Recently when I was in Hong Kong, I found out about a train that goes underwater from Hong Kong island to Kowloon peninsula. Likewise, living in the San Francisco Bay Area, the BART (Bay Area Rapid Transit) has a popular commuting train option that goes from San Francisco to Oakland through a tunnel placed under the bay—a distance of 6 miles when including the approaches from each station. Having lived in the Bay Area the last 9 years, surprisingly, I have never taken that train ride and for that matter, never taken a train trip through an underwater tunnel any where in the world. I’ve been wondering 2 things about these trains—1) what does it feel like to be on a train underwater and 2) how do they build such tunnels underwater in the first place.
This weekend I took my first BART ride from Oakland to San Francisco. I also discovered the way the BART tunnel was built and in general, the different methods of building underwater tunnels.
Riding the BART from Oakland to San Francisco
The tunnel is called the Transbay Tube and actually sits on the floor of the bay (vs being underneath the floor). It is surrounded by sand and gravel to keep the tunnel stable. As I expected, you can’t tell that you are riding the train on the floor of they bay, completely under the water. Looking out the windows of the train, all you see is black—no sci-fi images of seeing water through windows in the tunnel. The one thing that I did experience that told me it wasn’t a normal train ride was that my ears begun to plug, much like when one is in an airplane, or higher elevation. I can’t remember too many times when I was at lower elevations such as in a cave; but I suspect the ears would have similar experience there.
How was the BART (and other underwater tunnels) built?
As documented in Wikipedia, The tube is made of 57 individual sections that were built on land and towed out into the bay by a barge. They were then positioned above where they were to sit and lowered into a trench packed with soft soil, mud and gravel for leveling along the bay’s bottom. Once the sections were in place, bulkheads at each end of each of the sections were removed and a protective layer of sand and gravel was packed against the sides. I’m not sure exactly when it got lowered into the water, how it got placed precisely where it needed to be. I suspect several divers as well as advanced equipment was used to make that happen. It cost approximately $180 million in 1970 to build. More history on the construction of the entire BART system is available at BART’s official website. This is truly considered an engineering marvel.
Other ways to build an underwater tunnel include digging a tunnel far enough below the sea bottom that the rock over them keeps out the water. If the rock or soil isn’t able to keep out all the water while the tunnel is being dug, it’s possible to pump compressed air into the tunnel, so the pressure inside the tunnel is greater than the water pressure outside, and have workers seal the inside of the tunnel with a concrete or steel lining. (via Yahoo! Answers).
Here is a video describing the actual construction of the Transbay Tube
Vodpod videos no longer available.