Strength parameters which should be used for the calculation of slope stability analysis of landslide are discussed and a new method of determining the parameters for design use is proposed. The authors assume that the strength decrease due to the increase of pore water pressure along the potential slip surfase can be represented by a function of overconsolidation ratio (OCR) defined by the ratio of effective stresses before and after the increase of pore water pressure. It is also assumed that the strength decrease due to the state change of the material around the potential slip surface from soft rock to overconsolidated clay due to weathering etc. can qualitatively be represented, by the change of OCR.
Based on the assumption, the authors derive an equation representing the change of effective strength parameters c_d, φ_d, which varies in between peak and residual state, as a continuous function of OCR. In addition, a practical method of determining strength parameters for stability calculation is proposed, in which the parameters are given by combining the conventional reverse calcuation method with the laboratory shear test results.

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The geology in the catchment area of the Nitappugawa River consists primarily of the Upper Miocene Ikutahara Formation and Late Miocene rhyolite, andesite, and basalt dikes, as well as the Quaternary System. The Ikutahara Formation is widely distributed in the area and is composed of tuffaceous conglomerate, sandstone and mudstone, lapilli tuff, tuff breccia, tuff, rhyolite lavas, and andesite lavas. The clastic and volcaniclastic rocks occupy the valley and hillside whereas the rhyolite or andesite lavas cap the peaks of the mountains. The rocks of the Ikutahara Formation and a part of the rhyolite dikes were intensely affected by hydrothermal alteration related to the Ryuo epithermal gold-silver mineralization of the Late Miocene Age. The hydrothermal alteration related to an ancient neutral-type geothermal system is primarily characterized by smectite, zeolite, interstratified illite/smectite, illite, and adularia.
Seven ancient slide configurations are distributed on the interstratified illite/smectite and smectite zones. The Yasukuni slide and three slide configurations are closely related to the interstratified illite/smectite zone occurring abundantly in interstratified illite/smectite, which is a swelling clay mineral. The other three slide configurations are closely related to the smectite zone characterized by smectite, which is also a swelling clay mineral.
These facts are useful when evaluating contemporary slide hazards and constructing hazard maps of ancient hydrothermal fields.

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The effects of drainage well work, a construction method used to prevent landslides, were studied and the following results obtained.
(1) To understand the groundwater drainage effects of installing a drainage well on a sloping slide area, it is necessary to study the drainage caused by the drainage boring in addition to the drainage caused by the drainage well itself.
(2) The study confirmed that after installing a drainage well, the groundwater level is low close to the drainage well but rises as the distance from the drainage well increases.
(3) The overall decline in the ground water level accompanying the construction of the drainage well can be found using formula (1).
(4) The decline in the ground water level accompanying the construction of the drainage well is limited to the area where drainage boring has been performed.

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The geology of the Kanehana slide areas consists primarily of the Cretaceous-Paleocene Yubetsu Group, Upper Miocene Ikutahara and Yahagi Formations, Late Miocene andesite and rhyolite dikes, Lower Pliocene Hakugindai Lava, and Quaternary System. The Ikutahara Formation is distributed in the eastern part of the areas, and consists primarily of tuffaceous conglomerate, lapilli tuff, tuff, and rhyolite lavas. The Yahagi Formation is distributed in the western part of the areas, and consists primarily of tuffaceous conglomerate, sandstone and mudstone, pumice-tuff, and rhyolite lavas. The Hakugindai Lava unconformably covers the rocks of the Ikutahara and Yahagi Formations. In particular, the rocks of the Ikutahara and Yahagi Formations and andesite dikes were affected to various extents by hydrothermal alterations related to the Late Miocene epithermal gold-silver mineralizations of the Muka and Kunihana deposits.
Hydrothermal alterations are divided into two stages. The stage I neutral-type alteration zones consist of K-feldspar, illite, interstratified illite/smectite, chlorite, zeolite, and smectite zones, while the stage II acid-type zones include kaolinite, veined 7 A halloysite-kaolinite, and veined smectite-kaolinite zones.
Seven ancient slides and the Kanehana-toge slide occurred in hydrothermal alteration zones. The smallestscale Kanehana-toge slide and four intermediate-scale slides were closely related to the veined smectitekaolinite and smectite zones, respectively. Two large- and intermediate-scale slides were also closely related to the smectite zone, to the zeolite zone containing a relatively large amount of smectite, and/or to the kaolinite zone.
The largest-scale ancient slide configuration was approximately 1, 450 m in width of displaced material, and approximately 1, 550 m in total length, and was of the “Concave Poly-Hill type” consisting of roughly twelve parts. On the other hand, the smallest-scale slide was approximately 20 m wide and approximately 70 m long. The small- and intermediate-scale slides were of the “Concave Hill type”.
These data are useful for evaluating the degree of hazard of slides and for making a hazard map of slides in the hydrothermal alteration zones.

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In this paper, a new formulation of pile design is proposed, in which equilibrium of force and moment of a landslide mass including the pile to prevent landslide are considered. The method for evaluating slope stability basically follows that of Janbu in this formulation.
In order to make the equations of equilibrium statically determinate, it was assumed that the internal force between the adjacent slices should be minimized at equilibrium.
The distributed force was adopted as normal force acting on the slip surface, and therefore the point of application of force can be easily calculated.
The derived formula can be applied to both cotter and reinforcement type piles and has no restriction as to the length of the pile in the design.

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In the Shiretoko Peninsula, totally 645 landslide configurations were identified through aerial photographs. The total area of landslide mass was counted at approximately 10, 000 hectares, and corresponds to 10% of the whole area of the Shiretoko Peninsula. The characteristics of landslide configurations in the Shiretoko Peninsula are summarized as follows:
1) Small landslide configurations are found concentrically in the area underlain by Miocene strata. They consist mainly of “green tuffs” and shales interbedded with thin layers of tuff. Large and very large landslide configurations are arranged on the margins of Quaternary subaerial lavas overlying landslideprone Miocene strata, and both of them are found on the flanks of Quaternary stratovolcanoes.
2) The landslide configurations are identified as debris slump, rock slump, debris slide, rock slide, earthflow, collapse (debris/rock fall) and debris avalanche types. Although debris slump and rock slump types predominate in this area, bedrock geology and caprock structures make difference in frequency of the type and scale of landslide configurations.
3) In the Miocene area, most landslide configurations are debris slump type or debris fall type. Moreover, also recognized are large rock slump and rock slide types.
4) On the margins of Quaternary subaerial lavas, making up caprock structures, large landslide configurations are rock slump, collapse and debris avalanche types.
5) In the flank of Quaternary stratovolcanoes, there are large rock slumps and slides, giant collapses, and debris avalanches, which are so huge as to change remarkably the shape of volcanoes.
Recently, partial sliding in old landslide mass and falling from old landslide scars were triggered by heavy rainfall and earthquakes. Therefore, paticular attention should be paid to landslide configurations near houses and roads from the viewpoint of landslide damage prevention.

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