Neogene pyroclastic rock widely distributed in coastal areas of the Sea of Japan in Hokkaido has frequent occurrence of slope failure. This report aims to elucidate the failure mechanisms from geotechnical perspectives, using two typical examples (Toyohama and Daini-Shiraito failure). Toyohama failure took place in February 1996 on a scale of 11,000m3. It is believed that the major predisposing cause was the distribution of strata in dip slopes with the structure that makes it easy to provide surface water to highly permeable bedding planes from the rear side. The failure is believed to have been directly triggered by the freezing of spring water on rock surfaces below freezing temperatures, causing groundwater, whose outlets were blocked, to apply high pore water pressure to bedding planes and rock discontinuity such as sheeting joint and fissure, resulting in crack expansion. Daini-Shiraito failure occurred in August 1997 on a scale of 56,000m3. Accumulated rainfall over the three weeks prior to the failure reached 290mm. Geologically, heavily altered strata, distributed in dip slopes at the lower part of collapsed rock masses, were in wet conditions under which groundwater was easily provided, and the rock strength is believed to have deteriorated significantly. As a result, severe rainstorms weakened the strength of rock masses at the lower part and widened cracks, leading to the failure.
Large-scale slope failures occurred frequently in Neogene pyroclastic rock areas along the Sea of Japan in Hokkaido (Fig. 1, Table 1). Relations between scales of failures and geological features in these areas indicate that the maximum failure scales for lava and dyke are small in the order of 103 m3 whereas those for massive and bedded pyroclastic rock reach the order of 104 and 105 m3, respectively (Table 2). This is presumably because failures of lava and dyke are small in scale due to the dense distribution of joints and other separated planes, whereas largescale failures also occur in pyroclastic rock due to the rough development of separated planes. Geotechnical mechanisms for Toyohama and Daini-Shiraito failures (6 & 7 in Table 1) are detailed from geotechnical perspectives below.
The failure occurred on February 10, 1996. It blocked Toyohama Tunnel entrance and claimed the lives of 20 passengers aboard a passing bus. The scale of the failure is 60m in slope length, 50m in maximum width, 13m in maximum thickness approximately, and almost 11,000m3 in volume. The failure occurred in cloudy weather with occasional snow flurries and the outside air temperature was −5_, normal for that time of year. It should be noted, however, an unusual big chill (below −10_) was observed about 10 days prior to the failure, and at the time of the accident the snow cover was about 150cm, far deeper than the average figure (100cm) for that time of year. Although an earthquake of M7.8 occurred in 1993, its correlations with the failure remain unknown.