Toyohama Tunnel: Questions and Answers
All of us are concerned about the February 10, 1996 collapse of the Toyohama Tunnel entrance near Sapporo, Japan. Our hearts go out to the families and friends of the people who perished in the terrible tragedy. Is this a cause for concern about the potential tunnel at Devil's Slide?
SOC/CATS has contacted individuals in Japan in order to understand more clearly the details of the situation. The following are answers to the most commonly asked questions we've encountered during our signature gathering efforts:
1. What happened in Japan?
On Saturday, February 10, 1996, a landslide occurred on a snowy mountain on the west coast of Hokkaido, Japan's northernmost main island. A part of the landslide was an immense boulder (approximately 230 feet long and 100 feet thick) which struck the Toyohama Tunnel where it enters the mountain. When the boulder hit the tunnel portal, a bus and at least one car were trapped in the tunnel and crushed. 22 people were killed in this disaster.
2. Won't this same thing happen to a tunnel built at Devil's Slide?
No, this type of event is very unlikely to occur at Devil's Slide for several reasons explained below. There are very distinct differences in geology, climate, and tunnel architecture.
3. How is the geology different?
The area of the Toyohama Tunnel is primarily volcanic rock. According to Professor T. Ui of Hokkaido University, it is water cooled lava rock (it is an active volcanic area) and a mixture of hard and soft material. It is quite fragile, and during landslides very large pieces of rock can come down the mountainside.
The geology of San Pedro Mountain, where the Devil's Slide Tunnel would be built, is a combination of folded and fractured sedimentary rocks and moderately fractured granite. There is no volcanic activity. The height from the possible portals to the top of the ridge is less than 400 feet, the slopes are thickly vegetated, and the tunnel would be sited away from the unstable surface areas of the mountain and away from any earthquake faults, all factors that would preclude the type of catastrophic landslide that fell upon the Toyohama Tunnel.
4. How is the climate different?
Hokkaido, the location of the Toyohama Tunnel, is Japan's northernmost main island and is subject to severe winter weather. Temperatures range from 72F in the summer to 23F in the winter. Snows last from November to March and reach depths of 3 meters (9 feet) in the vicinity of the tunnel (Sapporo, the capital of Hokkaido, was the site of the 1972 winter Olympics.)
The Devil's Slide area of the California coast lies in a temperate zone, often described as "Mediterranean". Annual temperatures range from 32F to less than 90F, but these extremes are rare along the coast, where the ocean moderates the temperatures. There is little precipitation except during the winter months. During the winter, rain can be quite heavy, but there is only an occasional dusting snow on the higher mountains inland.
5. How is the tunnel architecture different?
The most important architectural difference is that the Devil's Slide Tunnel would be completely covered by the mountain. Both the north and south portals would be flush with the side of the hill. At Toyohama, approximately 40 meters of the tunnel extended outside the mountain. The large rock, which was so difficult to remove, slid down the mountain and hit this exposed portion of the tunnel. Thus weakened, the concrete near the tunnel entrance collapsed and crushed the vehicles that were there.
The loss of life in the Toyohama accident is tragic, but it is important to know the facts of the situation, how they apply to us, and how we can prevent such occurences in the future. It is our concern for a safe, reliable and appropiate highway project at Devil's Slide that has motivated us to be sure that the alternatives are fully studied and understood; it has led us to launch this initiative for what we believe to be the safest, most reliable, and most appropiate solution: The Devil's Slide Tunnel.
Information compiled and written by Janet Prochazka and Chuck Kozak
