Geology of a collapse of the sea-cliff at the western entrance of the Toyohama Tunnel, Hokkaido, Japan

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On 10 February 1996,a 11,000m^3 rock mass peeled off from the sea-cliff above the western entrance of the Toyohama Tunnel by a deep fracture plane trending NE to EW, fell down and destroyed the tunnel. Twenty people were killed in this incident. The sea-cliff, more than 150m high, is composed of Middle Miocene hyaloclastic rocks (the On'nenai Formation) grouped into three units - lower, middle, and upper. The lower and upper units are reworked hyaloclastite. The middle one is dacitic hyaloclastite, including abundant juvenile magmatic materials. Groundwater outlets are distributed along the unit boundaries, the main sources of the water supply having developed along the boundary between the middle and upper units at intervals of ca. 20m. A study of the surface structure of the cliff after its collapse revealed highly weathered wall, indicating that pre-collapse fractures had developed in the northeastern (left) side, uppermost part and western (right) side of the cliff. The northeastern side is the most widely weathered zone. Its right side is bounded by a distinct fracture, called Fr-7. The main portion of the collapsed rock-mass was peeled off from the right side of Fr-7. The collapse mechanism on the right side of Fr-7 is therefore most significant for considering the cause of the collapse. A main water source, which forms long icicles during the winter, is located to the right side of Fr-7. The surface structure of the right side of Fr-7 suggests that localized, subparallel fractures extended until finally they were connected, forming a continuous deep fracture with a nearly vertical dip. Interstitial water pressure and the freezing of water, as well as stress release, are assumed to have contributed to the extension of the fractures.
日本自然災害学会の論文

Geology of a collapse of the sea-cliff at the western entrance of the Toyohama Tunnel, Hokkaido, Japan

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