異常現象を示す樹木をｾﾝｻｰとする地すべり性開口ｸﾗｯｸの検出と解析
Detection and analysis of landslide open cracks using abnormal tree sensor
横山賢治･横山俊治
Kenji YOKOYAMA and Shunji YOKOYAMA |
|
Abstract
The trees received the damage by landslide open cracks show abnormal phenomena such as strained tree root, tilted trunk and intertwined trunk. Therefore, these abnormal trees are a excellent sensor for detection of the open cracks and their linear depressions. Additionally, the strained root is effective for the discrimination between hanging wall of the open crack and footwall, and for the estimation of the opening direction of the open crack. The distribution pattern and movement of crown cracks of the Yotsugi Landslide, Kochi Prefecture have revealed by means of the abnormal trees.
Key words:crown crack, open crack, abnormal tree, strained tree root, sensor
和文要旨
地すべり性開口ｸﾗｯｸによって損傷を受けた樹木は緊張した樹根や傾動した樹幹,絡み合った樹幹といった異常現象を示す。それゆえに,これらの異常樹木は開口ｸﾗｯｸやそれに起因した線状凹地の検出にすぐれたｾﾝｻｰである。さらに,緊張した樹根は開口ｸﾗｯｸの上盤と下盤の判定や開口方向の推定にも有効である。異常樹木を使うことによって,高知県代次地すべりの後背ｸﾗｯｸ群の分布様式と運動が明らかになった。
ｷｰﾜｰﾄﾞ:後背ｸﾗｯｸ,開口ｸﾗｯｸ,異常樹木,緊張した樹根,ｾﾝｻｰ |
間伐が森林の持つ表層崩壊防止機能に及ぼす評価手法の開発
Method for evaluating thinning influences on a forest's ability to prevent shallow landslides
阿部和時･黒川 潮･竹内美次
Kazutoki ABE, Ushio KUROKAWA, Yoshitugu TAKEUCHI |
|
Abstract
Thinning is indispensable for keeping planted forests healthy and producing timber of good quality. However, thinning is not being performed in increasingly large areas of planted forests in Japan due to stagnation of the forestry industry, and dense forests of thin trees are increasing. In such forests, trees can not develop roots properly, and their ability to prevent landslides may be adversely affected. This study aims to propose a method for evaluating the ability of thinned forests to prevent shallow landslides, based on past studies.
Planted forests of Cryptomeria japonica D. Don, which is the planted major tree species in Japan, were used for the analysis. Prior to the development of an assessment method, the distribution of roots was compared between a thinned forest where 1,400 trees grew per hectare and a non‐thinned forest where 3,400 trees grew per hectare. Since no clear differences were observed in the total root volume, distribution of root volume in the depth direction, and maximum root depth, it was decided to use the same method to estimate the amount of root distribution in the thinned and non‐thinned forests. The process by which a thinned forest loses the ability to prevent landslide was investigated by root pull resistance, which was found to disappear in about ten years after thinning. Based on these results, a method was developed to estimate changes in the ability of thinned and non‐thinned forests to prevent landslides. This method was used for simulation analysis, which showed that intense thinning, especially of matured 30‐ to 40‐year‐old trees, would lower the ability.
On the other hand, thinning of trees younger than 20 years, when forests slopes are prone to slides, caused no adverse effects, and thinning usually starts on the 15th year. The simulation also showed that typical thinning of 20‐year‐old or older trees would not cause the safety factor of the slope to drop below 1.0. These results indicate that thinning should be performed to ensure the growth of healthy forests where trees are resistant to diseases and meteorological damage, and where such healthy forests would naturally help to prevent landslides.
