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都市周辺山麓部の放置竹林の拡大にともなう土砂災害危険性
Risk of sediment related disasters due to the abandoned expanding bamboo
stands at the foot of slopes surrounding city area
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日浦啓全ａ）＊・有川 崇ｂ）・ドゥラ ドゥルガ バハドゥールｃ）
Hiromasa HIURA, Takashi ARIKAWA and Dura Durga BAHADUR |
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Abstract
The expansion of abandoned bamboo stand in the southern‐western region of Japan is remarkable. In September 1998 in Kochi city, due to the heavy rainfall, on the slopes surrounding Kochi city, many slope failures and debris flow have occurred. At almost 1/3 site of disasters, bamboo stands were found very close to the site or on that point, and the bamboo stands seemed to have important role on the sediment related disasters.
Researches on the relation between the expansion of bamboo stand and the sediment related disasters are very few, though authors have the conviction that, in the near future, the bamboo and bamboo stands will cause one of the severe problems in the sediment related disasters. Once the bamboo slope is cut for the cultivation or another agricultural usage or house building at the foot of the slope, having the root‐mat system good permeability, the infiltrated abundant rainwater through root‐mat system will cause severe erosion and induces the slope instability. Authors have investigated the soil of the bamboo stand in the laboratory and executed the penetration test in‐situ and subsequently, proposed three failure models of bamboo. These models will be useful for the treatment of the abandoned bamboo stand and for the measures of the instable slope on which bamboo grows.
Key words:abandoned bamboo stands, sediment related disaster, failure model, root‐mat system of bamboo, physical properties of soil in the bamboo stand
和文要旨
近年の関東以西の市街地周辺の山麓部での放置竹林の拡大は目に余るものがある。平成１０年９月に高知市を中心とする地域をおそった前線性豪雨では市街地の洪水被害のみでなく，都市を取り巻く山地の山麓部で多くの崖崩れ災害が発生し，しかもその約１／３の地点に竹林が見られた。竹林と崩壊現象の因果関係の解明は，まだ緒についたばかりであるが，今後，都市域での土砂災害の典型となることが懸念される。土壌の物理的性質からは表層部の透水性が大きく，そのため表層部に雨水が集中し，斜面の不安定化を惹起する懸念がある。筆者らは，竹林土壌の物理性を調べると共に竹林内において貫入試験を実施した。これらの結果をもとに崩壊モデルを提案した。このモデルは今後，放置竹林の管理のあり方を考えていく上で，さらには土地利用や斜面の不安定化と竹の存在の関連を考えていく上で有効なものとなることが期待できる。
キーワード：放置竹林，土砂災害，崩壊モデル，竹のルートマット，竹林土壌の物理的性質 |
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日本列島における地すべり地形の分布・特徴
Regional distribution of large landslide landforms in Japan
―Implication in geology and landform―
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藤原 治ａ）・柳田 誠ｂ）＊・清水長正ｃ）・三箇智二ｄ）・佐々木俊法ｂ）
Osamu FUJIWARA, Makoto YANAGIDA, Nagamasa SHIMIZU, Tomoji SANGA and Toshinori
SASAKI
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Abstract
Distribution map of large landslide landforms (longer than 200m) on whole Japanese islands was compliled by using landslide maps (mainly 1:50,000 scale) and detection of 1:25,000 scale topographical maps. The landslide landforms are widely distributed on Japanese hills and mountains, especially in the back arc zone of Tohoku district, Shin‐etsu, Hokuriku and Shikoku districts. Relationships between regional distribution of landslide landforms and geological and topographical features are analyzed by GIS (Geographical Information System) over the Japanese islands. Data on landslide landforms, geology, relief energy and slope gradient were prepared with 2 x 2km mesh system. The number of meshes for statistical analyses is about 71,000. Landslide landform exists in 43% of portion among these meshes. In 2602 meshes (3.7% of total meshes), landslide landforms cover more than 40% of each mesh area.
The landslide landforms are densely distributed in the Neogene sedimentary rocks, Green Tuff and the Paleogene and Neogene coalfields. Relative high dense distribution of landslide landforms are observed in the mountaineous region with 400―800m of relief energy and 15°―30° of slope gradient. The landslide landforms are scarcely distributed in the areas with dispersion of altitude more than 120m or less than 30m.
