地すべり土のリングせん断挙動に及ぼす鉱物組成の影響
The influence of mineral composition on the ring shear behavior of landslide soils
宜保清一a）＊・中村真也a）・佐々木慶三b）
Seiichi GIBO, Shinya NAKAMURA and Keizo SASAKI |
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Abstract
In the present study, ring shear tests were firstly performed for landslide soils which showed a considerable variation in physical and mineralogical properties. Influences of the mineral compositions on shear behaviors were analyzed, and classification of the stress‐displacement curve was discussed. A fall in the strength from the peak to the residual state was clearly observed for the soil containing a large amount of silt and sand particles, and clay fraction below ２０％. The variation of the strength reduction versus total contents of preferred‐oriented clay minerals was expressed as a convex curve. Stress‐displacement curves of landslide soils were classified into４types based on the total amount of minerals in soil.
Key words：landslide soil, ring shear, clay mineral
和文要旨
本研究では，物理的・鉱物学的性質を異にする地すべり土についてリングせん断試験を行い，せん断挙動に及ぼす鉱物組成の影響を分析し，応力〜変位曲線の類別化を試みた。シルト・砂分が多く，粘土含有量が２０％より少ない試料でもピークから残留への明瞭な強度逓減が認められた。強度逓減量は，配向性鉱物総量との関係において凸型変化を示した。地すべり土の応力〜変位曲線は，含有される鉱物総量に基づいて４タイプに類別できた。
キーワード：地すべり土，リングせん断，粘土鉱物 |
アンカー工による斜面の補強効果―極限平衡法と弾塑性FEMとの比較
Numerical comparisons of stabilizing effects of anchors in slopes between
limit equilibrium methods and elasto‐plastic finite element method
蔡 飛a）＊・鵜飼恵三a）
Fei CAI and Keizo UGAI |
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Abstract
The stability of slopes stabilized by anchors was analyzed by the three‐dimensional elasto‐plastic finite element method, and the results were compared with those of Fellenius' method and Bishop's simplified method. The comparison indicates that the three‐dimensional elasto‐plastic finite element method has the ability to evaluate the safety factor of the slopes reinforced with anchors. The safety factor of the three‐dimensional elasto‐plastic finite element method was almost the same as that of Bishop's simplified method, and was slightly larger than that of Fellenuis' method. Additionally, two simplified approaches are proposed to simulate anchors in slopes, and the results indicate that the safety factor of the proposed approaches was almost identical to that of the three‐dimensional elasto‐plastic finite element method where anchors and soil‐anchor interaction were simulated in details.
Key words：anchor, elasto‐plastic, finite element method, slope stability analysis
和文要旨
アンカー工で補強された斜面の全体安全率を３次元弾塑性FEMにより評価し，フェレニウス法と簡易ビショップ法に基づく結果と比較した。３次元弾塑性FEMがアンカー工で補強された斜面の全体安全率を精度よく表現できることが示された。３次元弾塑性FEMから定めたアンカー力を用いて簡易ビショップ法により計算した全体安全率は，３次元弾塑性FEMによるものとほぼ一致し，フェレニウス法により計算したものは，少し小さくなることがわかった。また，２次元弾塑性FEMに基づく簡易計算法を提案し，簡易計算法により，アンカー工で補強された斜面の全体安全率を精度よく算出できることを明らかにした。
キーワード：アンカー，弾塑性，有限要素法，斜面安定解析 |
すべり面せん断試験によるすべり面のせん断強度評価
Evaluation of the shear strength on undisturbed slip surface by newly
developed Slip Surface Direct Shear（SSDS）apparatus
眞弓孝之a）＊・柴崎達也a）・山崎孝成a）
Takayuki MAYUMI, Tatsuya SHIBASAKI and Takanari YAMASAKI |
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Abstract
This paper describes both the principles of Slip Surface Direct Shear apparatus（SSDS）that would be able to adjust the position of slip surface to coincide with the shear surface of the apparatus and the results of the test in use of it. The number of tested slip surfaces is１２４．These were taken from Tertiary strata landslides, shear zone landslides and solfataric landslides. From the test results, a typical stress path on the slip surface is recognized. The results obtained from Tertiary strata landslides clearly show that the residual shear strength is affected by slip surface covered with thin smectitic clay. As for the shear zone landslides, test results show that the residual shear strength is strongly affected by the coarse size grain content in the slip surface clay. Comparing SSDS data with that of ring shear tests, authors have found that if the sample prepared for a ring shear test has the same clay content as the slip surface clay, both results show the same strength parameter.
