Yoshiki WATANABE

Study on seismic stability method of earth structures based on inversed analysis of collapsed slopes in 2004 Mid-Niigata Prefecture Earthquake

Satoru OHTSUKA

Seismic design method of infrastructures has been modified in Japan since the disaster of South-Hyogo Prefecture Earthquake in 1995. However, it has not been established for earth structures and it is left for the further subject. In this study, case studies of collapsed and non-collapsed slopes of fill at Takamachi housing complex and of rock at Yokowatashi were conducted in order to investigate the applicability of seismic stability method of earth structures, especially on the relationship between the seismic intensity coefficient in design and the actual earthquake wave.
At Takamachi housing complex, four cases of collapsed slopes and two cases of non-collapsed slopes were analyzed. The factors of static safety were obtained from 2.1 to 3.0, and the seismic intensity coefficients were obtained as 0.33 to 0.42. From the site observation collapsed fill was seen to include much amount of water. The seismic intensity coefficient was analyzed with using the repeated strength of Takamachi fill conducted by Yoshii(2007). It was obtained as about 0.2 in case of fully saturation condition at the boundary between fill and natural slope. The Mid-Niigata Prefecture Earthquake is well known as the complex disaster of earthquake and rainfall. However, this result indicated the fills were possible to fail in spite of low ground water level.
At Yokowatashi two cases of collapsed slopes and one case of non-collapsed slope were analyzed. Since they failed along the thin tuffaceous sand layer, the soil at slip line was directly sampled and tested. The mechanical constants were obtained as the angle of shear strength, 37.8 degrees and the cohesion as 0 kN/m2. Since the source of groundwater supply for slip line did not exist from geometrical condition and the water conductivity coefficient of thin tuffaceous sand layer was higher, the effect of groundwater was not considered in the stability analysis. The factor of static safety was obtained from 2.4 to 2.7 and the seismic intensity coefficients as 0.38 to 0.4 for collapsed slopes. On the other hand, the factor of safety was obtained as 3.2 and the seismic intensity coefficient as 0.45 for non-collapsed slope. This concluded that the earthquake motion observed at Myouken was equivalent to the seismic intensity coefficient of 0.4 in conventional seismic stability method

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