The central part of Kochi Prefecture is geologically situated in the Outer zone of South west Japan. Several largescale belt-shaped geological tectonic lines are developed from east-north-east to west-south-west. The distribution of sliding areas and collapsed places reflects this belt-shaped structure of the tectonic lines. Along the tectonic lines such as Kiyomizu tectonic zone and Mikabu tectonic line, a number of sliding areas exist. Since the erosion of the ground proceeds rapidly there, the height of summit level becomes low and the gradient of slopes gentle. The Shikoku mountain ranges have been upheaved by from 500 to 1000 meters since the beginning of the Quaternary, affected by the eustatic movements and climatic changes. Thus, several stages of gentle slopes, in which the secondary deposits are accumulated and the rocks are weathered, are developed. These marginal areas are referred to as “an erosion front”. A number of landslides occurred in these marginal areas a: the attack of the typhoon No.5, 1975 and the typhoon No.17, 1976, and, subsequently, landslide disasters were caused. At the typhoon No.5, 1975 and No.17, 1976, the rainfall zone was almost the same, but great differences were seen in the rainfall intensity and the duration time of rainfall. Thus, the occurrence of landslide disasters at the typhoon No.5, 1975 was greatly different from that at the typhoon No.17, 1976. That is, the typhoon No.5, 1975 was of the medium scale, but it was a summer-type typhoon with heavy rainfall. Landslide disasters were caused by large-scale collapses and mudflows occurred along the tectonic lines. The typhoon No.17, 1976 was of the very large scale. Since the typhoon No.17, 1976 stayed for about one week with a front, the rainfall time was very long and the precipitation reached 50% of the annual precipitation. After the latter half of the typhoon No.17, large-scale landslides occurred in the gentle slopes along the tectonic lines.

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It has been revealed by geologic survey that the landslide phenomena at the Asari river happened in only the altered rhyolite and rhyolitic tuffs, relating to hydrothermal alteration subsequent to acidic volcanism.
Data on clay minerals by chemical, thermal and infrared absorption analysis, and X-ray diffraction method indicate that the alterations can be classified into three types, which are (I) the hydrothermal alteration characterized by the mineral assemblage of pyrophyllite-kaoline-alunite, (II) the hydrothermal alteration characterized by the mineral assemblage of sericite-chlorite and (III) the regional propylitic alteration.
It may be considered that the hydrothermal alteration zone were formed along the shear zone, in which layers consisting clay minerals were concerned with the cause of the landslide phenomena in this area.

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Sarukuyoji Landslide consists of a series of several sliding units and moves seasonally 2-5 meters per year at the head of a sliding unit.
The active source of the unit is the thrust of the upper sliding unit and water pressure filling tension cracks.
Displacement of the unit involves both creep movement on sliding surface and deformation due to consolida tion of the sliding unit.
In this paper, displacement of a sliding unit in Sarukuyoji Landslide area is analysed on the base of a consolidation theory.
The stresses in the sliding mass are calculated by force-polygon method. Void ratios to the stresses are got through e-log p curve, and strain and deformation of the sliding mass are calculated.
Slight over-consolidation of the sliding mass causes its consolidation deformation.

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Soil tests for shear strength determination are summarized. The procedure for making undisturbed, pre-cut, slurried and slikenside samples are showen. Test methods for peak and residual shear strengthes on these samples in reversal direct shear test, triaxial test and ring shear apparatuses are discribed. The results of soil tests on samples of Sarukuyoji and Abekura landslide areas are also showen.

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The procedures to increase the factor of safety of a landslide by excavation involve flattening of slopes and benching. Back analysis is often used to determine the factor of satety by assuming c' or φ'. As variation of factor of safety by excavation methods depends on the parameters of shear strength on slip surfaces, determi nation of them must be done carefully.
Considering quantity of excavation, benching of the slope for remedial works on landslides is more available than flattening.

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