuring the underground temperature.

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The safety factor of an experiment slope is discussed herein with regards to the rise and drawdown of reservoir water level.
The time-dependent saturated-unsaturated seepage problem was analyzed by finite element method. and solutions thus obtained were compared with the pore pressure observed in the experiment slope.
Conclusions reached are as follows.
(1) The pore pressure distributions as calulated by the F. E. M. agree with the experimental results. (see fig. 8, fig. 9)
(2) The safety factor of the slope has a minimum value both at the rising and drawdown stages of the reservoir water level, the minimum value at the drawdown stage being smaller than the one at the rising. (see fig. 15)
(3) Noteworthy is that the saturation of the toe of sliding mass at the rising stage of water level is likely to trigger a total collaspe of the sliding mass.

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