Gentle‐slope movements induced by the 2003 Tokachi‐Oki
Earthquake, in the Kyowa area of Tanno Town, Hokkaido, Japan
Yoji ITOa)*, Satoshi YAMASHITAa), Teruyuki SUZUKIa) and Hiroaki HIRATAb) |
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Abstract
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 10,000 m3 spouted 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.
Key words:liquefaction, artificial volcanic ash ground, ground collapses, lateral spreads, lateral sand boils, waste‐filled valleys, geomorphic effect |
極限平衡法に基づく三次元斜面安定解析：簡便法vs厳密法
Three‐dimensional limit equilibrium slope stability analysis:
simplified methods vs rigorous methods
蒋 景彩ａ）＊・山上拓男ａ）
Jing‐Cai JIANG and Takuo YAMAGAMI |
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Abstract
Existing three‐dimensional（３D）slope stability analysis methods based on limit equilibrium are comprehensively reviewed, and studies which have been conducted to evaluate the computational accuracy of these ３D methods are briefly summarized. The end effect（i.e. ratio of the ３D factor of safety to the ２D value）estimated using different methods of columns is also examined. It may be concluded that simplified analyses based on an extension of the ordinary method of slices（e.g. Hovland method）are not adequate for ３D slope stability computations because they assume zero intercolumn forces, do not satisfy equilibrium conditions, and more importantly, predict overconservative results leading to a partial or complete neglect of the end effect that should be evaluated properly by a ３D analysis. Consequently, the use of a rigorous method of columns that satisfies both force and moment equilibrium is strongly recommended for a realistic ３D evaluation of the slope stability.
Key words:Limit equilibrium, ３D stability analysis, rigorous column methods, sliding direction, end effect, Hovland method
和文要旨
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。
キーワード：極限平衡法，三次元安定解析，厳密コラム分割法，すべり方向，三次元効果，Hovland法 |
金沢市山科町における長雨を誘因とする流動性地すべりの特徴と運動機構
Feature and sliding mechanism of a flowslide triggered by
continual rainfall in Yamashina area, Kanazawa City, Japan
田中康博ａ）ｄ）・汪 発武ｂ）＊・中村佳代ｃ）・松本樹典ａ）
Yasuhiro TANAKA, Fawu WANG, Kayo NAKAMURA and Tatsunori MATSUMOTO |
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Abstract
Due to a continual rainfall, a flowslide occurred in Yamashina area, Kanazawa City, Japan on November ８, ２００２. The sliding mass was fully fluidized after slope failure and had deposited thickly in a bamboo area. The stratum around the landslide area was massive mudstone of Tertiary period. According to field investigation, the mudstone was classified into strongly weathered mudstone layer at top, densely cracked moderately weathered mudstone layer at middle and fresh mudstone layer at bottom. It is confirmed that the flowslide was initialized along the sliding surface between the densely cracked layer and the fresh layer, and the sliding mass moved in the strongly weathered mudstone layer. Based on the landslide motion simulation reproducing the Yamashina flowslide, the shear resistance at steady state in the strongly weathered mudstone area and the bamboo area were estimated as １０ kPa and ５０ kPa respectively. It is concluded that the landslide with high mobility was caused by the low mobilization of shear resistance in the saturated strongly weathered mudstone with low permeability.
Key words:Tertiary mudstone formation, flowslide, continual rainfall, sliding mechanism, case study
和文要旨
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。
キーワード：第三紀泥岩層，流動性地すべり，長雨，運動機構，事例研究 |
Preliminary study of landslide risk evaluation by micro landform
interpretation on southern slope of Siwalik Hills, Nepal, using
aerial photo interpretation
-A case study of Shrawan danda landslide Butwal, Rupandehi, Nepal-
Bishnu Prasad GYAWALIa) and Toyohiko MIYAGIb)* |
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Abstract
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.
Key words:micro landform feature, aerial photo interpretation, GIS, risk evaluation, AHP |
A new method for estimating the shear strength parameters at the critical
slip surface*1
Teuku Faisal FATHANIa)* and Hiroyuki NAKAMURAb) |
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Abstract
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.
Key words:shear strength parameters, critical slip surface, dynamic programming, submerged landslide |
樹木年輪幅を用いた地すべり変動解析の試み
−北海道薄別川地すべりを例として−
Analysis of landslide movement using tree ring‐width variation:An example
from the Usubetsu‐gawa Landslide, Hokkaido, Japan
菊池俊一ａ）＊ |
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Abstract
In May ２０００, a very substantial landslide occurred in Sapporo. It was later named the Usubetsu‐gawa Landslide. A dendrochronological study using the tree ring‐width of living trees in this landslide area was used to examine the response of the rings to disturbances owing to previous landslides. The tree ring‐width chronologies in the active landslide area were poorly correlated with the average Usubetsu‐gawa chronology made using living trees growing outside the landslide area. This was explained as resulting from repeated landslides that altered tree thickening owing to root breakage caused by surface cracking, and to canopy gaps produced by fallen or tilted surrounding trees. The variation in the tree ring‐width chronologies on various slopes and micro‐landforms suggests differences in the movement history within the same landslide area.
Key words:landslide, disturbance history, tree ring width, response, correlation
和文要旨
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。
キーワード：地すべり，撹乱履歴，樹木年輪幅，応答，同調性 |
モデル斜面を用いた杭のFEM感度解析結果とくさび杭の設計式の比較
Comparison of simulated behavior of pile for landslide by FEM and dowel pile formula
石井靖雄ａ）＊・藤澤和範ａ）・太田敬一ｂ）
Yasuo ISHII, Kazunori FUJISAWA and Keiichi OTA |
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Abstract
To clarify the applicable conditions of the design formula of landslide pile from a modulus of deformation of landslide soil mass and pile bending stiffness, sensitivity analysis by the finite element analysis using the model slope which is changed by modulus of deformation of landslide soil mass, the pile bending stiffness and the piling position was carried. As a result of analysis, When β・ｌ which is calculated by modulus of deformation of landslide soil mass and the pile bending stiffness become three or less, the moment obtained by finite element analysis become bigger than the moment obtained by long pile formula and finite length pile formula of the dowel pile.
Key words:landslide, pile works, finite element method, modulus of deformation
和文要旨
地すべり土塊と杭の変形特性，地すべ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，地すべり土塊の変形係数，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べた。その結果，地すべり土塊の変形係数，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，地すべり土塊の変形係数と杭の曲げ剛性，地すべり土塊内の杭長により求められるβ・l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。
キーワード：地すべり，杭工，有限要素法，変形係数 |