Abstract
The large rock block failure had been occurred on the rock slope above the west side entrance of Toyohama Tunnel, Route 229, located in the west coast of Hokkaido, Japan, at about 8 : 10 a. m. on 10 February 1995. The mechanism of the rock block failure is discussed with several facts concerning geotechnical characteristics of the field and natural conditions around the field, especially the seasonal change of groundwater behavior, based on the results of field observations, field tests, laboratory tests and field measurements. In conclusion, the main factor of the rock block failure is considered as what the hydro-mechanical pressure, occurred by the phenomenon that groundwater had been sealed by the ice lens on the slope under extreme low temperature condition, has been applied to the discontinuity behind the rock block.
