F-Line to Dudley
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How far down would we have to go to do WMATA or even Copenhagen style tunnels?
Probably not comparable at all. Most of D.C. sits on hard rock outcrop at the heavily eroded shoreline base of the Appalacians. The only silt mush is in the immediate Potomac drainage area. The whole capital region is basically an ex-mountain range sticking into the Atlantic that was ground completely flat by the glaciers. Instead of a lumpy debris field like Boston where the glaciers just pushed a lot of refuse from higher elevations into the water...and then the humans did more of the same over a 250-year span.
The Big Dig didn't really make much use of traditional TBM's either. That was mostly hella deep cut-and-cover, some faux boring through ground around SS that was artificially cryogenically frozen to give it more hardness, and sunk precast tunnel sections for the Ted and the Pike tunnel under the Ft. Point Channel. To the extent that they'd be boring anything for the Link, it's more like a rotary dirt scoop to excavate the space under 93 and maybe more of that freeze-dried business around SS where things have to weave above/below other tunnels.
The last time anything resembling traditional bedrock boring happened was the Porter-Davis Red Line tunnel, which is the only reason it was possible to go so deep and travel inocuously under property lines. North Cambridge is a considerably higher-elevation pre-glacier rock outcrop. I don't think there's any place in Boston-proper comparable to that unless you were burrowing the Red Line from Mattapan Sq. under property lines to get onto the Fairmount ROW. The Neponset watershed around Mattapan-Milton-Hyde Park is where the hard granite deposits are.
Beyond that, just about everywhere has to be cut-and-cover and reinforcement of the mush for any underground structures including building pilings.
Some reading about Boston geology I found on 'th Google: http://written-in-stone-seen-throug...chitectural-geology-of-boston-roxbury_27.html.
I don't understand half the technical jargon, but the maps are instructive. You can see on the overhead map that little green toothpick...it's an ancient hard volcanic feature stuck in the middle of mud on all sides. That is precisely what they bored the Porter-Davis Red Line through. The basin in pale tan is all the tidewater mush...that's all of downtown, the Charles watershed, most of Cambridge and Somerville, Watertown, Everett, Chelsea, Eastie. All recent in origin. Then the speckled tan in Brookline, Roxbury, and Dorchester is somewhat rockier sediment (boulders and crap chucked in this direction off the Worcester Hills)...also recent glacial in origin, but more of a crushed pebble/boulder debris field than the tidewater mush. Then the dividing line at around Mattapan Sq between the glacial deposit and the hard-ass and very ancient volcanic and flattened-mountain rock in Dedham and Milton.
Then you can see on the cross-section map how the mush is pretty much like a lake sitting on top of firmer rock..."shoreline" bordered by the hard volcanic rock and that Porter-Davis toothpick sticking vertically upright. Then the black-and-white map showing the ages of it all, with the glacial mush all being very recent compared to the rest, the little volcanic dikes subdividing the mush in Cambridge and around Beacon Hill like a fence, and then the hard-ass Neponset granite being well over half a billion years old.
It pretty much confirms everything we know...the Central Business District is some soft, silty mush bordered by lots of lumpy mush. And it really doesn't get any better until you're at the city limits and the ring of inside-128 inner 'burbs.