東北地方における珪質泥岩層の異常堆積構造と地すべり
Anomalous sedimentary structure with siliceous mudstone beds
and landslides in Tohoku district, Japan
阿部真郎a)*･森屋 洋a)･金子勝比古b)･米田哲朗b)
Shinro ABE, Hiroshi MORIYA, Katsuhiko KANEKO and Tetsuro YONEDA |
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Abstract
Many landslides with Neogene siliceous mudstone beds in Tohoku district are characteristically observed in the form of large‐scale weathered‐rock slide or bedrock slide. The main purpose of this study is to clarify the anomalous sedimentary structure of the siliceous mudstone beds distributed around Ou mountains and Yokote basin, and to identify if there exists any relationship between landslide occurrence and such structural formation. We added a new observation result to the previous researches done by Abe et al.(1994)and Abe(1996). It was verified that the anomalous sedimentary structure of the siliceous mudstone formation was mainly formed by marine slumping at the time of piling up of Sannai‐formation layers. Moreover, stratum of the mountain range is found to be characterized by remarkable crushing. It was clarified that the main cause of landslide in the mountain range was the existence of 'weak stratum' according to the anomalous sedimentary structure of the siliceous mudstone formation. Since these types of landslide do not often exhibit clear surface topography, analysis of underground sedimentary structure becomes important for the forecast of these landslides.
Key words:anomalous sedimentary structure, slumping, clastic dike, siliceous mudstone
和文要旨
東北地方の新第三系中新統珪質泥岩層に発生する地すべりは,大規模な風化岩〜岩盤地すべりの様相を示すものが多い｡
本稿は,奥羽山脈西縁部と横手盆地の境界付近に分布する珪質泥岩層の異常堆積構造と地すべりの関連性の解明を目的としたものである｡阿部ほか(1994),阿部(1996)のこれまでの研究結果をもとに,筆者らの新たな事例観察や試験結果を加えて検討した結果,珪質泥岩層の異常堆積構造は主として山内層堆積時の海底古期地すべりによるものであることと,特に山脈縁辺部で破砕が著しいことが検証された｡現在山脈縁辺部で発生している地すべりは珪質泥岩層の異常堆積構造とこれに伴う弱面を素因として発生している可能性のあることが明らかになった｡また,これらの地すべりは地すべり地形を伴なわない場合が多く,そのため地下構造からの発生予測が重要となる｡
キーワード:異常堆積構造,スランピング,砕屑岩岩脈,珪質泥岩 |
Modified Newmark analysis of seismic permanent displacements of slopes
Viet Bao NGUYENa), Jing‐Cai JIANGa)* and Takuo YAMAGAMIa) |
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Abstract
Newmark sliding block models are commonly used for seismic stability analysis of slopes. However, estimation of permanent displacements in conventional Newmark methods is based on a distinct and fixed slip surface which is usually located using a pseudostatic limit equilibrium analysis. By combining a finite element seismic analysis and dynamic programming, this paper first shows that critical slip surfaces within a slope greatly change their shapes and locations over a wide area with time during an earthquake. Then, a modified Newmark analysis is presented in order to consider the effect of variable critical slip surfaces on estimated seismic displacements of slopes. Results of analysis for a hypothetical slope and an actual slope failure caused by an earthquake indicate that the neglect of variation of critical slip surfaces during an earthquake may result in an improper sliding mass and as a result incorrect estimation of permanent displacements. It is also shown that permanent displacement may occur along a relatively wide slip band rather than a distinct slip surface.
Key words:slope stability, earthquake, permanent displacement, FEM, dynamic programming, critical slip surface |
中性子水分検層と環境同位体による第三紀泥岩地すべり地の
排水トンネルの抑制効果の評価
Evaluation of the drainage effect by drain tunnel in Tertiary mudstone landslide area using
a neutron moisture logging and environmental isotopes in groundwater
石田 聡a)*･原 郁男b)･土原健雄a)･今泉眞之a)
Satoshi ISHIDA, Ikuo HARA, Takeo TSUCHIHARA and Masayuki IMAIZUMI |
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Abstract
In order to develop the evaluation method for the drainage effect by drain tunnel, monitoring of water content using neutron moisture logging in the drainage boring pipe for 19 months and environmental isotope concentrations(Tritium and Radon:222Rn)in groundwater from the drainage boring pipes for five years were carried out after the construction of drain tunnel. The drain tunnel is 1.25m(diameter)×500m(length)at 50m underground with drainage boring pipes, which are expected to drain superfluous water around the sliding surface of an active landslide in Tertiary mudstone area, Niigata Prefecture. Consequently, the monitoring of water content showed a remarkable decreasing with about 8% of water content around the sliding surface for the first four months and subsequently slight decreasing. The tendency of decreasing of water content corresponds with the fluctuation pattern of discharge water volume from drainage boring pipes. The monitoring of tritium and 222Rn concentrations revealed that the groundwater around the tunnel was recharged from rain before 10 years and the unsaturated zone around tunnel was occurred. These change of groundwater flow after construction of drain tunnel certifies that the monitoring techniques are useful methods for evaluation of the effect by the drain tunnel.
