■すべり面におけるせん断強度と物理的性質との関係
Correlation between physical properties and shear strength of undisturbed
slip surfaces
眞弓孝之・山崎孝成・岩尾雄四郎
Takayuki MAYUMI, Takanari YAMASAKI and Yushiro IWAO |
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Abstract
This paper describes the relation between physical properties and shear strength of undisturbed slip surfaces in Tertiary strata landslides, shear zone landslides and solfataric landslides. The number of tested slip surfaces is 124 and almost all samples were taken from the drainage well. Both Slip Surface Direct Shear test(SSDS)that would be able to adjust the position of slip surface with the shear surface of the apparatus and ring shear test were executed. Slip surfaces and shear surfaces at residual state were observed by scanning electron microscope. The roughness of these surfaces was measured by laser focus displacement sensor. For the investigation of physical properties grain size analysis and Atterberg plasticity limits tests were conducted. From the test results, it became clear that all samples taken from both Tertiary strata landslides and solfataric landslides included smectite in the slip surface clay and such slip surfaces were covered with smectite. The relation between residual friction angle and plasticity index for such samples were divided into three groups. Also it was shown that the sample contaminated by sand fraction built coarse grains into rough shear surface with high shear strength.
Key words:slip surface direct shear test, ring shear test, grain size distribution, consistency, average of roughness
和文要旨
新第三系堆積岩地すべり，変成岩地すべり（破砕帯地すべり），温泉地帯地すべりに属する日本各地から採取した全１２４試料に対してすべり面せん断試験，およびリングせん断試験を行い，すべり面せん断試験から実際のすべり面において発揮されるせん断強度の発現機構を探り，リングせん断試験で得られた結果との比較を試みた。具体的には，すべり面や試験後せん断面を，走査型電子顕微鏡を用いて構造観察し，面に確認された起伏構造の程度を評価すべく，レーザー変位計を用いて面の起伏量を計測し，面粗度（算術平均粗さ）を求めた。さらに，採取したすべり面試料の粒度特性，コンシステンシー特性を求め，これらとせん断試験の結果の関係についても考察を行った。本研究の成果として，新第三系堆積岩地すべりや温泉地帯地すべりに属する全てのすべり面試料中にスメクタイトを検出し，スメクタイトがすべり面やリングせん断試験後のせん断面を被覆する形態を確認した。また，スメクタイトを含むすべり面粘土のせん断抵抗角と塑性指数との関係には三つのグループ分けが出来ることを見いだし，さらに相対的に粗い粒径成分が混入することで生じた面の粗さがせん断強度を増大することを確認した。
キーワード：すべり面せん断試験，リングせん断試験，粒度組成，コンシステンシー特性，算術平均粗さ |
■斜面のレオロジー挙動と疑似地すべり
Rheological deformation in slopes and quasi landslides
玉田文吾・福田順二・山本哲朗
Bungo TAMADA, Junji FUKUDA and Tetsuro YAMAMOTO |
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Abstract
Tension cracks often occur on the ground surface of steep slopes after heavy rain. However, the expansion of these cracks does not continue for a long time. This phenomenon of displacement is called a“quasi landslide"because it does not indicate the occurrence of new landslides.
The phenomenon is caused by the rheological deformation of soft clay layers in debris deposits and the increase of shear stress by saturation of the soil near the ground surface results in the deformation. The amount of rainfall, pattern of rainfall and the formation of the slope are closely related to the phenomenon.
By observing this kind of deformation using a tiltmeter within the hole, special landslide prevention measures in the slope are not needed.
Key words:rheology of clay, quasi landslide, tiltmeter within the hole, amount of rainfall
和文要旨
豪雨後，急斜面の地表面に引張亀裂が発生することがある。しかし，これらの亀裂は長時間にわたって拡大しない。この変位現象を疑似地すべりと呼称する。なぜならば，これは新規地すべり発生の前兆ではない。
この現象は崩積土中の複数の軟質粘土層のレオロジー変形に基づく。これは，地表面付近の地層の飽和によるせん断応力の増加が原因である。この原因は，降雨量と雨の降り方および斜面地形が関係する。
この種の変状が孔内傾斜計を用いて確認されたとき，斜面内での特別な地すべり防止対策の必要はない。
キーワード：粘土のレオロジー，疑似地すべり，孔内傾斜計，降雨量 |
■貯水池地すべりの変状予測と対策工実施の判定法の開発
Assessment of landslide activities in the reservoir slopes and its application to decision making of prevention works
杉村淑人・野中樹夫・山辺 正
Yokito SUGIMURA, Mikio NONAKA and Tadashi YAMABE |
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Abstract
Data on the landslide activities in the reservoir slopes and the causes of such conditions were collected for 84 reservoir landslide slopes composed of Paleozoic Mesozoic strata. Four active levels were defined, and the landslide activities were classified. To investigate the predictability of active levels, ten items affecting the landslide activities were chosen, and a formula for evaluating the relationships between the active levels and the ten items were developed by using a statistical method called “quantification theory type\外字(8152)". In predicting active levels resulting in collapse or ground surface fall, the formula has a prediction accuracy of around 80 percent. Then, the feasibility of decision making of reservoir landslide prevention works appropriate for the active level was investigated.
