The effect of the antecedent precipitation is pointed out as a reason of geo-disaster in the previous great earthquakes such as 2004 Niigata-ken Chuetsu Earthquake. However, the effect of the rainfall is not considered in the seismic guidelines. In other words, the change of the geomaterial characteristics due to a rainfall is not considered. In this study, seismic FEM analysis method of a slope to consider the effect of the antecedent precipitation is proposed. We introduce the simulation results of two case histories; one is the shaking table test and the other is a damaged embankment in field. The proposed analysis method is useful for the performance evaluation of a slope/embankment considering the combination effect of a rainfall and an earthquake.

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A slope failure occurred at an embankment of Tomei Expressway in the earthquake at Suruga-bay, in 2009. The impact of the disaster to the society was so large that the emergency check-ups for embankment slopes were conducted. The target of the check-up are slopes for the embankment with 10m height or more, located at the mouth of drainage basin and reclaimed with material possibly deteriorated by slaking phenomenon. However, there is no clear standard for the definition of drainage basin, and no reasons for the limitation to the embankment with 10m height or more.
In this research, the effects of topological condition for slope failure were statistically analyzed based on the case histories of Noto Toll Road damaged in 2007 Noto-Hanto Earthquake. The application of the orderd-probit model clarified which kind of topological condition is important to assess the seismic risk of slope failures of embankment.

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Yokowatasi landslide was a translational rock slide on a monoclinal dip-slope at 25 degrees in geologic structure. It was induced by the combination of the 2004 Niigata-ken Chuetsu earthquake (MJMA: 6. 8) and the preceding precipitation, three days prior to the earthquake, brought by the typhoon No. 23. The slope of the landslide site mainly consists of Pliocene massive sandy siltstone layers intercalated by a thin sensitive seam of tuffaceous sandstone. It developed into slide surface of the landslide during the earthquake. We found in the slightly moved siltstone block remained at the north side flank of the landslide, many narrow sand dykes connected with the seam of tuffaceous sandstone. The seam contains 90% of pumice with very low compressive strength. lt is presumed that the structure of the seam was destroyed by the earthquake stress, then the overlying siltstone block started to move and that excess pore water pressure was immediately produced and increased in the seam. This procedure generated high velocity of the landslide. We calculated the velocities of the sliding as 10m/sec in average and 20m/sec at the maximum. More than four seams of tuffaceous sandstone including the seam were found around Yokowatashi area through our geological survey. Those seams indicate evidences of former occurrences of landslides and development of cuesta topography in this area.

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The M7. 6 earthquake occurred in West Sumatra, Indonesia on 30th Sept. 2009 triggered many long runout landslides in Padang Pariaman and Agam districts and caused numerous fatalities. It is found that long runout landslides occurred on steep slopes mantled by a pumice layer. A stiff clay layer with low permeability was present beneath the pumice layer. The most important factor was that it had been raining for three hours before the earthquake occurred, although the rainfall was of medium intensity. Laboratory tests on the pumice sample with ring shear apparatus showed that the pumice layer could be liquefied by earthquake triggering under fully saturated and undrained conditions. This study shows the worst scenario of the combination of rainfall and earthquake in the distribution area of the pumice.

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