Key words:drain tunnel, radon, tritium, neutron moisture logging, water content
和文要旨
新潟県板倉町の活動中の大規模地すべりの抑制工として建設された排水トンネル内の観測孔において，排水トンネルの効果判定のために，中性子水分検層による含水比変化と環境同位体（トリチウムとラドン）濃度変化のモニタリングを行った。その結果，すべり面付近の含水比は４ヶ月間で約８％低下しその後は漸減した。含水比の低下は排水ボーリングからの排水量の低下と調和的であった。また環境同位体の測定から，排水ボーリングから排水される水はかん養後１０年以上経過している事が明らかになると共に，排水が定常状態に達してトンネル周辺地山に不飽和の部分が生じていると推定され，中性子水分検層と環境同位体によるモニタリングは排水トンネルの効果判定に有効であることが実証された。
キーワード：排水トンネル，ラドン，トリチウム，中性子水分検層，含水比 |
間隙水圧の繰り返し載荷を受ける過圧密粘性土のせん断強度
Shear strength of overconsolidated clay for repeats of pore water pressure loading
大塚 悟a)･宮田善郁b)･岩部 司c)*
Satoru OHTSUKA, Yoshifumi MIYATA and Tsukasa IWABE |
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Abstract
This paper represents the change in shear strength of clay for repetitive pore water pressure loading with the use of both experimental tests and model simulation. Pore water pressure was increased under constant deviator stress to simulate the soil behavior in landslide. Yield stress in pore water pressure was obtained constant for repeats of pore water pressure loading, but failure stress was obtained to decrease. Failure stresses constituted the failure envelopment line in void ratio and effective mean stress space. This exhibits the process of change in both shear strength and state variable of soil caused by repetitive failures during pore water pressure loading cycles. In pore water pressure loading, the strain softening of clay develops due to dilation and the shear strength decreases. On the contrary, the shear strength recovery develops by consolidation effect in unloading. Failure stress varies due to two opposite effects of pore water pressure loading cycle. It was found the failure stress in pore water pressure approached to the balanced one between two opposite effects in repetitive pore water pressure loading cycles.
Key words:shear strength, repeated load, pore water pressure, failure mechanism, constitutive equation
和文要旨
本研究は地すべり斜面における粘性土のせん断強度が，間隙水圧の載荷によって繰り返し破壊することによりどのように変化するのか明確にするために，三軸試験と構成式によるシミュレーションから検討した。供試体の軸差応力を一定に制御して間隙水圧を繰り返し載荷すると，降伏応力は繰り返し載荷によらずに一定値となる一方，破壊応力は繰り返し載荷によって減少した。得られた破壊応力は間隙比〜平均有効応力空間にて包絡線を描き，間隙水圧載荷を受ける粘性土の状態変化の過程が明らかになった。これは間隙水圧を載荷すると粘性土は吸水せん断のために軟化することによるが，逆に間隙水圧を除荷すると圧密による強度回復が生じる。このために破壊応力は間隙水圧の繰り返し載荷における２つの異なる作用を受けて変化する。繰り返し載荷を継続すると両者の影響が平衡する破壊応力に漸近することを示した。
キーワード：せん断強度，繰り返し載荷，間隙水圧，破壊機構，構成式 |
二列の壁を有する斜面の安定性評価法に関する提案
Stability analysis of slopes with two stabilizing walls
神田幸正ａ）・山上拓男ｂ）・蒋 景彩ｂ）＊・グエン バオ ヴェトｂ）
Yukimasa KANDA, Takuo YAMAGAMI, Jing‐Cai JIANG and Viet Bao Nguyen |
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Abstract
A limit equilibrium method is presented to evaluate the stability of slopes reinforced with two stabilizing walls(e.g. sheet pile walls). The basic idea of the method is to allow presence of three different slip surfaces in upslope, midslope and downslope earth masses separated by the two walls. Contrary to conventional limit equilibrium analyses which predict a safety factor for a given sliding mass, the proposed method estimates forces acting on the stabilizing walls by specifying a target factor of safety for the stabilized slope.
