CHOMPERS Tunnel boring machine arrives at Nicolai Shaft, top; west side tunnel will be matched by east side twin in late phase. (Photo courtesy of City of Portland Bureau of Environmental Services)

Contractors in Portland, Ore., are using two 16-ft-dia, variable pressure tunnel boring machines to build a 4.2-mile storage tunnel to handle the city’s combined sewage overflows. The $293-million project includes a 220-million-gallon-per-day pump station to send storage to a treatment plant and reduce the city’s annual number of overflow incidents to five per year. The work is driven by a 1991 stipulation and order the city signed with the state’s Dept. of Water Quality.

The gravity line, driven through difficult soft-ground conditions, will divert combined stormwater and sewage overload from an interceptor on the west side of the Willamette River, under the river to a pump station on the other side. Dubbed the "Big Pipe Project," the rate-funded work is the most expensive capital construction job in Portland’s history.

The project represents a three-year phase midway through a 20-year, $1-billion overhaul of the century-old system. The program aims to reduce annual discharge of sewage mixed with stormwater from an estimated 6 billion gallons in 1991, to 250 million gallons by 2011.
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Under a two-phase contract, contractor Impregilo/ S.A. Healy Joint Venture, Chicago, teamed on preconstruction services with tunnel designer Parsons-Brinckerhoff. It is now in the second phase for construction services. The company will use two TBMs supplied by Germany’s Herrenknecht AG operating with a pressurized bentonite clay slurry at the face to keep back groundwater. Giuseppe Quarta, project director for the contractor, says Herrenknecht claims this is the first U.S. use of a full-sized, manned, slurry-shield TBM.

Driving the tunnel is expected to be challenged by mixed ground that includes multiple layers of recently consolidated sand and silt alluvium underlain by permeable cemented gravel with pockets of plastic, clayey silt. Portland is also in a high-risk earthquake zone. "There’s a whole range of soft ground conditions," says Paul Gribbon, principal engineer with the city’s Bureau of Environmental Services. "We anticipate several spots where there will be mixed phases where we’ll be tunneling from one type of material into another."

In August, TBMs were barged in sections upriver and lowered into the 120-ft-deep Nicolai Shaft, the primary access. Four other similar vertical shafts with interior dimensions from 39 to 59 ft dia for air venting and tunnel access, also are under construction. A 129-ft-dia, 150-ft-deep shaft and pump station at Swan Island is also being built under the contract. TBMs are now being assembled. They will drill in opposite directions for about two years to construct the 14-ft i.d. tunnel."We’re estimating a rate of about 34 ft per day," says Gribbon.

Precast concrete lining segments will be bolted and erected in the tail of each TBM. The 151,000 cu yd of excavated alluvium will be pumped through each machine and to the surface, where the bentonite will be separated for reuse. "Occasionally, we will add fresh bentonite, but it’s basically a completely closed system," says Quarta.