Huh? There is a provisioned space below the CA/T for the N-S Link. That's the way to do it.
Yes. When they sank the slurry walls to frame the CA/T tunnel those walls extend twice as deep as the highway tunnel to support it all through mushy landfill and waterfront silt. There's no bedrock to speak of so the walls had to go impossibly deep. But the space between the walls below the highway was excavated all the way to the bottom, then re-filled when they built the tunnel. So it is not an empty space per se, just fresh clean dirt 100% clear of utilities and other impacts that they can easily scoop back out. That wasn't intentionally done for the Link...it was simply a happy circumstance of the only way they could physically build the CA/T was using that novel slurry wall method and going hella deep with the walls.
To add the Link tunnel underneath all they would have to do is re-scoop the fill below 93 between those slurry walls, lay a new tunnel floor, and pour a new tunnel lining all around. The Link tunnel would not have the same waterproofing issues plaguing the CA/T tunnel because tracks are far narrower than highway lanes + shoulders and they would be able to do a regular-thickness tunnel lining. In the current tunnel the bare slurry walls
are the tunnel walls and they have no double-thick lining, because that was the only way to fit a wider highway in the same footprint as the old surface artery. It's a 'feature', not a bug, that the walls leak. They're supposed to and are mitigated from it because there's no waterproofing lining. The thing they didn't anticipate was just
how much it was going to leak.
Most of the project expense is tied up everywhere
except the actual tunnel. On a minimum build they would need:
-- 2 tracks in the main tunnel under 93. The way that tunnel would be built is 1 bore per each 2 tracks. And if they build it 4 tracks there would be 2 bores with a thick wall between them to support I-93 above and add an extra waterproofing layer. Would cover the same space as the 93 tunnel but have much narrower ROW in up to 2 tunnel bores because of all the extra wall lining the 93 level doesn't have. At minimum they could just build 1 bore and leave the rest of the space empty, but they'd probably want to spring for both bores to get 4 tracks since they only have one shot at this. Even if the second bore remains vacant for several years until they get more money to lay the extra tracks.
-- South Station approach. Requires deep-bore tunneling under Dot Ave. and the Ft. Point Channel retaining wall from roughly the Pike ventilation building to Northern Ave. where it would join under 93 where the CA/T makes it due-north curve. That is tricky because there's a very locked and precise lateral trajectory they have to weave through to get between the levels of the Pike, Red Line, Silver Line, that cross it above/below. And they have to dance around the Independence Wharf building pilings by jutting a few feet out underneath the Channel. Most of this will be deep underground so the surface impacts aren't going to be major. Going on the other side under Atlantic Ave. is theoretically feasible but way too invasive to building pilings and has to dance around the 93 exit tunnels. So there's pretty much only one way they can build it.
-- SS lead tunnels. At bare minimum they have to build a mile-long, 2-track curving tunnel from the Pike vent building to the NEC spitting to the surface where the NEC and Worcester Line split. This is also locked into a precise trajectory because it must portal to the NEC by the Washington/Shawmut block to avoid the Orange Line tunnel that crosses and portals on the Shawmut/Tremont block. It must avoid the Pike tunnel and associated ramps of that interchange, and must avoid the pilings for the Pike vent building where it joins the SS/Dot Ave. segment. There is literally only one way they can build this. And they can never ever extend it to an expanded "Back Bay Under" because the Orange Line tunnel blocks further extension.
-- South Station Under and North Station Under. The SS underground station is offset from the main station underneath Dot Ave. Need 8 tracks worth of 1000 ft. platform space 100+ ft. underground, concourses interfacing with the surface, and to underpin the corner pilings of the office tower on Summer/Dot Ave. Plus I imagine they want this to
look like an opulent downtown union station, so it'll be a lot more than a utilitarian bunker. NS Under would have the same track layout, but would be more or less underneath the current surface platforms so it's not as expensive building a cavern there.
-- NS approach. Deviates from 93 at about N. Washington St. where the CA/T starts inclining to the surface, slides along northeast-facing side of the Garden. Much cheaper and less invasive than the SS approach. Tunnel to surface goes more or less under the current tracks, spitting up on the surface where the NH Main/Lowell Line and Eastern/Western Route tracks diverge. Since this is all under barren T-owned land with nothing but tracks, minimally invasive and not real expensive. And it goes pretty deep under the Charles. At minimum the NH Main/Eastern/Western portal has to be built.
That's just the minimum build. And would probably cost $6-8B. They
want to also do this:
-- Central Station. A 6-track cavern with 800 ft. platforms at Aquarium and Blue Line transfer that's going to be in the deepest part of the tunnel on a slight incline. This one is going to be so messy and constrained it probably shouldn't be built at all, and probably will be the first thing cut.
-- Old Colony portal. Another mile-long portal splitting from the NEC lead tunnel at the Pike vent building and going underneath Southampton Yard to a portal by Southampton St. Not real surface-invasive because it's also under nothing but T land and tracks, but the length of the tunnel is quite long and it won't be cheap.
-- Fairmount portal. Splits from the Old Colony portal under S. Boston Bypass road, spits on the surface on the other side of the Amtrak maint building right before the Fairmount bridges over 93. Not real long since it shares most of the Old Colony tunnel and cut-and-cover under barren T-owned RR track land since the OC tunnel is almost up to the surface at this point, but if they build this the OC tunnel is non-optional.
-- Fitchburg portal. Splits from the NS approach at about Austin St. and curves under the Fitchburg tracks to spit out on the south side of Boston Engine Terminal. Like Fairmount not real long and cut-and-cover because the approach tunnel is almost at the surface here.
This is like multiple $B in 'extras'. Central Station cavern a couple $B at least and the OC portal probably adding close to $1B. CS they really really need to cut because it's so compromised.
And I think the OC + Fairmount and Fitchburg portals are way surplus to a requirement for the initial build. You can reach Middleboro and the Cape via the NEC tunnel, Stoughton and Taunton...only Fairmount, Greenbush, Plymouth, and Braintree-Bridgewater on the Middleboro Line don't get a one-seat to NS and that's a pretty sharp minority of commuter rail ridership with no intercity destinations. Ayer-Wachusett is fully reachable via Lowell and the Stony Brook Branch to Ayer at about the same schedule if Lowell were upgraded >100 MPH on an express run and the Stony Brook Branch had its speed limit raised to 65 MPH or greater. Only Belmont Ctr.-Littleton are omitted, a very small % of commuter rail.
I would seriously just build tunnel cuts at the underground line splits to provision, and if they really need these go add them years later in separate funding commitments. They compromise nothing in a future tack-on to omit at the start to save cost on the heinously expensive base requirements.
But unfortunately because of the engineering they have to dance through this is just about the only routing and the only station builds they can undertake. So there's not a lot of room for brainstorming alternates. None of them are feasible at all. And none would be even as close to (relatively) easy or cost-saving as going under 93...which can only be done with ONE (and one only) trajectory outlined above. That's it. It was planned this way with the CA/T to not block a Link add-on, and this precise routing was the literal only way they could build 93 without blocking a future Link add-on.
