The Mushu-Iwado and Momokawa failure-type slides of the Joetsu region, Niigata Prefecture, Japan, developed upon diagenetic alteration zones within sedimentary soft rocks of the Nakanomata Mudstone Member of the upper Miocene Noudani Formation and the Tsutsuishi Alternation of Sandstone and Mudstone Member of the Pliocene Kawazume Formation, respectively. The sedimentary soft rocks consist primarily of mudstone, sandstone, conglomerate and volcaniclastic rocks.
Core samples of sedimentary soft rocks from the Mushu-lwado and Momokawa landsLide areas were leached in aerated conditions to measure pH and concentrations of SO₄^2-, HCO₃^-, Cl^-, Na⁺, K⁺, Ca²⁺, Mg²⁺etc.
The leachate pH of fresh rock ranges from 7.05 to 8. 90, while weakly, moderately and strongly weathered rocks record pH values of 6. 25 to 7. 22, 3. 01 to 6. 82, and 6. 20 to 6. 90, respectively. The leachate anions are predominantly SO₄^2-. The chemical characteristics of leachate are Na-SO₄ type in fresh, weakly weathered and strongly weathered rocks, and Ca-SO₄ type in moderately weathered rocks. Hexadiagrams of leachate from fresh, weakly weathered, moderately weathered and strongly weathered rocks reveal the chemical characteristics of each rock type. Na⁺ concentrations exceed Ca²⁺ in leachate from fresh rocks, but Ca²⁺ is dominant in leachate from weathered rocks.
Smectite and smectite layers of interstratified illite/smectite minerals change from Na-type through Na-Ca-Mg-type to Ca-Mg-type during the progressive chemical weathering of the sedimentary soft rocks.
During chemical weathering, the porosity of the sedimentary soft rocks increases by approximately 5 to 10%, while the bulk density of the rocks decreases by approximately 0.2 to 0.4g/ml, and shear strength is reduced. These changes result from the consumption of pyrite, plagioclase and other minerals during the chemical weathering process.
Exceptional care should therefore be taken in managing and preventing Tertiary-type landslides within sedimentary soft rocks, as they are an especially hazardous environment.

View full abstract

In this study, the new simplified discrete limit analysis technique for a three-dimensional slope stability problem is proposed, using the divided column as a three-dimensional element of RBSM (Rigid Body Spring Model). This approach is as easy as to use the slice method currently used in the conventional slope stability analysis and it helps to develop a planning of very effective slope protection work taking into account displacement of each column. In this model, a sliding surface constitutes a square, since a column is square. Then, the concept of the isoparametric element is introduced to define this surface. As a result of analyzing a three-dimensional slope stability problem using this model, we proved that the safety factor of a landslide can be calculated, such as a traditional approach, and it became possible to predict a motion of the whole body of the slope from displacement direction of each column.

View full abstract

Evaluation of the maximum resistance and corresponding pile-head displacement of a landslide prevention pile in arbitrary landslide mass, based on a few significant factors such as the geometrical shape of the landslide mass, the construction position of the pile and the mechanical properties of the ground, is very effective in design. Such a solution may be directly linked on a cost cut and safety disposition of landslide prevention projects. In this study, a parametric study of a simple landslide model based on the 3-dimensional elasto-plastic finite element method (FEM) is carried out. In each analytical case, the maximum resistance and corresponding pile-head displacement of a landslide prevention pile is estimated and a consideration about the relationship between those obtained values and various mechanical conditions is made. Moreover, some important indications are examined from comparison with the solutions based on the present design code.
Finally, in order to save the time cost spent on the 3-dimensional FEM, the neural network assembled based on the FEM results is built, and the idea for which such a method is used in business is proposed. Compared with the amount of work accompanying the analysis processing by FEM, execution of the neural network is much lighter. This induces the decisive difference at the time of examining the application possibility to a design. In this study, the possibility of development of the computer-aided design system of landslide prevention pile on network computing is shown.

View full abstract

Many types of mass movements frequently occur on volcanic edifices. The most dangerous is a gigantic sector-collapse with its following debris avalanche, as in the Mount St. Helens and Mt. Bandai cases. A total of 128 debris avalanches are described for 67 Quaternary volcanoes in Japan. Two-fifths of Japanese volcanoes show evidence of debris avalanches. Debris avalanches have occurred at the rate of one every 60 years during the last 500 years in Japan. Most are larger than 0.2km³. Deposits usually spread widely around the foot of volcanoes. One-third of debris avalanche deposits are accompanied by hummocky hills, which provide a good index of debris avalanches. Only 48 debris avalanches have obvious amphitheaters. Geological or geomorphological evidence indicates source positions of 65 debris avalanches. Gigantic sector-collapses tend to occur on the upper part and crest of volcanoes. Two-thirds of debris avalanches did not have a clear source area. Topographic features of amphitheaters are easily eroded or buried. The causes of destruction of volcanic bodies are not well known. Most height-to-length ratios (H/L) of debris avalanches range from 0.2 to 0.08 and indicate high mobility. Gigantic sector-collapses and debris avalanches are usual processes of destruction of volcanic edifices. Debris avalanches tend to occur repeatedly on active stratovolcanoes, which have the cone shape characteristic of Mt. Fuji. Debris avalanches on Quaternary volcanoes are usually larger than those in other geologic regions.

View full abstract