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.

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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 gradi-ent 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.

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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.

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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.

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Many landslides occurred during the Chi-chi earthquake (M_L=7.3) on 21 September 1999 in western central Taiwan. These land-slides 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.

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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.

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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.

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