Abstract
In Recent Hokkaido, two giant rockfalls destroyed tunnels of highways of Route No.229 running on the coast facing Sea of Japan. The first rockfall occurred on Toyohama Tunnel in Furubira Town on February 10, 1996 (Toyohama Tunnel Rockfall). The rockfall resulted into killing 20 persons mostly in a bus which was just passing through the tunnel. The second rockfall occurred on the 2nd Shiraito Tunnel on August 25, 1997 (2nd Shiraito Tunnel Rockfall), followed by rockfall on August 28. We describe the two rockfalls and compared them briefly, added by Echizen Beach Rockfall which occurred in 1989. Both of the rockfalls in Hokkaido took place from steep sea-cliffs of Neogene andesitic hyaloclastite. The Toyohama Tunnel Rockfall occurred from 70 m high and the 2nd Shiraito Tunnel Rockfall from 160 m high. The total volume of the former and latter is 11, 000 m3 and 65, 000 m3, respectively. The former occurred in mid-winter, whereas, the latter in mid-summer. However, for the both rockfalls, geological discontinuity is important, and their triggers related to groundwater discharge in common.
Finally, we tried to do image processing analyses of the data of the rockfall scarp and fallen debris in order to clear the mechanism of the 2nd Shiraito Tunnel Rockfall. As the results, we have obtained a computer simulation model such the following process as crack opening-toppling-slight sliding-falling-toppling down -slidingdispersion. The process is complex compared with a simple fall of the Toyohama Tunnel Rockfall.