Key words：slip surface direct shear test, constant volume shear test,
ring shear test, residual strength, grain size distribution characteristics
和文要旨
すべり面に発揮されるせん断強度を直接的に計測することを目的とし，不撹乱含すべり面試料中のすべり面を試験時せん断面に一致させられる一面せん断試験装置を開発した。本試験装置を用いて，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試料に対して試験を行い，すべり面のせん断強度特性として典型的な応力径路の線形を確認した。また，試験結果を地質毎に分類し，対象試料の物理的特性や電子顕微鏡を用いた観察結果等を踏まえて考察を行い，新第三系堆積岩の多くの地すべりの場合に，すべり面上に見られるスメクタイトの存在が残留強度に強く影響を及ぼしていること，さらに変成岩地すべりではすべり面粘土中に含まれる粗粒径成分の多寡が強度に強く影響すること等を明らかにした。さらにリングせん断試験で得た残留強度との比較を行い，すべり面粘土の粒度組成がリングせん断試験用試料の採取，および粒度調整処理によって大きな変化を生じない場合に，両試験結果が一致することを明らかにした。
キーワード：すべり面せん断試験，定体積せん断試験，リングせん断試験，残留強度，粒度組成
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岩石色彩値の深度変化からみた
山形県白鷹火山周辺の風化環境と山体崩壊の可能性
Weathering environments based on rock color changes with depth and the
probability of recent sector collapses in the Shirataka Volcano area, Yamagata,
Japan
横田修一郎a）＊・西山賢一b）・原口 強c）・井口 隆d）
Shuichiro YOKOTA, Ken‐ichi NISHIYAMA, Tsuyoshi HARAGUCHI and Takashi INOKUCHI |
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Abstract
Rock color values a＊ and b＊（CIE L＊a＊b＊ color measurement system）of borehole cores from the caldera of the Quaternary Shirataka Volcano, Yamagata, Japan, were measured with a colorimeter, and their changes with depth and lithofacies were analyzed to understand weathering environments in volcanic areas. In general, the weathering environment is reflected in oxidative changes to iron minerals in rocks. The correspondence of a＊ and b＊ color values to the colors of goethite, which is brownish as a result of low‐temperature oxidation, and hematite, which is reddish brown, reflecting high‐temperature oxidation, makes it possible to distinguish the spatial distribution of these iron minerals by examining color distribution.
The distribution of a＊ and b＊ values in rocks of the caldera indicates that a goethite zone of low‐temperature oxidation extends widely, but hematite zones of high‐temperature oxidation are localized. b＊‐values, indicating goethite, were higher at shallow depths and decreased with depth, reaching a constant low value at ５０ to １５０m depth. Low‐temperature（i.e., normal atmospheric temperature）oxidation has thus prevailed in this volcanic region for a long time period, as indicated by the thick weathered zone represented by high b＊ values.
The thickness of the high‐b＊‐value zone seems to be independent of lithofacies or geologic structures; instead, it is almost concordant with the present‐day topographic relief in the caldera. The continuity of the weathered zone suggests that the caldera has not been exposed to recent large‐scale surface sedimentation or erosion. Therefore, recent large‐scale avalanches or sector collapses, which would have caused large‐scale sedimentation, probably have not occurred in this area.
Key words：volcano, sector collapse, weathering, Shirataka Volcano, color values
和文要旨
火山地域における風化環境変化の実態をみるため，山形県白鷹火山カルデラにおいてボーリング・コアの岩石色彩値を連続的に測定し，岩相と対応させて色彩値a＊，b＊の深度変化を調べた。一般に岩石中の鉄鉱物は風化環境に敏感に反応し，色彩値の違いとなって現れやすい。a＊，b＊値と含有する鉄鉱物との関係に基づけば，それらの空間分布から黄褐色化をもたらす常温風化の酸化作用と赤褐色化をもたらす熱水変質等の高温酸化作用を識別できる。
本カルデラ内では後者を特徴づけるヘマタイト（hematite）ゾーンは局所的であるが，前者のゲータイト（goethite）は広い範囲に及んでいる。黄褐色化の指標であるb＊値は地下浅部では高いが，深度とともに低下し，地下数１０m〜１５０mで一定値に収束する。したがってカルデラ内では地表から地下深部にいたる風化作用が存在し，この風化帯の厚さから長期間継続してきたことが推定される。
風化帯は特定の岩相分布や地質構造に規制されたものではなく，むしろ現在のカルデラ内地形に調和的である。風化帯構造の連続性から，カルデラ内では物質移動による大規模な埋積が最近生じたとは考えられず，したがって，これをもたらすような山体崩壊が最近発生した可能性は低いと推定される。
キーワード：火山，山体崩壊，風化，白鷹火山，色彩値 |
神居古潭帯に分布する蛇紋岩の不攪乱状態および
再構成粘性土試料の力学特性
Mechanical properties of undisturbed and reconstituted serpentinite originated
from the Kamuikotan belt in Hokkaido
三田地利之a）＊・藤澤久子b）・和智真太郎c）・大河原正文d）・工藤 豊a）
Toshiyuki MITACHI, Hisako FUJISAWA, Shintaro WACHI, Masafumi OKAWARA and Yutaka KUDOH |
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Abstract
Various laboratory tests such as triaxial test, reversal direct box shear test, swelling pressure test, slaking test were carried out to investigate the mechanical properties of serpentinite using the undisturbed and reconstituted samples originated from the Kamuikotan belt in Hokkaido. From the test results, it was concluded as follows.
