Preliminary Survey for the Toyohama Tunnel Repair Work

The rock-bed falling accident of the Toyohama Tunnel of national highway Route 229 in Furuhira-cho, Hokkaido which took place in February of this year 1996 resulted in a tragedy of killing 20 people. It is said that the accident was caused by the progress of weathering due to the ground water penetrating into cracks made in the slopes. In addition to the visual survey, a survey using TVS was conducted to obtain data to be used as a reference for the repair of the Toyohama Tunnel. Photo 1 is a visible image of the fallen site. Photo 2 is the same scene taken by TVS. This thermal image explains that the remainder of the fallen rock-bed is the low temperature portion containing moisture and that the ground water is affecting these areas. In the severe natural environment of Hokkaido, the moisture containing portions go through repeated cycle of freezing and melting. As a result, cracks are made and expanded, and the probability for eventual falling is high. For such reason, they are considering to blast the remaining portion with dynamite before starting the repair work.
These days, people think that it is desirable to continue survey of various buildings and constructions, including tunnels, visually and by using various equipment, such as the infrared imaging equipment, from immediately after completion of such buildings or constructions.

Photo 1 and 2

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