Key words: Paleozoic Mesozoic strata, quantification theory type\外字(8152), landslide activity, reservoir slope
和文要旨
中古生層の地質帯に位置する貯水池周辺の無対策地すべり８４地区について，貯水位操作にともなう変状発生状況とその要因に係わるデータ収集を行い，変状レベルを４つに区分し変状実態の整理を行った。次に変状レベルの予測の可能性を把握するため，変状に影響する１０アイテムを選定し，変状レベルと各アイテムの関係について，数量化２類を用いて判別式を作成した。その結果，崩壊や地表に段差が生じる変状について８割程度の判別的中率となる判別式が見出された。そこで，変状レベルを考慮した貯水池地すべり対策工の可能性について検討を行った。
キーワード：中古生層，数量化２類，地すべり変状，貯水池周辺斜面 |
■四国，中央構造線沿いの熱水変質帯における地すべりの初期発生機構－ミクロの見地から－
Initial mechanism of landslide occurrence in hydrothermal alteration zone
near MTL, Shikoku Island, Japan-From microscopic point of view-
宮原正明・北川隆司・矢田部龍一・横田公忠
Masaaki MIYAHARA, Ryuji KITAGAWA, Ryuichi YATABE and Kinutada YOKOTA |
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Abstract
Specimens collected from light greenish gray colored alteration zones found at the outcrops and boring cores in the Umanosedanigawa River landslide area(Iyomishima city, Ehime Prefecture)near MTL were microscopically investigated to elucidate the initial mechanism of landslide occurrence. The light greenish gray colored alteration zones are generally observed in Sanbagawa pelitic schist. It is inferred that the alteration zones were formed by the hydrothermal alteration during rhyolitic intrusive activities. A lot of green fine‐grained quartz was characteristically recognized in the light greenish gray colored alteration zones. Smectite was crystallized in interstices developed as network pattern in green fine‐grained quartz. It was confirmed that slaking‐like destruction was occurred in green fine‐grained quartz by swelling of smectite. It is considered from these results that the expanding of smectite formed in green fine‐grained quartz serves as a trigger of landslides in the investigated area.
Key words:landslide, MTL, hydrothermal alteration zone, smectite
和文要旨
中央構造線に近い愛媛県伊予三島市における地すべり地域での地表踏査とボーリング調査によって確認された地すべりの素因と考えられる淡緑灰色変質帯をミクロ的に調べ，地すべり発生のメカニズムを推察した。淡緑灰色変質帯は，三波川帯泥質片岩中に見られ，流紋岩質貫入岩の貫入時における熱水変質作用で形成されたと推定される。淡緑灰色変質帯には，特徴的に緑色細粒石英が認められ，その内部にスメクタイトが網目状に生成している。緑色細粒石英には，スメクタイトの膨潤によるスレーキングに似た破壊が発生していることが確かめられた。この地域の地すべりをミクロ的に見た場合，スメクタイトの膨潤による石英粒子のミクロ的破壊現象が地すべり発生の引き金になっていると考えられる。
キーワード：地すべり，中央構造線，熱水変質帯，スメクタイト |
■Development of a risk assessment methodology for landslides in Sri Lanka
Bandara ABEYSINGHE, Yushiro IWAO and Senerath BANDARA |
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Abstract
The central hilly areas of Sri Lanka underlaid by highly folded and fractured metamorphic rocks of different grades of weathering have a high probability for landslides. Intense precipitation is a major factor contributing to the landslides. Despite repeated occurrences of landslides in Sri Lanka inflicting losses in terms of life and property, very little has been done towards introducing scientific practices for delineating the degrees of hazard, identifying elements at risk, and landslide risk assessment. A National Building Research Organisation(NBRO)has already commenced production of landslide hazard zonation maps(1:10,000)covering the areas prone to landslides. The proposed methodology for risk assessment in this paper will facilitate determination of risk through scientific analysis of landslide hazards in future. The demonstration of method was based on a case study carried out at Yatiyantota in Kegalle district. The expected annual total risk cost due to landslide disaster at specific site and/or annual risk cost on specific element at risk(property or person)could be calculated through the given methodology. It may assist in the designing of cost effective solutions and mitigation actions for the area. It is also expected to help non‐technical decision‐makers to assess the situation before taking appropriate futuristic measure. This methodology may be used for assessment of the risk at any specific site in landslide‐prone areas in Sri Lanka. Assessment of risk before commencement of major infrastructure development projects and establishment of human settlements in landslide‐prone areas based on the risk criterion will be essential for sustainable development of the central hills of Sri Lanka.
