 |
Re: The Last SP Train #75, The Lark April 8, 1968 at LA
Author: OldPoleBurner
Date: 01-03-2010 - 21:54
Actually, I believe the earhquake issue is way over-rated. I hope my explaination as to why, is not too off topic.
I know this concern has been mentioned here several times, without any opposition. But from an engineering standpoint, it is very manageable. If fact, no matter where you put the track in California, the difference in seismic risks won't be enough to matter much. Full seismic safety must be engineered in anyway - no matter what route you take.
Indeed, California has two tunnel structures that have already performed very well in earthquakes, both on BART.
The Transbay tube passes directly through a serious seismic risk zone, in that it passes under the bay in a fairly loose and muddy sand and gravel mix; right through a focal point of the parabolic bedrock formation, that allowed the bay to be created in the first place. Such places greatly amplify the shaking beyond that experienced elsewhere - even at the fault line.
Indeed, this particular focusing phenomenon was responsible for the damage to the Bay Bridge in the '89 quake. The bridge also was embedded in the same bay mud near the parabolic focal point. Despite this very harsh and unstable environment, the tube has performed flawlessly over the years during several major quakes in the 5.5 to 7.1 range. It was designed for up to a 9.x quake.
BART's base tunnel through the Berkeley Hills is another example. The Hayward Fault is also considered to be a very dangerous fault. About 3/4 of a mile in from the west portal, the BART tunnel passes directly though a fault zone of fractured rock and rubble several hundred feet thick. This tunnel has also endured nearly forty years, all nearly flawlessly. And is still very safe to be in, even during an earthquake.
However, I did say nearly flawless. My sources tell me it has one issue. That of the track shifting in the fault zone. The west side of the fault is shifting to the north a fraction of an inch per year. Fortunately, the shift is not concentrated at a precise point, but is spread over the rather thick fault zone. But after nearly 40 years of shifting, speed reductions from 79 mph down to 50mph have been required through the fault zone. As you ride through, you can still feel the shift in the tracks. But it is rather subtle, due to the reduced speed.
It is expected that enlarging each bore in the fault zone will be necessary within 5 to 10 years, to compensate. But hey, 50 years before major repairs are needed is still very tolerable.
Note that tracks crossing an earthquake fault at the surface, also present the exact same problems. But it is usually much easier to fix on the surface, than in a tunnel, which eventually may have to be enlarged in order to properly realign the track.
All in All, it is a quite manageable engineering challenge, and well worth it where needed to cross a mountain range.
OPB
I know this concern has been mentioned here several times, without any opposition. But from an engineering standpoint, it is very manageable. If fact, no matter where you put the track in California, the difference in seismic risks won't be enough to matter much. Full seismic safety must be engineered in anyway - no matter what route you take.
Indeed, California has two tunnel structures that have already performed very well in earthquakes, both on BART.
The Transbay tube passes directly through a serious seismic risk zone, in that it passes under the bay in a fairly loose and muddy sand and gravel mix; right through a focal point of the parabolic bedrock formation, that allowed the bay to be created in the first place. Such places greatly amplify the shaking beyond that experienced elsewhere - even at the fault line.
Indeed, this particular focusing phenomenon was responsible for the damage to the Bay Bridge in the '89 quake. The bridge also was embedded in the same bay mud near the parabolic focal point. Despite this very harsh and unstable environment, the tube has performed flawlessly over the years during several major quakes in the 5.5 to 7.1 range. It was designed for up to a 9.x quake.
BART's base tunnel through the Berkeley Hills is another example. The Hayward Fault is also considered to be a very dangerous fault. About 3/4 of a mile in from the west portal, the BART tunnel passes directly though a fault zone of fractured rock and rubble several hundred feet thick. This tunnel has also endured nearly forty years, all nearly flawlessly. And is still very safe to be in, even during an earthquake.
However, I did say nearly flawless. My sources tell me it has one issue. That of the track shifting in the fault zone. The west side of the fault is shifting to the north a fraction of an inch per year. Fortunately, the shift is not concentrated at a precise point, but is spread over the rather thick fault zone. But after nearly 40 years of shifting, speed reductions from 79 mph down to 50mph have been required through the fault zone. As you ride through, you can still feel the shift in the tracks. But it is rather subtle, due to the reduced speed.
It is expected that enlarging each bore in the fault zone will be necessary within 5 to 10 years, to compensate. But hey, 50 years before major repairs are needed is still very tolerable.
Note that tracks crossing an earthquake fault at the surface, also present the exact same problems. But it is usually much easier to fix on the surface, than in a tunnel, which eventually may have to be enlarged in order to properly realign the track.
All in All, it is a quite manageable engineering challenge, and well worth it where needed to cross a mountain range.
OPB