Key words:landslide map, GIS, geology, geomorphic quantity
和文要旨
既存の地すべり地形分布図と１／２．５万地形図の新たな読図結果を編集して，日本列島全体の地すべり地形分布図を作成した。分布図では奥行きが約２００m以上の地すべり地形を対象として作成した。分布図をみると地すべり地形は日本の山地・丘陵に広く分布し，とくに東北地方背弧側，信越，北陸，四国で密に分布する。次に地すべり地形の分布と地形，地質との関連性をGISを用いて統計的に解析した。
解析には２km四方のメッシュで作成された地すべり地形，地質，起伏量，傾斜量のデータを用いた。統計解析に用いたメッシュの数は約７１０００個であり，そのうち４３％のメッシュは地すべり地形を含む。ひとつのメッシュの中で面積４０％以上が地すべり地形で占められているメッシュを多地すべりメッシュと呼び，これは２６０２個（全体の３．７％）ある。地すべり地形が多い地質区分は，新第三紀海成堆積岩，中新世火山岩類（グリーンタフ火山岩類），古第三紀〜新第三紀挟炭層である。起伏量が２００〜６００m，あるいは平均傾斜が１５°〜３０°の山地では地すべり地形の分布密度が高い。
キーワード：地すべり地形分布図，GIS，地質区分，地形量 |
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Simulation of groundwater fluctuation in fracture‐zone landslides
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Akhilesh Kumar KARNAａ）＊，Hiroyuki NAKAMURAｂ） and Katsushige SHIRAKIｂ）
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Abstract
Landslide is one of the major geotechnical problems in the mountainous regions. Pore water pressure is one of the main factors which affect the shear strength of the soil at the slip surface, thereby making the slope more unstable. Therefore, it is necessary to predict groundwater table to forecast the stability of a slope. Further, in order to understand the groundwater fluctuation, it is necessary to know the hydraulic properties of the landslide soil. Hydraulic conductivity and soil‐water retention function of landslide material are the two most important factors determining the seepage flow characteristics. As the fracture‐zone landslides are highly discontinuous due to existence of cracks through which major groundwater flow occurs, the use of experimentally measured soil hydraulic parameters are quite limited. This article attempts to find a suitable method to estimate these properties using the rainfall and groundwater level measured in‐situ using finite element method. In order to achieve this, parametric analysis was done considering multilayered soil matrix. The result of analysis was applied to an actual landslide and it was found that agreeable groundwater fluctuation can be obtained by assuming homogeneous soil properties with multilayered system.
Key words:fracture‐zone landslide, groundwater level prediction, seepage flow simulation, soil hydraulic properties |
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神戸層群凝灰岩の切土掘削に伴う地盤変形と遅れ破壊
Occurrences of delayed failure and deformation of ground with excavation
on the tuff of Palaeogene Kobe Group, Kobe district, SW Japan
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中川 渉ａ）＊・遠藤 司ｂ）
Wataru NAKAGAWA and Tsukasa ENDOU
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Abstract
The Kobe Group consists of frequent alternations of conglomerate, sandstone, mudstone and tuff beds, and has been known the site of many landslides. The tuff of the Kobe Group is often generated delayed failure, which is the collapse with the excavation of ground. The occurrences of collapse are not always related to the rainfall and an ascending level of the underground water. The collapse occurs in general after the excavation with wide period from one to ten years. The authors investigated natures of 28 landslides collected from the Kobe Group through the recent twenty years, and revealed the mechanism of delayed failure within the tuff.
１）The forms of collapse with the delayed failure show always a linear‐type, and also have 3‐13 in ratios of wide (W) and depth (H).The range of W／H ratio is similar to them of the common landslides.
２）Seventy percents of studied landslides were collapsed after one year from the excavation.
３）The surface of landslides with delayed failure is recognized in the soft‐tuff and the layer‐parallel shear zones, which are formed around the boundaries between the tuff and other sedimentary beds.
