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.

View full 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.

View full 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.

View full 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.

View full 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 inslope 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.

View full 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 displacemen 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_s=1. Tree root functions like reinforcement material and improves factor of safety of forested slope, F_s≥1. However, target factor of safety (also called design factor of safety Fds) of a forested slope must be equal to or higher than a standard value (such as Fd_s= 1.2 in this paper). Magnitude of F_ds 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.

View full 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.

View full abstract