Key words:landslides, hazard identification, elements at risk, risk assessment, Sri Lanka |
■四万十層群の斜面崩壊の性状と植生の役割に関する研究
Studies on the characteristics of the slope failure of Shimanto formation
and the role of vegetations
潘 暁波・日浦啓全・篠 和夫・江崎次夫
PAN Xiaobo, Hiromasa HIURA, Kazuo SHINO and Tsuguo EZAKI |
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Abstract
In recent years, many slope failures occurred in the zone of Shimanto formation, which produce the sediment amounted to be in the range of 104-105m3. By the comparative investigation of the failure mechanism with those occurred in the mountains of Granitic rocks which will produce one order less volume of sediment(<104m3)and assuming that the ratio of depth against longitudinal length of moving soil mass in both geology to be almost the same, authors found that the important morphological factor is the steep and longer slope length, whereas the inclination has similar tendency of the distribution. In Granitic rock mountains, slides are generally thin and the underlying base rock will never collapse, on the other hand, for the case of Shimanto formation, slip surface will develop deep beneath the surface until the depth of loosened fissure rich base rock. By the analysis of measured records and the video of a slope failure occurred in Aki county, Kochi prefecture in 1998, precursive movements show the creep characteristics and the function of root network system which is believed to work for defending the slope failure could not recognized in case of the reforested Sugi(Cryptomeria japonica). The growing effects of a single tree onto the soil were simulated in use of the direct shear test apparatus.
Key words:Shimanto formation, loosened base rocks, slope failure, root network system
和文要旨
本論では，四万十層群の斜面で生起する崩壊現象を取り扱っている。近年，四万十層群の斜面崩壊事例が多発しているように思われる。これらの現象を理解するために，崩落土砂量の規模ではひとつオーダーの低い花崗岩類山地の表層崩壊を対比させたて考察した。発生場の条件のとして斜面傾斜の分布にそれほど違いはないが起伏量と斜面長に相違のあること，表層のポーラスな土層の構造については地質帯の違いはあるものの共通した構造を持つことを確認した。その後，平成１０年に高知県安芸郡安田町与床で発生した斜面崩壊についてのビデオ映像と観測記録を解析し，その特徴としてクリープ的な前兆現象を示すことを確かめた。崩壊時には表土の崩落に止まらず，緩みを生じている基盤岩の深さまで進むことを指摘した。さらに，降雨を直接の引き金としないで発生した事例であることを指摘した。ついで，ビデオに写された樹木の崩落時の挙動より砂質土的な取り扱いが可能であることを指摘した。人工林として植栽されたスギでは根茎のネットワークによる崩壊抑止効果は期待できないが，単木が成長していく過程で土層全体の強度発現に一定の役割を果たしていることを，一面せん断試験器を用いて実験で検証した。
キーワード：四万十層群，斜面崩壊，基盤岩盤の緩み，根茎のネットワーク |
■長野県北部，切久保地すべりの後期更新世以降の活動履歴
Tephro― and archaeo‐chronology of the Kirikubo Landslide, northern Nagano Prefecture, central Japan
植木岳雪
Takeyuki UEKI |
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Abstract
The Kirikubo Landslide is a Late Pleistocene large‐scale landslide in the northern Nagano Prefecture, central Japan. The Tateyama E Tephra Bed just conformably overlies the main slip scarp, suggesting that the Kirikubo Landslide occurred primarily at 60 to 75 Ka. Secondary smaller‐scale landslide cut the primary landslide mass at 7 to 10 Ka, based on the age of archaeological site on the slide blocks.
Key words:large‐scale landslide, tephrochronology, archaeochronology, Pleistocene, Nagano Prefecture
和文要旨
長野県北部，八坂村切久保には後期更新世に発生した大規模地すべりがあり，本研究ではこれを切久保地すべりと呼ぶ。滑落崖の直上を立山Ｅテフラが覆うことから，切久保地すべりは６～７．５万年前に発生したと考えられる。また，移動ブロック上に立地する考古遺跡の年代に基づいて，１万～７０００年前には，初生的な移動ブロックを切る２次的な地すべりが発生したことが明らかになった。
キーワード：大規模地すべり，テフロクロノロジー，考古編年，更新世，長野県 |