This paper introduces a method for calculations of minimum total safety factors of slopes by means of the elasto-plastic FEM, considering tension cracks. Soils are assumed as elasto-plastic materials which obey the Mohr-Coulomb equation as the yield condition (or failure condition) and the Drucker-Prager equation as the flow rule.
The procedure of calculations is as follows. First, the shear strength of soils is assumed as τ_f=c'/F+σ'tanφ'/F. The initial value of F is taken so small that the initial state of a slope is almost elastic. Next the value of F is increased step by step, and finally the ultimate value of F where slope failure occurs, is determined as the minimum total safety factor of the slope. If large tensile stresses occur in the slope, the above procedure is repeated again after thin elements with small strength and low Young's modulus are inserted into the places where tension cracks are assumed to appear.
It is shown that the method presented here can take account of tension cracks as well as pore water pressures properly which always reduce the safety factor of slopes.

View full abstract

Some improvements on analytic procedure of “three dimensional tracking method” used for presumption of underground-slide-surface-geometry in landslide area is discussed. Compared with the previous procedure (cf. J. of JLS, Vol.26, No.3, 1989), main improvements are summarized as follows:
1)to debug the singular matrix appeared in normal equation derived by least-square-method,
2) to process arbitrary number of boundary-point-data mathematically, and
3) to consider positional error of the boundary-point-data and tracking-point-data.By these improvements, the procedure became simple and rational, thus the presumption reliable. Through computer simulation, precision of the presumption affected by selection of the boundary-points and tracking-points is studied. At the same time, influence of the positional error is examined.
Application of the improved procedure to actual landslide in Komoro district is demonstrated.

View full abstract

A large number of landslide track is topographically recognized in the southwestern district of Iwate prefecture, which is situated in the eastern part of the “Green Tuff region”. Understanding the conditions of landslide generation, geological features in the district have been investigated by comparing the distribution map of landslides with the distribution map of gentle slopes, the rostored contour map, and the geological map.Although geologic condition of the district is somewhat peculiar since it had suffered strong mineraliza-tion in addition to the “Green Tuff” alteration, follow ings have been clarified;
1) In the case that a flat surface exists on a summit or mountainside of the mountainous area, the flat comes into a recharge area of groundwater. Therefore, where is a slope of low gradient lower in altitude than the flat, the watertable under the slope is raised and become easy to generate landslide.
2) In the area in which a potential lifting zone exists, landslides are ready to generate because unstable slopes are formed along the zone.
3) In the area where faults or fault zones area are developed, landslides with a linear headscarp, instead of a crescent-shaped scarp, are formed owing to the effect of structural controle in the fractured Neogene strata.
4) Strate including layers of muddy rock and/or acidic tuff are ready to cause landslide, because the lalyers are easy to suffer argillization.
5) In the studied district, landslides are developed well especially in the area which had suffered deep weathering, geothermal alteration and mineralization, as compared with areas without alteration.

View full abstract

We have found a large-scale landslide in a hilly mountainous area of Kikonai Town, southern Hokkaido, by recent aerophotographs. Through previous aerophotographs of the landslide area, we can determine that the landslide occurred after June 23 and before July 27, 1988. Further investigation into the aerophotographs of ca. 1948, 1959, 1973, 1976, 1988 has revealed that the landslide has a historical movement related with cutting of needle-leaf forests (man-made cedar forest). In addition, we have researched the landslide in the field and analyzed the landslide volume in comparison between the maps made through aerophotographs in 1959 and those in 1988. As the results, the landslide is geologically dip-slipping within a clayey rhyolitic tuff and was triggered by upheaval of groundwater level in the old landslide, followed by considerable high precipitaion in June and July, 1988.

View full abstract

The relationships between the differential erosion topography and the slope failures on the Mt. Kano-zan and its environs in the Boso peninsula, Japan have been discussed. The results are as follows.
The geology consists of sedimentary rock formations can divided into two types. First one is sandstone rich formation type and the other type is mudstone rich formation. The former area is formed high relief, low slope angle and low valley density. The later is formed low relief, high slope angle and high valley density. It is considered that the main facter is based on the low potential of erosion derived from decreasing surface stream which caused on permeability of unconsolidated sandstone.
The phenomena of slope failures are main agency of the processes beneath the surface layer on the mountain slope of the Kano-zan and its environs. The distribution of the slope failures are consolidated on the mountains which composed of low relief, high slope angle and valley density. This is very important phenomena of the main agency of the aifferetial erosion topography in the humid temperature area.

View full abstract