Key words:shallow landslide, forest, root system, root, thinning, pull‐out resistance of root
和文要旨
間伐は健全な人工林を成立させ,良質な木材を生産するために欠くことのできない重要な施業である。近年,日本の人工林では森林･林業を取り巻く情勢が悪化して間伐を実施することができず,細い木が林立した立木本数密度の高い人工林が増えている。このような林分では根系の発達が悪く,森林が持つ表層崩壊防止機能が劣るのではないかとの懸念が強い。このため,既往の研究をもとに間伐の影響も加えた森林の崩壊防止機能を評価できる方法を提案することを目的とした。ここでは日本の主要造林樹種であるｽｷﾞの人工林を対象とした。本手法を提案するに当たり,立木密度1400本/haの間伐林分と3400本/haの非間伐林分における根系分布状態を比較調査した結果,根の全体積や深さ方向への体積分布,最大分布深さ等について両者の間に明瞭な違いがなかったため,根の分布量推定には同じ方法を用いることにした。また,間伐された木の根が徐々に腐朽して崩壊防止機能が減少する過程を根の引き抜き試験によって調査したところ,間伐後約10年で根の引き抜き抵抗力は消失することが明らかになった。これらの調査結果を加え,間伐･非間伐林分における崩壊防止力の変化を評価できる手法を完成した。この手法を用いてｼﾐｭﾚｰｼｮﾝしたところ,強い間伐を多く実施した林分の方が崩壊防止力は低いことが示された。特に,30～40年生以上の壮齢林における間伐は崩壊防止力の上昇傾向を大きく鈍らせた。しかし,一般的に間伐が15年生頃から行われることを考えると,表層崩壊が多発しやすい20年生以下の幼齢林では間伐の影響が現れることはなかった。また,20年生以上の林分で一般的な強度の間伐が行われても,斜面安全率が1.0を下回らないことが示された。この結果から,間伐は病中害や気象害などに強い健全な森林を造成するために実施し,このような健全な森林が形成されることで必然的に崩壊防止機能も発揮されると考えるべきであることを指摘した。
ｷｰﾜｰﾄﾞ:表層崩壊,森林,根系,根,間伐,引き抜き抵抗力 |
アテ形成履歴からみた薄別川地すべりの変動履歴と水文･地質構造
Dendrochronological analysis of the Usubetsu‐gawa Landslide movement with special reference to hydrogeological structure of mass movement blocks
中村友輔･菊池俊一･北口勇作･藏田昭美
Yusuke NAKAMURA, Shun‐ichi KIKUCHI, Yusaku KITAGUCHI and Akiyoshi KURATA |
|
Abstract
We examined that the integration of the two methods-Dendrochronological analysis and hydrogeological survey-can be effective measures to grasp the phenomenon of landslide with a view to preventing landslide disasters.
For this study, the Usubetsu‐gawa landslide in Sapporo, northern Japan, was selected as the study site. Firstly, we investigated mass movement history by way of compression wood analysis to identify the activity of small blocks that were divided in the landslide area to take account of geomorphic conditions. Secondly, we conducted hydrogeological survey to estimate the movement of each landslide blocks. The survey shows that the area has a single block which has suffered from many repeated slide action, that is, a highly active landslide block can be divided from other lower active landslide blocks.
The findings suggest that compression woods analysis is a reliable method to identify active landslide blocks and priority landslide blocks, which must be of vital importance for the prevention of landslide disaster. In addition, through hydrogeological survey, some blocks are assumed highly active within harder weathering zone or in the area where the effective stress was notably declined by precipitation. The assumption was proved to be reliable by the findings of machine observation. It shows the possibility of estimating the movement of each landslide block through hydrogeological survey.
It can be said that the integration of the two survey methods is effective in order to grasp the landslide phenomenon for the prevention of landslide disaster in the landslide area with unknown slide action frequency.
Key words:landslide, dendrochronological analysis, hydrogeological survey
和文要旨
地すべり現象の把握と災害の予防のための調査手法として,樹木年代学的手法と水文地質学的手法という二つの手法の統合を考え,北海道薄別川地すべりに対し,両手法の統合の有用性を明らかにするため調査を行った。まず樹木年代学的手法,アテ形成履歴解析により,長期的な地すべり変動の把握と重点的な調査対象領域として捉えられる地すべりの活発領域の抽出を試みた。次に,水文･地質構造調査を行い,将来的な地塊動態の予測を試みた。その結果,アテ形成履歴解析により,複数の地すべり土塊から構成されていた薄別川地すべりのある土塊ですべりが繰り返されていたことが判り,薄別川地すべりを活発領域と非活発領域の二つに区分でき,災害予防のための重点的調査対象領域をアテ形成履歴解析により抽出できることが明らかとなった。また地すべり地の水文･地質構造調査結果から,強深層風化帯,強度の有効応力低下域であり活動性が高いと想定した土塊では,実際に地表伸縮計観測によってその妥当性が明らかになり,現在の水文観測と地質構造解析から,地すべり土塊の動態を想定することが可能であることが実証され,活動頻度が未知の地すべり地で,現象の把握と災害予防のために両手法を統合することは有用性が高いと考える。
ｷｰﾜｰﾄﾞ:地すべり,アテ形成履歴解析,水文･地質構造調査 |
四国御荷鉾地すべりの多様な生態系
Diverse ecosystem in Mikabu landslide, Shikoku district
稲垣秀輝･小坂英輝･平田夏実･草加速太･稲田敏昭
Hideki INAGAKI, Hideki KOSAKA, Natsumi HIRATA, Hayata KUSAKA and Toshiaki INADA |
|
Abstract
In our country, many parts of the land are mountain area. It is difficult for people to live where mountain area consists of many steep slopes. However, agriculture has been performed since ancient ages in the landslide area of mountain terrain, because there are many gentle slopes, much groundwater and fertile soil. We have studied recently on the countermeasures against the landslide, the lifestyle that is coexisted with the landslide phenomenon, and the land use.