Estimation of interactive forces between earth masses and walls can be made using an existing method of slices. In this paper, the Bishop simplified method is applied, and a repeated trial procedure is established in order to find the most dangerous state for the walls and the corresponding set of three critical slip surfaces. An illustrative example is presented to demonstrate the effectiveness of the proposed method.
Key words:stabilizing walls, interactive forces, slope stability, limit equilibrium, critical slip surfaces
和文要旨
二列の壁（例えば矢板壁）を有する斜面の極限平衡法に基づく安定性評価法が提案されている。提案法の基本概念は二列の壁によって仕切られた上流，中間及び下流土塊中に異なる三つのすべり面が存在し得ると仮定するところにある。そして通常の極限平衡法では与えられたすべり面に対する安全率が算定されるのに対し，提案法では安定性を確保するための目標安全率が指定され，逆に壁に作用する力が評価される。
壁で仕切られた三つの領域中にそれぞれすべり面を仮定すると，既存の極限平衡法から，目標安全率に対応した壁に作用する力を算定することができる。本稿ではビショップ（Ｂｉｓｈｏｐ）法を応用して，壁にとって最も危険な状態とそれに対応する一組の臨界すべり面を探索する手法が構築されている。最後に，適用事例から提案法の有効性が確認されている。
キーワード：抑止壁，相互作用力，斜面安定，極限平衡法，臨界すべり面 |
四国室戸半島の海食崖における崩壊特性
Features of slope failure at sea cliff in Muroto peninsula, Shikoku district
稲垣秀輝ａ）＊・平田夏実ａ）・日浦啓全ｂ）
Hideki INAGAKI, Natsumi HIRATA and Hiromasa HIURA |
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Abstract
The features of slope failure at the sea cliff of the Muroto peninsula, Shikoku district, were analyzed by the aerial photo interpretation and the field investigations. Some essential characteristics of the slope failure became clear. Those slope failures at the sea cliff often occurred at the time of great earthquake. While the slopes of terraces, which are covered by forest keep being stable, though abandoned terraces which had been used for cultivation will often collapse on the occasion of the heavy rainfall because of the lack of maintenances. As for the slopes which consist mainly of unconsolidated terrace deposits, the cycle of the slope failure is about 20 years. The erosion rate of basement rock, Shimanto groups, is about 5.6cm/year.
Key words:slope failure, sea cliff, Muroto peninsula, vegetation
和文要旨
四国室戸半島の海食崖の崩壊特性を空中写真判読と現地調査により解析した。海食崖の崩壊は地震時に多く発生するほか，斜面上方の土地利用とも関連があり，森林による斜面保全効果が認められた。未固結の段丘堆積物斜面には約２０年周期で崩壊を繰り返すところがあり，基盤岩として四万十層群が分布している海食崖の侵食速度は約５．６ｃｍ／年であった。
キーワード：斜面崩壊，海食崖，室戸半島，植生 |
空中写真による新潟県戸沢川崩壊の変遷と計測
Sequence and measuring of landslide using aerial photographs
-Tozawagawa Landslide, Niigata, Japan-
山岸宏光ａ）＊・丸井英明ａ）・Ayalew LULSEGEDa）・関口辰夫ｂ）・堀松 崇ｃ）・畑本雅彦ｄ）
Hiromitsu YAMAGISHI, Hideaki MARUI, Ayalew LULSEGED, Tatsuo SEKIGUCHI, Takashi HORIMATSU and Masahiko HATAMOTO |
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Abstract
On January 5, 2000, a landslide of debris avalanche type(Cruden and Varnes, 1996), took place on the steep slope along the Tozawagawa river, upper tributary of Aganogawa river in Niigata Prefecture, Japan. This landslide blocked the Tozawagawa River, forming a natural dam 500m long temporarily. This landslide has been changing in form in short times just after the catastrophic event on January 5, 2000. In this paper, we are describing the sequence of the landslide using the aerial photographs taken in 1947 up to 2001, and are showing the scale and volume calculation using the aerial photographs.
Key words:landslide, aerial photographs, sequence, scale measuring, photogrammetry
和文要旨
２０００年１月５日，新潟県阿賀野川上流の戸沢川の急斜面から，岩屑なだれタイプの崩壊（Ｃｒｕｄｅｎ ａｎｄ Ｖａｒｎｅｓ， １９９６）が発生した。この崩壊は戸沢川を堰き止め，一時的に長さ５００ｍの天然ダムを形成した。この崩壊はまた，２０００年１月５日の大崩壊以来，短時間に形態を変化させてきた。本文では，１９４７−２００１年に撮影された航空写真を使用してその変遷をたどり，あわせてスケールや規模の計算を行ったものである。
キーワード：ランドスライド，空中写真，変遷，規模測定，写真測量 |