The 2003 Tokachi-Oki Earthquake caused extensive ground collapses accompanied by slightly lateral displacement . subsidences, and lateral spreads of gentle slopes in the Kyowa area of Tanno Town, situated about 230km away from the epicenter. The direct inducement of such slope movements was the liquefaction of the volcanic ash that had artificially covered the valley. Especially, at Locality No.1 in the Kyowa area, liquefied volcanic ash of about 10, 000m³spouted from the slope to the sides, and a wide farmland collapsed. Such large ground collapses caused by lateral jetting of the liquefied volcanic ash are new phenomena and new landslide disasters induced by the earthquake.
The old landforms of the liquefied and moved area were narrow waste-filled valleys and swamps formerly utilized as paddy but these were converted into the gently-sloping farmland by the land fill of the volcanic ash. Though the field of similar landform transition and ground condition is large in number, it is characteristic that the slope movements caused by the liquefaction of volcanic ash were confined to the fields where the reclaimed planes tilted to the south or south-southwest. In addition to, the dip direction of the reclaimed planes coincided with the direction of the main lateral displacement of the surface in each liquefied area. The dip direction of the reclaimed planes might have brought about the geomorphic effect.

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本総説は極限平衡法に基づく三次元斜面安定解析法について, 種々の観点から検討と考察を加えたものである。先ず, 既往の解析法を厳密法と簡便法; すべり方向を予め既知とする理論と規定しない理論に分類しつつ概観している。次いで, コラム分割法の計算精度を考察し, さらに三次元安全率と二次元安全率の位置付けについて議論している。そして最後に, 我が国で地すべり解析にしばしば用いられている土研のHovland法を取り上げ, その問題点を指摘している。
以上の結果, 釣合条件が満たされていないHovland法などの簡便法は, 三次元効果を無視する程に低い安全率を与えるため, より実態に即した解を得ようとして多大な労力を費やす三次元解析の意義を損ねる解法となっていることが知られた。
本文を通して著者らは, 三次元解析に際しては, その目的がより正確に斜面の安定性を評価することであるから, 精度上問題の少ない厳密コラム分割法を採用すべきことを強く主張するものである。

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2002年11月8日に金沢市山科町で長雨を誘因とする流動性地すべりが発生した。発生直後の地すべり土塊は完全に飽和されており.竹林の分布している範囲では地すべり土塊が厚く堆積していた。地すべり地は第三紀泥岩層 (犀川層) からなり.現地調査より泥岩は下位から新鮮な泥岩, 亀裂の発達した風化泥岩, 粘土化した強風化泥岩の三層に分類され, 風化泥岩と新鮮部との境界で地すべりが発生したことがわかった。また, 運動機構を明らかにするために行った運動シミュレーションより, すべり面の定常状態でのせん断抵抗が強風化泥岩層では10kPa, 竹林部では50kPa (竹林の土塊のせん断抵抗値ではなく, 地すべり土塊中に含まれる竹の地すべり運動による衝突が地すべりを減速させる効果を表現した値) であることがわかり, 竹林が分布している範囲に, 地すべり運動を減速させる何らかの要因があった。山科地すべりの流動化した原因の一つとして, 長雨によって飽和された低透水性の強風化泥岩が, 地すべり土塊の衝撃載荷により, 非排水せん断され, そのせん断抵抗が低くなり流動化したと考えられる。

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Landslide induced disaster is a serious problem in the mountainous terrain of Nepal. The landslides and debris flows cause losses of lives and properties, damage the nature and other infrastructures in the monsoon every year. The risk evaluation of landslides is to be most important for landslide hazard reduction . This paper is to focus on evaluating the potential landslide topography of Shrawan danda landslide which lies on the southern slopes of the Siwalik Hills and has created serious problems in Butwal, Nepal. The series of the investigation methods have been carried out to identify the characteristic of each factor of the micro landform of the landslide through interpretation of aerial photographs incorporated with the information of field survey and historical records. The maps and related tables were converted to digital format from the database that was the part of the geographical information system (GIS). The information of aerial photo interpretation has been used for risk evaluation of the landslide based on the AHP method.
Firstly, Shrawan danda landslide was divided into three landside bodies (A, B and C) based on the geomorphological features of the landslide area. Secondly the distribution maps of the micro landform features were prepared by using GIS. Thirdly, the length, area and density of the micro landform features (scarps, blocks, bare lands and cracks) of the landslide bodies were calculated. After that, the detailed information of instability of the landform features of the landslide bodies were analyzed for risk evaluation. Finally the weight of risk value of the micro landform features produced by the AHP method were used for classifying the landslide bodies as one with high, moderate or low risk.
The results show the weight of risk value of the landslide body A=44, B=90.4, and C=76.4 points. The weight of risk value of the landslide body B and C were found more than 70%. These landslide bodies are highly contributed by the number, length and density of the micro landform features which are perceived as high risk . The weight of risk value of the landslide body A was found between 30 to 70% which is moderately controlled by the number, length, area and density of the micro landform features that is perceived as moderate risk. Thus results of qualitative and quantitative estimation of the micro-landform features led by using GIS and numerical function, and the weight of the risk value of the micro landform features led by using Analytical Hierarchy Process (AHP) method synthesizing information of the aerial photo interpretation (API) provide a precise contribution to evaluate risk of a landslide in various environmental conditions.

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It is highly important to estimate the shear strength parameters at the critical slip surface of landslide in order to design effective countermeasures. This study aims to propose a new method for estimating a set value of shear strength parameters which satisfies two conditions: the calculated critical slip surface coincides with the actual slip surface and the obtained safety factor is equal to 1.0. Dynamic programming using simplified Janbu's method was applied as a tool to locate the critical slip surface that yields minimum value of safety factor. A specific case of submerged landslide was examined to demonstrate the efficiency and accuracy of the proposed method. Furthermore, the same method of analysis was applied to examine another 47 cases of landslides. It found that two relationships, one between the estimated cohesion (c) and the average depth of landslide and the other one between the internal friction angle (φ) and the inclination of slip surface, can be used for practical estimation of shear strength parameters. Further, the estimated shear strength parameters of submerged landslides have a reasonable agreement with the change in safety factor by the rising of reservoir level.

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札幌市の薄別川地すべり地では2000年5月に大規模な滑動が生じた。この地すべり地を対象に, 2000年5月滑動以前の地すべり撹乱履歴を, 樹木群の年輪幅変動解析から検討した。変動域の樹木群の年輪幅系列は, 非変動域から得られた薄別平均年輪幅系列との同調性が低かった。これは, 土塊の破砕・移動が繰り返される地すべり地では亀裂発生による根系切断や, 周辺個体の傾倒による林冠疎開などの立地環境変化が樹木群の肥大成長状況を変化させたと説明される。また, 斜面上の位置や微地形分布等が異なる領域間で年輪幅変動に相違が見られることは, 地すべり土塊の運動履歴の相違を示唆している。

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地すベり土塊と杭の変形特性, 地すベりの滑動とこれに伴い杭に作用する荷重を考慮できるFEM解析モデルを構築し, 地すベり土塊の変形係数, 杭の曲げ剛性, 杭の打設位置が杭の変形に及ぼす影響とくさび杭の設計式との関係を調ベた。その結果, 地すベり土塊の変形係数, 杭の曲げ剛性, 杭の打設位置によって杭の変形形状が変化し, 地すベり土塊の変形係数と杭の曲げ剛性, 地すベり土塊内の杭長により求められるβ-llが概ね3以下になると, FEMにより得られる最大曲げモーメントはくさび杭の設計式で求められる最大曲げモーメントよりも大きくなる場合が見られることが明らかとなった。

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