In this paper, the ecosystem feature of animals and plants in a landslide area was investigated as an example of the large‐scale landslide of Shikoku Mountains. The landslide area has various micro‐topographic features, that is, diverse ecosystems and land use are accepted by comparing a landslide area with the adjoining non‐landslide area. The conservation methods of the diverse ecosystems are important without serious natural disaster in the landslide area, where is distributed as like belt and island in the steep mountains area.
Key words:landslide, ecosystem, geoecological section, Shikoku, species diversity
和文要旨
我が国は急峻な山岳地が多い。この山岳地の中で緩斜面の発達する地すべり地は地形･地質的な理由で多くの地下水をもち,土壌も肥沃で重要な生活の場となっていることが多い。従来から我々はそこでの地すべり災害への対応策や,地すべり現象と共存した暮らしや土地利用のあり方を研究してきた。今回,四国御荷鉾地すべりを例として,地すべり地域の植生を中心とした動植物の生態系調査を行なった。その結果,地すべり地域では隣接する非地すべりの山岳地に比較して,その独特の地すべり地形や地質の特徴を反映して土地利用が多様であること,それに対応して二次的環境に関わるさまざまな植生相をもち,動植物の多様性が高いことがわかった。
さらに,地すべり地域という災害地の中での防災対策の必要性と,急峻な山岳地の中で島状あるいは帯状に分布する地すべり地の多様な生態系を保全していくことの重要性について述べる。
ｷｰﾜｰﾄﾞ:地すべり,生態系,地生態断面,四国,種の多様性 |
Analytical study on the role of tree roots system in slope stability
Kanji KONDO,Toshiro NONODA,Setsuo HAYASHI and Shinya NUMAMOTO |
|
Abstract
The objective of this paper is to investigate the role of a tree roots system in slope stability through an analytical method. Variations of the diameter and the area ratio of the roots depending on its position in the soil were determined using two actual Japanese cedar trees(Cryptomeria japonica D. Don). The shape of the whole roots of a tree was assumed to be a hemisphere centered at the middle of the tree stump, thus the diameter and the area ratio of the roots is distributed on the surface of the hemisphere with an arbitary radius. The role of the tree roots system was clarified by the Spencer method for two‐dimensional analysis. The conventional studies of many researchers have adopted fixed and linear slip surfaces in stability analysis. In this study, the safety factor against slope failure was optimized to determine the critical slip surface of the non‐circular shape. The analytical examination using model slopes of homogeneous soil material revealed that a tree located at the toe of a slope is far more effective in slope stability than at any other position of the slope, and the role of this system has the function of increasing the critical safety factor against slope failure, on the contrary the depth of the slip surface(i. e. the scale of slope failure)will increase.
Key words:tree roots system, slope stability, critical slip surface, two‐dimensional analysis, Spencer method |
Slope stability of forested slopes considering effect of tree root and
steel bar reinforcement
Quang Minh NGHIEM, Hiroyuki NAKAMURA and Katsushige SHIRAKI |
|
Abstract
A tree root crossing through upper soil layer into bedrock increases shear strength of soil and renders the slope more stable. The effect of root reinforcement on soil strength has been well researched through the mechanistic and empirical studies, but yet few studies have examined how tree roots in combination with steel bar influence slope stability. Further, most of previous research lacked consideration of effect of slope displacement on the reinforcement of tree roots and steel bars.
