This paper introduces a method for calculations of minimum total safety factors of slopes by means of the elasto-plastic FEM, considering tension cracks. Soils are assumed as elasto-plastic materials which obey the Mohr-Coulomb equation as the yield condition (or failure condition) and the Drucker-Prager equation as the flow rule.
The procedure of calculations is as follows. First, the shear strength of soils is assumed as τ_f=c'/F+σ'tanφ'/F. The initial value of F is taken so small that the initial state of a slope is almost elastic. Next the value of F is increased step by step, and finally the ultimate value of F where slope failure occurs, is determined as the minimum total safety factor of the slope. If large tensile stresses occur in the slope, the above procedure is repeated again after thin elements with small strength and low Young's modulus are inserted into the places where tension cracks are assumed to appear.
It is shown that the method presented here can take account of tension cracks as well as pore water pressures properly which always reduce the safety factor of slopes.

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地すべり地の地表の変位追跡測量データを解析して, 地下すべり面の三次元形状をひとつの曲面方程式で近似して推定する。これまでの手法に改良を加え, 滑落崖や脚部隆起点の位置のデータおよびボーリング等で得られた地下すべり面の位置のデータを解析に取り入れた。これらのデータの精度に応じた重みを付けて解析することにより, すべり面推定の精度をかなり向上させることができた。

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A large number of landslide track is topographically recognized in the southwestern district of Iwate prefecture, which is situated in the eastern part of the “Green Tuff region”. Understanding the conditions of landslide generation, geological features in the district have been investigated by comparing the distribution map of landslides with the distribution map of gentle slopes, the rostored contour map, and the geological map.Although geologic condition of the district is somewhat peculiar since it had suffered strong mineraliza-tion in addition to the “Green Tuff” alteration, follow ings have been clarified;
1) In the case that a flat surface exists on a summit or mountainside of the mountainous area, the flat comes into a recharge area of groundwater. Therefore, where is a slope of low gradient lower in altitude than the flat, the watertable under the slope is raised and become easy to generate landslide.
2) In the area in which a potential lifting zone exists, landslides are ready to generate because unstable slopes are formed along the zone.
3) In the area where faults or fault zones area are developed, landslides with a linear headscarp, instead of a crescent-shaped scarp, are formed owing to the effect of structural controle in the fractured Neogene strata.
4) Strate including layers of muddy rock and/or acidic tuff are ready to cause landslide, because the lalyers are easy to suffer argillization.
5) In the studied district, landslides are developed well especially in the area which had suffered deep weathering, geothermal alteration and mineralization, as compared with areas without alteration.

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空中写真により, 南部北海道の木古内町釜谷の丘陵性山地で, かなり大規模な地すべり (幅300m, 長さ700m) が最近発生していたことがわかった。それ以前の空中写真の判読により, この地すべりは1988年の6月23日以降, 7月27日以前に起こったことが明らかになった。さらに, 1948年頃, 1959年, 1973年, 1976年, 1988年に撮影されたこの地すべり付近の空中写真を判読すると, 針葉樹林 (杉の人工林) の伐採と関連した地すべり地形の変化が読み取れる。さらに, 筆者らはこの地すべりの発生前と後とを比較して量的な解析を行い, 現地調査をも実施した。その結果, 地質的な素因としては, 緩傾斜の新第三紀泥岩層にはさまる粘土化した流紋岩質凝灰岩から発生した流れ盤型であり, 誘因としては6月-7月始めのやや強い降雨による, 古い地すべり面の間隙水圧の上昇が推定される。

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The relationships between the differential erosion topography and the slope failures on the Mt. Kano-zan and its environs in the Boso peninsula, Japan have been discussed. The results are as follows.
The geology consists of sedimentary rock formations can divided into two types. First one is sandstone rich formation type and the other type is mudstone rich formation. The former area is formed high relief, low slope angle and low valley density. The later is formed low relief, high slope angle and high valley density. It is considered that the main facter is based on the low potential of erosion derived from decreasing surface stream which caused on permeability of unconsolidated sandstone.
The phenomena of slope failures are main agency of the processes beneath the surface layer on the mountain slope of the Kano-zan and its environs. The distribution of the slope failures are consolidated on the mountains which composed of low relief, high slope angle and valley density. This is very important phenomena of the main agency of the aifferetial erosion topography in the humid temperature area.

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