It’s no exaggeration to say that an enormous piece of tunneling machinery was shipped to Providence, RI, recently. The hard rock tunnel boring machine weighs 690 tons (621 t) and came all the way from Japan on a ship large enough to haul it.
The machine will be used to bore an immense tunnel beneath Providence, which is designed to store up to 62 million gal. (235.6 million L) of sewage and prevent it from flowing into the nearby Narragansett Bay.
The job is scheduled to take 22 years to complete. The 3.5-mi. (5.6 km) tunnel will sit 250 ft. (75.8 m) below the ground and measure 30 ft. (9.1 m) in diameter. It will be bored along with six shafts going down from the surface and a number of other smaller side tunnels. Only an apparatus so large could handle such a task. The Hitachi machine came in several different pieces and will be assembled now that it is on U.S. soil.
“Putting it together is a battle, but it’s something we do all the time,” explained Steve Minassian, project manager of Shank/Balfour Beatty, the primary contractor on the project. M.L. Shank is a San Francisco-based tunnel construction specialist that has joined forces for the project with Balfour Beatty, a construction company from England.
The $700-million project should keep sewage from washing into the bay, a situation that has become a major problem whenever it rains in Providence.
“Any time we have a half inch of rain in 24 hours, the sewers are overwhelmed and sewage and storm water is washed into the bay,” said Jamie Samons, public affairs manager of the Narragansett Bay Commission (NBC). It is of particular concern for the local shellfishing industry because the sewage shuts down thousands of acres of beds for as long as seven to 10 days, as well as public beaches along the shore.
But the tunnel, which is part of the Combined Sewer Overflow Abatement Plan (CSO), will store the effluent until it can be transported to the Field’s Point Wastewater Treatment Facility.
“With this in place, you won’t see floatable sewage in the bay anymore,” said Samons. “That is the legacy of the 19th century. The facility was great in the 1800s, but there are a lot more people here now.”
When completed, the city will comply with the Federal Clean Water Act of 1972. The CSO will reduce the annual volume of discharge by approximately 40 percent. It also will reduce shellfishing closures by 47 percent in the northern half of the upper bay and 77 percent in the southern half.
“Americans today feel that safe and clean water is not a priority, but a necessity,” commented Paul Pinault, NBC executive director, “so from a public health as well as an environmental perspective, the CSO project is arguably the most important construction project you’ll never see.” Only several relatively small holes will ever be visible from above ground.
Work has yet to start on the actual boring. As of early July, the shafts going down were still being excavated. Shank/Balfour Beatty is using a Tramac V45 breaker, complete with a specially made moil point tool, mounted on a Komatsu PC300 excavator.
The digging is in itself a complicated process. To begin with, the sedimentary soil in the area is rather unstable, so it has been frozen with the help of briny water cooled with ammonia. It will remain in that state throughout the entire excavation. The drilling compressor is made by Atlas Copco of Sweden.
The work done by the Tramac breaker also is quite awkward. The two machines take up just about the entire space in the hole, so when 2 ft. (.6 m) of rock have been broken, they are actually lifted, allowing muck buckets to be lowered and filled with dirt. Progress is made at approximately 8 ft. (2.4 m) per day.
Once the shafts are completed, the boring machine will have to be lowered. Minassian estimates that in all, 500,000 tons (450,000 t) of dirt will be moved, approximately 80 percent of which will come from the main tunnel.
Boring will be done primarily through sandstone and shale, rock that is relatively easy to cut through. “It is not supposed to be difficult … it is medium strength stone. It is not like the Big Dig in Massachusetts where they are digging through granite,” Minassian said.
It’s hard for him to estimate what kind of pace will be kept during the duration of the boring, something that depends on what type of rock is encountered and how much water there is to remove. However, he figures the machine will move anywhere from 70 to 140 ft. (21.2 to 42.4 m) of rock per day. Work will be done on two shifts, five days a week, with a maintenance crew active on the third shift.
The project will be done in three phases. The first tunnel will be completed in either 2007 or 2008, after which there will be a two-year break. Authorities will then evaluate how things are progressing and whether there are advances in technology and any changes in Environmental Protection Agency rules. Two near-surface interceptors will then be constructed, followed by the boring of a second major tunnel. That part of the work will probably not start until 2010 at the earliest, according to Samons.
“It’s exciting. This is the biggest public works project in state history,” Samons said. “This is really good for a city like Providence, because it is a really old city.”