４）The strains had been generated in the soft‐tuff and layer‐parallel shear zone from the cut‐slope, which is deformed elastically by the excavation. The deformation of cut‐slope has been accumulated throughout a couple of years.
５）The deformation of cut‐slope occurs consequentially as swelling and slaking with increase the water content of soft‐tuff through a few years from the first. The tuff of the Kobe Group has commonly swelling minerals such as Ca rich‐type and middle‐type of smectite. Generally, the period of deformation on the tuff bearing the middle‐type of smectite is long up to ten years.
The delayed failure within the tuff and the layer‐parallel shear zone of the Kobe Group is caused by the release of stress in the ground as a result of the excavation. From the release of stress, the cracks are developed in the cut‐slope. Then, the underground water penetrated into the cracks where occur the continuous reaction to underground water and middle‐type of smectite. Thus, the cut‐slope is collapsed suddenly with accompanied by decreasing the shear strength and increasing the water content and strain softening in the soft‐tuff for the period from a few to ten years after the excavation.
Key words:delayed failure, Kobe Group, layer‐parallel shear zone, soft‐tuff, smectite, swelling
和文要旨
神戸層群は礫岩，砂岩，泥岩および凝灰岩の互層から構成され，地すべりが多発する地層として知られている。神戸層群の凝灰岩には切土掘削に伴う遅れ破壊が認められる。この遅れ破壊は，降雨や地下水上昇などの影響にかかわらず生じており，凝灰岩を含む地山を掘削してから数年〜１０年の期間をおいて発生することが多い。筆者らは，神戸層群の凝灰岩に見られる遅れ破壊について，その発生機構を推定するために，ここ２０年間に確認された地すべりのうち２８箇所の遅れ破壊事例を詳細に検討し，その特徴を明らかにした。
１）遅れ破壊の崩壊形態は，大部分が直線型を示しており，断面的には層理面に沿ったすべり面と断層・節理等の分離面の２平面で形成されている。その地すべりの深度（d）と幅（W）の比率は３〜１３で，一般的な地すべりの値と一致する。
２）地山の掘削から１年後に発生した遅れ破壊は全体の７０％に達し，掘削後７〜８年を経て崩壊するケースもある。
３）遅れ破壊によって形成されるすべり面は，軟質な凝灰岩や地層境界に発達する層状破砕帯中に認められる。
４）軟質な凝灰岩や層状破砕帯を有する切土面は，掘削されてから数年の間，その面内に歪みが蓄積され，弾性的な変形の後微小変形が継続する。
５）神戸層群の凝灰岩層には軟質凝灰岩，層状破砕帯に膨潤性鉱物のスメクタイトが含有されている。神戸層群のスメクタイトには，中間型と崩壊型が存在し，前者は掘削後の変形が長期に継続し，遅れ破壊の原因となる。一方崩壊型はCaが豊富で切土後比較的早期に崩壊する。
神戸層群の凝灰岩及び層状破砕帯に生じる遅れ破壊の多くは，地山の掘削に伴い応力が解放されることによって発生する。掘削された切土面には亀裂が生じ，そこに地下水が浸透する。その後，地下水と軟質凝灰岩や層状破砕帯に含まれる中間型のスメクタイトが反応して，膨潤し，切土面を変形させる。軟質凝灰岩や層状破砕帯は，含水比の増加と変形の進行に伴い徐々に剪断強度を低下させ，掘削後数年〜１０年を経て突然崩壊すると考えられる。
キーワード：遅れ破壊，神戸層群，層状破砕帯，軟質凝灰岩，スメクタイト，膨潤 |
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Occurrences and displacements of landslides by an earthquake with
a subsequent rain:the 1999 Chi‐chi earthquake in central Taiwan
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Wen‐Neng, WANGａ），Hiroyuki NAKAMURAｂ）＊，Satoshi TSUCHIYAｃ），
Shang‐Chih WUａ） and Shoung OUYANGａ）
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Abstract
Many landslides occurred during the Chi‐chi earthquake（ML=7.3）on 21 September 1999 in western central Taiwan. These landslides have been discerned from either aerial photographs or SPOT images, digitized, and coregistered with topographic maps at 1:25,000 scale. The data of the landslides were manipulated using ArcView GIS. Results indicate that the rock formation consisting of sandstone with shale interbeds in Miocene age was most susceptible to landslides, whether by the earthquake or by the rain. Most earthquake‐induced landslides occurred on the upper part of the slopes and many transported displace material to river channel. The average travel ratio of the earthquake‐induced landslide was 1.2.
