As I apparently haven’t posted about the Viaduct since May 12th, I guess it’s time for an update!
Some have suggested that the tunnel could be used later for a new transit corridor. I want to explain why that’s so unlikely – and it’s a simple explanation.
The viaduct deep bore tunnel would run from 60 to 200 feet underground. The points where it’s deepest are also where we need transit – under the center of downtown, where the endpoints of all the potential transit trips in such a tunnel would be. Unfortunately, these would also be where station platforms and entrances would be most expensive to construct.
More after the jump…
Given the cost of similar deep stations, such as Beacon Hill, Washington Park in Portland, and the projected costs for First Hill, building five or six stations to connect to the deep bore tunnel would probably cost more than building an entirely new shallow tunnel instead.
For Capitol Hill, for instance, the route has quite an s-curve before downtown. This is in part because it’s cheaper to make the tunnel longer, giving the trains more distance to climb, than to build a deeper station.
There’s also an issue of grade. Stations need to be relatively flat, but much of the tunnel is not, severely restricting where stations could be. Mitigating this would, again, cost a lot of money.
Finally, capacity. Westlake, University Street, International District, these all need high capacity entrances and exits. With a deep station, most (if not all) users would be entering the station by elevator. The four elevators at Beacon Hill probably wouldn’t have the capacity to handle the long-term needs of the downtown core. Again, more money for a larger excavation – and a lot more real estate cost.
Suffice it to say, a viaduct tunnel will never be usable for transit.