Every city has a construction site that looks like it quit. Cones out, fence up, a crane sitting there, nothing visibly moving for weeks at a time. Usually it’s a permit fight or a money problem.
Under the Queen City Avenue bridge in Manchester, New Hampshire, it means a 281-ton machine is parked at the bottom of a hole waiting on its plumbing.
The machine is a tunnel borer, it’s named Granite Janet, and the plumbing is a slurry plant. I’ll get to what that means, because it’s basically the reason the thing can work at all.
So what’s actually down in the hole?
A tunnel boring machine isn’t one machine. It’s a train. The cutterhead and the shield are what you’re looking at in photos, and behind that runs a string of trailing platforms called gantries. They carry the power packs and transformers, the ventilation, and the refuge chambers the crew climbs into if something goes wrong up front.
Manchester public works director Tim Clougherty has put the whole assembly at about 570 feet long once its 17 to 19 gantries are hooked on. The front end weighs 281 tons dry, according to Engineering News-Record, which watched a 700-ton crane lower the sections into the launch pit on June 18.
So you’ve got a 570-foot industrial train sitting in a trench beside a river, fully assembled, pointed at a wall. Calling that a machine is a bit of a stretch. It’s closer to a small factory that happens to move forward a few dozen feet on a good day.
The slurry plant is the holdup
Slurry sounds fancier than it is. It’s water mixed with bentonite clay, pumped under pressure up to the cutterhead, and it does two jobs at once. It holds up ground that would otherwise collapse into the face, and it picks up the chewed rock and hauls it back out of the tunnel as a liquid. A plant on the surface strains the muck out and sends the clean stuff back around the loop.
And no, you can’t skip it and just push harder.
The city runs a public work status page for the project, and as of this week it still has the connection of the slurry plant and the muck handling system to the machine listed as active work, along with commissioning of the water treatment plant. The machine itself, the push frame and the slurry plant are all down as finished.
Which raises the obvious question. Wasn’t this supposed to start in August?
It was. ENR reported in July that the launch was set for August, and the tunnel’s design manager at CDM Smith had said in the spring that boring would begin in early August. Parsons, the construction manager, updated its own project page in August and had it down as a launch in late summer. I can’t find any announcement that the machine has started, and the city’s work list still shows those hookups in progress, so as far as I can tell Granite Janet hasn’t gone anywhere yet.
The first thing it grinds isn’t rock
Now for my favorite detail on this job.
The ground right in front of the machine is a mess, geologically speaking. Down there you get old fill, glacial deposits and a till layer packed with boulders, and under all of that, bedrock: schist, gneiss and granite. At the launch that means soft stuff up top and hard rock underneath, which is roughly the worst thing you can hand a cutterhead. It tends to ride up into the soft, stall against the hard, and the ground overhead can settle while it sorts itself out. Manchester’s subsurface investigation found that layering along long stretches of the line, and boulders sitting in glacial till are a headache all by themselves, as a machine that chewed through more than 2,000 of them under a Canadian city found out.
So the crew filled that ground with concrete. Clougherty has explained that they solidified the whole launch area on purpose, so the cutterhead starts against one uniform vertical wall instead of biting into loose material up high and bedrock down low. Steve Minassian, a tunnel sector manager at Parsons, gave the Union Leader the counterintuitive version of the same idea: “sometimes a bad thing is a good thing.”
I’d have assumed you want the softest ground you can get. Apparently hard rock is the friendly option, because when the machine breaks down in rock the crew can back it off a little, walk up to the face and swap cutters. Do that in soft ground under the water table and you’ve got a cave-in waiting to happen, so they have to pressurize the chamber first.
It runs in two different modes
You’ll see this machine described two ways depending on who’s writing. Parsons calls it a hard-rock tunnel boring machine. The safety partnership the contractor signed with the Labor Department calls it a slurry Mixshield. They’re both correct, and that’s the point of the design.
In solid rock it can run open. The face isn’t pressurized, the cutterhead openings are wider, and the disc cutters do what disc cutters do to granite. When the line drops into soft ground or a mixed face, the crew closes it up and runs a pressurized, slurry-supported face instead. CDM Smith’s design write-up flagged the first 6,500 feet or so as the stretch where the bedrock profile is most undulating, and that’s the stretch where it’ll be switching back and forth.
The finished tunnel is 12 feet across on the inside. The concrete rings that line it run 14 to 14.5 feet on the outside, so the hole being cut is noticeably bigger than the tunnel you’d end up standing in. If you see 12.5 feet quoted somewhere, that’s an older figure from before the design was locked down.
Where it’s going
The route ends on Mammoth Road, where the machine comes up at a former CVS across the street from the new one. That’s a pretty funny spot to surface a 281-ton machine after two years underground. At least it comes up on dry land, which is more than you can say for an 83-ton borer that had to be dug out through the ocean floor off an Australian beach.
So what are the seven shafts for?
Seven drop shafts sit along the route, and they’re there to get street water down into the tunnel. Clougherty has put their depth at 50 to 70 feet or so, and Parsons says Drop Shaft 6, the one on Massabesic across from Elliot Hospital, goes down 92 feet.
Water falling that far down a vertical shaft drags a serious amount of air down with it, and that air has to get back out somewhere or it comes shoving back up the shaft. That’s why the city’s work list has a de-aeration chamber going in at Drop Shaft 2, separate vent shafts at Drop Shafts 1 and 6, and a supplemental vent over by Maple and Silver. It’s a stormwater tunnel, but a fair chunk of the engineering is about moving air.
They’re not all anywhere near done. The city has blasting and rock support running at Drop Shaft 1, concrete going in at Drop Shaft 2, ground improvement at Drop Shaft 3 and site prep at 4 and 5. Drop Shaft 7 out on Mammoth Road doesn’t start until 2027, and the entrance box culvert on the Rockingham Trail waits until 2028.
Once it’s all tied together, the tunnel is meant to keep more than 150 million gallons of combined sewer overflow out of the Merrimack River, under a consent decree the city signed with the EPA and the state.
One more thing about the name, since it isn’t a marketing exercise. Two Hillside Middle School seventh graders, Aaron Nicholas and Jullien Santos, won a naming project run across all four Manchester middle schools. They picked Granite Janet for Janet Bonnema, an American engineering technician hired onto the Eisenhower Tunnel job who became one of the first women allowed to work inside a tunnel under construction in this country. For a machine about to spend two years under people’s houses, that’s a better name than most of these get.
Parsons still has the launch listed as late summer. The city’s work status page, as of this week, still shows the slurry plant and muck handling hookups as active work. Summer ends on September 22.