By using actual distribution of tree root, an analysis of slope stability considering tree root reinforcement as a function of groundwater level and slope displacement will be proposed. Larger displacement of slope results in greater reinforcement of tree root. The resisting force and displacement increase until the slope becomes stable. For some reasons(external load, more rainfall... etc), if the slope became unstable again, the process of landslide repeats until F sub s =1. Tree root functions like reinforcement material and improves factor of safety of forested slope, F sub s ≧1. However, target factor of safety(also called design factor of safety F sub \{ d s \} )of a forested slope must be equal to or higher than a standard value(such as F sub \{ d s \} =1.2 in this paper). Magnitude of F sub \{ d s \} depends on the importance of slope. Thus, we need extra reinforcement even if the slope has tree root reinforcement. There are many methods to reinforce slope. In this paper, steel bar is chosen for two reasons:1)steel bar has the same behavior as a root in a landslide;2)steel bar is one of very few methods that can sustain the vegetation on slope. By applying model of inclined root proposed by Nghiem et al.(2001, 2003), a design method is presented for root fibers reinforcing slope in combination with steel bars.
Key words:forested slope, landslide, root reinforcement, target factor of safety, slope stability |
異なる林相及び森林伐採状況における樹木地上荷重の斜面安定に及ぼす影響
Influence of the forest tree load on the slope stability with different forest felling
久保田哲也,大村 寛,奥村武信,多田泰之,プレム・プラサド・パウデル
Tetsuya KUBOTA, Hiroshi OMURA, Takenobu OKUMURA, Yasuyuki TADA and Prem Prasad PAUDEL |
|
Abstract
Forests are among the best land use types for erosion control as well as environmental conservation. They reinforce not only the soil surface to reduce surface erosion but also the surface layer to control shallow failures. The effects of roots on slope stabilization emerged into the spotlight with only a few considerations of negative effects of forest load. The portion over the slope surface, i.e., their trunks and canopies, act as surcharge load for slope stability. Although roots have no effects on deep slides that sometimes appear with heavy rainfall and severe earthquakes, these negative effects have seldom been examined. Hereby, an assessment of slope stability with forest surcharge has to be taken in order to ensure the slope stability analysis when it comes to planning against landslides. Three different types of forest slope(conifer forests and a broad‐leaved tree forest)were chosen as test slopes in this study, and analyzed by non‐circular slip surface analysis with different forest felling types, i. e., 50%, 80%, 100% felling from the top and no felling.
Consequently, it is found that there are some cases in which its effects reaching up to approximately 4% of their safety factor. Notwithstanding, it is found that the 50% felling from the top makes sufficient effect on the slope stabilization in some slopes. However, almost cases, the forest surcharge was found to be virtually harmless to the slope stability even with severe seismic forces.
Key words:slope stability, forest load, forest felling, root effect, landslide
和文要旨
森林斜面の崩壊･地すべりに対する安定性や対策を考える際,環境面に与える影響を極力抑える目的で,森林を保全しながら斜面安定を図る手法,あるいは,森林を積極的に利用するｸﾞﾘｰﾝﾍﾞﾙﾄ構想のような手法が増加している。従って,森林の斜面安定に与える影響を評価することが重要となっており,根系の斜面補強効果などの研究に進展が見られる。すべり面がある程度深い場合などには,これら根系の影響は無視できるが,そのような場合に森林地上部荷重(樹木地上部荷重)が及ぼす影響についてはまとまった研究が少ない。ここでは,実斜面および実際の林分に関して,異なった林相･伐採条件を仮定した安定解析を用い,斜面安定に対する森林地上部の影響について検討した。
主な結果は次の3点である。①森林地上部荷重によるF s の減少は,最大でも無森林時の3～4%にとどまる。しかし,安全率があまり大きくない場合には,森林地上部荷重を無視すると,安全率を過大に見積もる可能性がある。②森林地上部荷重を取り除くことを目的とした森林伐採方式の差については,斜面上方半分(面積率50%)の伐採をすれば,100%伐採した場合と同じ斜面安定効果が生じるｹｰｽが見られた。③森林地上部荷重の影響は,無地震時と比べて地震時の方が小さくなった。
ｷｰﾜｰﾄﾞ:斜面安定,森林地上部荷重,伐採,根系,地すべり |