Focusing on the most frequent angle of landslide slope, in this study we examined the engineering features of Tertiary formations along the Japan Sea in the eastern part of Shimane prefecture, based on 50-m DEM (Digital Elevation Model) topographical analysis. Our main conclusions are summarized as follows: (1) In the study area, the distribution and angles of slopes composed of stratum in the Iwami and Izumo groups can be affected by the direction of the slope. However, the most frequent angle of the slopes is not related to the slope direction. (2) The most frequent angle of landslide slopes is very similar to that of natural slope. (3) The most frequent angle of landslide slopes tends to decrease with an increase in stratum order. (4) For the stratum of the Iwami groups, the landslide slopes were distributed more often in the areas in which small slope angles were more prevalent. The most frequent angle of the landslide slope showed a particular value that was dependent on the kind of stratum. Therefore, the most frequent angle of the landslide slope can be a useful index for estimating the strength parameter of a landslide's slip surface.
A landslide is a three-dimensional movement of a mass of earth and rocks, involving a limited length and width of the mass. Stability of a landslide can be discussed more comprehensively if it is rendered three-dimensionally. Simplified three-dimensional stability analysis is a quasi three-dimensional method using the expanded results of stability calculation of two-dimensional cross-sections for the evaluation of landslide stability. This method can be executed by using existing spreadsheet and two-dimensional slope stability calculation software. This paper describes the geometric modeling scheme of the simplified three-dimensional stability analysis method and also discusses a series of analysis procedures on the basis of example analyses. The results of the analyses have compared favorably with the results of the methods proposed by Hovland (1977) and J. Erik Loehr (2004) . The authors believe that the simplified three-dimensional stability analysis method can be used to estimate the magnitude of three-dimensional effects when no other three-dimensional method is applicable, though the calculation estimates approximate values.
Rock specimens consisting of smectite-bearing fine tuffs of the Upper Miocene Ikutawara Formation from the Ikutahara-Minami landslide area in Engaru Town, northeastern Hokkaido, Japan, were analyzed. The rocks belong to a hydrothermal clinoptilolite zone, and consist of clinoptilolite and minor quartz, feldspar, smectite and pyrite phases. The rocks are divided into two groups on the basis of their smectite content; approximately 7 wt% and very small amounts of smectite-bearing fine tuff. For rock core specimens perpendicular and parallel to the lamina plane, point load (axial test) and uniaxial compression tests were performed using laboratory testing equipment with specimens under a forced dry-state, dried in an electric oven at 60 ± 3 °C for1day, and a forced wet-state, saturated with distilled water for 10 days. For these rock specimens from the boundary area of soft and semi-hard rocks, comparative tests revealed that the smectite content influences the point load and uniaxial compression strengths, according to discrepancies in the coefficient of variation of both strengths. The uniaxial compression strength is approximately 20.0-22.6 times as much as the point load strength under the forced dry-state, and approximately 10.8-11.4 times that under the forced wet-state.
The Typhoon Meari (T0421) passed through northern Mie Prefecture September 28 to 29, 2004. Many landslides and slope failures occurred by the heavy rainfall during the Typhoon in Miyagawa village. Using aerial photography, we examined 233 landslides and slope failures from the points of geology, totalr ainfall, and total rainfall intensity-most were slope failures. The frequency distribution of landslides and slope failures in the Sanbagawa and Chichibu belts were not significant. It was clear that a larger number of slope failures occurred in the areas where the hourly rainfall was in the 110-120-mm range. However the slope failures were not dependent on accumulated rainfall. In the Sanbagawa belt, many slope failures were generated in dip slope areas. While in the Chichibu belt many of the slope failures were generated in reverse dip slope areas. The heavy rainfall and weathering were generally restricted to the surface, so slope failures occurred rather than landslides.