Study results indicate that the Toraji typhoon‐related landslide in 2001 was a secondary disaster of the Chi‐chi earthquake. Due to relatively high water content in the displaced material, the average travel ratio of the typhoon‐related landslide reached 1.5,greater than the earthquake‐induced one.
Key words：Chi‐chi earthquake, Toraji typhoon, earthquake‐induced landslide, typhoon‐related landslide, travel ratio |
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中栗地すべりにおける鮮新世泥岩の化学的風化
Chemical weathering of Pliocene mudstone in the Nakaguri landslide area,
Niigata Prefecture, Japan
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丸山清輝ａ）＊・武士俊也ａ）
Kiyoteru MARUYAMA and Toshiya TAKESHI
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Abstract
The Nakaguri landslide of Pliocene mudstone in Niigata Prefecture is a reactivated landslide. Chemical and mineralogical analyses of the drilled cores from the Nakaguri landslide area clarified the following facts:
The degree of chemical weathering of the movement layer is highest near the surface and declines with increasing depth. In the oxidation layer, sulfuric acid is produced by oxidizing pyrites, the solid soil is made acidic, and this acidification process reaches the layer in the subordinate position.
Moreover, there is more smectite in soils in contact with the assumed slip surface and the narrow band near it than in the soil outside of these parts. It is assumed that minerals change in quality and become smectite as the movement layer acidifies. Chemical weathering of the mudstone is dominated by the oxidation of pyrite and the dissolution of rock‐forming minerals by the resultant sulfuric acid, which leads to the deterioration of the mudstone. The increase of smectite is assumed to be one of the main causes of the movement of the landslide.
Key words: landslide, mudstone, chemical weathering
和文要旨
新第三紀層泥岩地帯で発生した再滑動型地すべりである新潟県中栗地すべりにおけるボーリングコアの化学分析とX線回折を実施し，地すべり移動層及び基岩の化学的風化状況を明らかにした。
移動層の化学的風化の程度は，地表面近くが一番高く，深度が増すにしたがって低くなっている。酸化層では，黄鉄鉱の酸化により硫酸が生成され，土塊が酸性化しており，その酸性化は下位層まで及んでいる。また，スメクタイトの量は，推定すべり面付近でその周辺より多くなっている。鉱物は酸により移動層内で変質し，スメクタイト化していることが推定される。
キーワード：地すべり，泥岩，化学的風化 |
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東野名地すべり地の地下水特性
―孔内水位観測と水質調査の結果から―
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Characteristics of groundwater from observations of the
groundwater levels and water quality in Higashi‐nomyo landslide area
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稲葉一成a）＊，吉田英二a），早川嘉一b）
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Kazunari INABA, Eiji YOSHIDA and Kaichi HAYAKAWA
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Abstract
This paper deals with the result of the observations regarding the groundwater levels and water quality in Higashi‐nomyo landslide area.So far it has been considered that the groundwater overflowed from the groundwater basin would permeate into the landslide slope located downward.This investigation revealed that the overflowed groundwater only exists at the top of the landslide slope while the water in the lower slope is derived from the deep layer and surface layer.
Key words:landslide, groundwater level, drainage well, water quality
和文要旨
東野名地すべり地C−５ブロックにおける地下水分布・流動を明らかにする目的で，孔内水位観測と水質調査を行った。当ブロックの地下水流動として，これまでは，地下水盆からオーバーフローした地下水が下方に位置する地すべり斜面へと浸透していくものと考えられていた。本調査では，地下水盆に由来する水は地すべりブロックの頭部に存在し，中腹部や末端部では，地下深部に由来する水と，表層部に由来する水が分布していることを明らかにした。
キーワード：地すべり，地下水位，集水井，水質 |