This study presents a numerical procedure by means of shakedown analysis to assess the seismic stability of slope based on the pseudo-static concept. The factor of safety was defined with the strength reduction coefficient as commonly employed in slope stability. The applicability of the numerical procedure was examined firstly to the static slope stability assessment through the comparison in factor of safety with the Taylor's friction circle method. It was clearly shown that the factors of safety of two methods were in good agreement and the proposed method assessed the static slope stability properly. Secondly, the seismic stability of slope was investigated in the frame of pseudo-static concept. The effect of earthquake load direction on the seismic stability was widely investigated for cohesive and frictional soils. The factor of safety obtained for the horizontal seismic coefficient was found to be close to the critical factor of safety in direction. Thirdly, the effect of repeated change in both direction and magnitude of earthquake load on the factor of safety was especially examined. It was found that the repeated change in earthquake load affected the seismic slope stability significantly, especially in the case of frictional soil. However, it was found that the factor of safety for the horizontal alternating load was close to that for the alternating load in every direction in the same way with the monotonic earthquake load.

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Seven ancient slide topographies in the Kanehana landslide area have been identified as concave polyhill- or concave monohill-type.
The geology of this area consists primarily of the Cretaceous-Paleocene Yubetsu Group, the Upper Miocene Ikutawara and Yahagi Formations, Late Miocene-Early Pliocene rhyolite, andesite and basalt dikes, Lower Pliocene Komatsuzawa and Hakugindai Lavas, and Quaternary System. The Quaternary System consists of the Rubeshibe Formation and pumice flow, river terrace, talus, slide, alluvial fan and recent fluvial deposits. The Ikutawara and Yahagi Formations consist primarily of clastic rocks, acidic volcaniclastic rocks and rhyolite lavas. The rocks of the Ikutawara and Yahagi Formations and rhyolite and andesite dikes have been affected to various extents by hydrothermal alteration related to Late Miocene-Early Pliocene epithermal goldsilver mineralization.
The hydrothermal alteration zones are divided into smectite, zeolite, propylitic, interstratified illite/smectite minerals, illite, K-feldspar, steam-heated alteration and advanced argillic alteration zones, based on the mineral assemblage of hydrothermally altered rocks. The seven ancient slides and the Kanehana-Toge slide occurred in the hydrothermal alteration zones, and were mainly related to the smectite zone.
The slope instability evaluation system for slides used here is based on topography, slope geology, geologi-cal structure and the type of hydrothermal alteration according to 1: 5, 000-scale topographic and geologic mapping of the area. Using this evaluation system slope areas can be classified into four ranks ; stable hard-rock slope (class I), stable soft-rock slope (class II), potential slide area (class III) and ancient slide area (class IV). Based on the evaluation, a slide hazard map of the area was created.

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We have developed a new resonance method which enables us to measure resonant frequency and consequently to obtain propagation velocity by imparting vibrations to samples. And by this new method we can also estimate the strength of the materials through the obtained propagation velocities.
Over the past twenty-two years we have conducted seveal researches, using this new method. In our research in 1988 into steel tower footings whose underground concrete foundations dangerously deteriorated in the course of many years, we diagnosed their conditions without conducting any digging or excavation. Since 1995 we have been engaged in the preservation and the restoration of Angkor Monuments and carried out diagnoses on the rocks and the concretes of the monuments by this method.
The present article is the report about the excellent results we have gained by applying our new method to the studies of the soil samples from landslides in the Tohoku district. By studying the relationship between propagation velocity and water content, we have found that the strength of the soil declines with its changing process from elastic body to plastic body. Our findings are applicable to landslide prediction and protection measures.

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[in Japanese]

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