Extension of concealed fractures: Why did a collapse of the sea-cliff at the western entrance of the Toyohama Tunnel, Hokkaido, Japan occur?
A 11,000m3 rock mass, which was peeled off from the sea-cliff above the western entrance of the Toyohama Tunnel by a deep fracture plane trending NE to EW, slid/fell down and destroyed the tunnel. The deep fracture had been concealed by the collapsed rock mass. The fracture had been partly initiated probably by release of stress in the high and steep cliff; formation of incipient sheeting jont or nectectonoc joint. The extension of the fracture, however, is considered to be accelerated by water supply, which leads to inceases of interstitial water pressure during winter, freezing-thawing process, and rapid change of strain of the rocks in the temperature below zero, make the rocks weaken. The surface structure of the cliff-wall after the collapse suggests that localized, subparallel fractures extended until finally they were connected, forming a continuous deep fracture with a nearly vertical dip.