Quote:
Originally Posted by OhioGuy
On the Georgetown side, it could enter into a hillside tunnel, thereby eliminating the need to go extra deep to get across the river and therefore avoid having overly deep stations in Georgetown.
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I'm not qualified to speak extensively about geology or construction technique, but a few comments:
- huge Metro trains can't take L-shaped curves like that, certainly not quietly
- that L adds a lot of additional track length
- apparently, much of the cost of boring tunnels is getting the TBM on site; once it's there, the marginal cost of tunneling declines significantly
- similarly, a deep tunnel vs. a shallow tunnel doesn't save much money unless you're doing cut & cover, which will not happen in Georgetown
- the Rosslyn station will have to be pretty deep anyhow
The Red Line's Connecticut Avenue line was to have a Bloor-like* bridge across Rock Creek, but the NPS vetoed that.
* Toronto's shallow, east-west Bloor subway crosses the deep Don Valley ravine on a bridge whose lower deck actually predated the subway.
http://transit.toronto.on.ca/subway/5104.shtml