The residual angle of shearing resistanceφ′r as well as the peak angle of shearing resistanceφ′p were unexpectedly large by showingφ′r＝２７〜２８°and φ′p＝３１±２°irrespective of the state of undisturbed or reconstituted. But the undrained strength of the reconstituted specimen was less than one fourth of the undisturbed one due to marked difference of dilatancy characteristics of those specimens. The angle of shearing resistance between the pre‐existing foliated flakes of serpentinite exhibited ６．５°at minimum.
Key words：triaxial compression test, reversal direct box shear test, residual state angle of shear resistance, swelling pressure, slaking
和文要旨
北海道神居古潭帯で採取された蛇紋岩の不攪乱状態および再構成の試料について，三軸圧縮試験，繰返し一面せん断試験，膨潤圧試験，スレーキング試験などの各種力学試験を実施した。試験の結果，蛇紋岩のピークせん断抵抗角φ′pは不撹乱，再構成によらず，３１±２°程度の値をとり，残留状態のせん断抵抗角φ′rは２７〜２８°と予想外に大きい。しかし，蛇紋岩粘性土の非排水せん断強度はダイレタンシー特性の違いから，不攪乱蛇紋岩の４分の１以下と大幅に低下する。また，予め葉片に沿って二分し，試料の表面を磨いた状態での試験結果から得られた葉片に沿う最小のせん断抵抗角は６．５°を示した。
キーワード：三軸圧縮試験，繰返し一面せん断試験，残留状態せん断抵抗角，膨潤圧，スレーキング |
Analysis of root reinforcement at slip surface
Quang Minh NGHIEMa）＊, Hiroyuki NAKAMURAa） and Katsushige SHIRAKIb） |
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Abstract
In this paper, a theoretical model is proposed to investigate the behavior of tree roots subjected to mass movement and to determine the reinforcement capacity of tree roots at slip surface of landslide. Tree roots function as a type of reinforcement material to minimize landslide. By modelling a root system as a cluster of root fibers inserted through the unstable soil to the bedrock, the increase in the shear resistance of soil can be reliably estimated. Properties and distribution of the roots are taken from a number of previous field experiments. Roots used in the model are Japanese cedar（Criptomeria japonica）roots, the most popular species in Japan. With the inclined roots classified by inclinations and diameters, the relationship between reinforcement capacity of root fibers and horizontal displacements is analyzed under two main conditions: ultimate stress and pullout resistance. The computed results of the tree root model are used in the prediction of root reinforcement at slip surface of a previous soil‐shear test. There is, in general, a reasonable agreement between predicted results and experimental data.
Key words:inclined root, root reinforcement, ultimate stress, pullout resistance, landslide |
別当谷側壁でみられる白山甚之助谷地すべりの内部構造
Internal structure of the Jinnosuke‐dani landslide, observable on the Betto valley wall
藤井幸泰a）・堀伸三郎b）＊
Yukiyasu FUJII and Shinzaburo HORI |
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Abstract
The Central‐ridge large‐scaled block of Jinnosuke‐dani landslide is well exposed at the left bank of the Betto valley. Accessibility of the exposure is limited to the bottom part along the valley floor, because the exposed wall is nearly vertical and more than １００m high.
The geological structure of the wall is clarified by both direct observations and analysis of the photographs taken from the opposite ridge. The observable internal structure suggests that the Central‐ridge large‐scaled block has three major slip surfaces.
Key words：internal structure, slip surface, Jinnosuke‐dani landslide, Mt. Hakusan
和文要旨
別当谷左岸の山腹斜面には，中間尾根大規模ブロックの側面が連続的に露出している。露頭は標高差１００m以上の急崖で，近接して観察できる部分は河床付近に限られる。
対岸の尾根部から撮影した連続写真の解析と，河床付近での調査結果を併せて，露頭全体の地質構造を把握できた。その結果，中間尾根大規模ブロックは３層のすべり面で構成されることがわかった。
キーワード：地すべり内部構造，すべり面，甚之助谷地すべり，白山 |