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Re: Monday Rambles - link to report
Author: OldPoleBurner
Date: 03-04-2010 - 10:08
The Garlock fault is certainly of no more concern than is the Hayward Fault in the East Bay. Indeed, a big one (7.0 to 8.0) is expected in the next thirty years along the Hayward Fault.
But BART bored two separate side by side single track base tunnels right through it anyways - 40 years and many quakes ago. Not only that, the Hayward fault in the area of the tunnels is in continuous motion, with the Oakland side moving about .125 to .25 inches per year northward in relation to the Orinda side. And indeed, as Graham Buxton suggests for the Garlock fault, BART did enlarge the bore in the Hayward fault zone, to account for fault movement.
Contrary to common understanding, an earthquake fault is not just one singular fracture in the rock, but is hundreds of fractures spread over several hundred feet. Thus a tunnel crossing a fault actually passes through a "fault zone", typically 500 to 1500 feet long. The shifting in the tunnel at the fault zone is not abrupt at all, but is instead a relatively gentle swerve. The re-enforced concrete tunnel lining is amazingly flexible, and just shifts around like a rubber hose.
Only in the area of the fault zone, and perhaps 1000-1500 feet on either side, needs an enlarged bore, to allow room for track adjustments, and for extra side clearances. The rest of the tunnel can be normal size.
The BART tunnel has served well for about forty years now, but sources tell me that it is expected to need serious maintenance in about 10 years; as all the extra room afforded by the oversize bore will have been about used up by then. That will involve enlarging the bore for about 3000 feet in each of the 4 mile long tunnels. Of course, by having two separate tunnels; as one bore is being enlarged, trains will still run in the other bore during the work.
BART's Orinda tunnel will then be good for another fifty years. Pretty good for BART! Pretty darn good anywhere else too!
OPB
But BART bored two separate side by side single track base tunnels right through it anyways - 40 years and many quakes ago. Not only that, the Hayward fault in the area of the tunnels is in continuous motion, with the Oakland side moving about .125 to .25 inches per year northward in relation to the Orinda side. And indeed, as Graham Buxton suggests for the Garlock fault, BART did enlarge the bore in the Hayward fault zone, to account for fault movement.
Contrary to common understanding, an earthquake fault is not just one singular fracture in the rock, but is hundreds of fractures spread over several hundred feet. Thus a tunnel crossing a fault actually passes through a "fault zone", typically 500 to 1500 feet long. The shifting in the tunnel at the fault zone is not abrupt at all, but is instead a relatively gentle swerve. The re-enforced concrete tunnel lining is amazingly flexible, and just shifts around like a rubber hose.
Only in the area of the fault zone, and perhaps 1000-1500 feet on either side, needs an enlarged bore, to allow room for track adjustments, and for extra side clearances. The rest of the tunnel can be normal size.
The BART tunnel has served well for about forty years now, but sources tell me that it is expected to need serious maintenance in about 10 years; as all the extra room afforded by the oversize bore will have been about used up by then. That will involve enlarging the bore for about 3000 feet in each of the 4 mile long tunnels. Of course, by having two separate tunnels; as one bore is being enlarged, trains will still run in the other bore during the work.
BART's Orinda tunnel will then be good for another fifty years. Pretty good for BART! Pretty darn good anywhere else too!
OPB
