For officials in Portland, Ore., the Big Pipe project is no longer a dream. The recently completed $1.4-billion, 20-year combined sewer overflow (CSO) control program was implemented in response to an Oregon Dept. of Environmental Quality order to reduce significantly CSO events by December 2011. Thanks to large-diameter underground tunnels installed along the Columbia Slough and on both sides of the Willamette River, the city now meets the overflow frequency criteria set forth in 1991 by the DEQ. Annual CSO volume to the Columbia Slough and the Willamette River has dropped by 99% and 94%, respectively, according to city officials.
The city’s Big Pipe program consisted of three major phases and more than 300 individual projects to meet the Oregon DEQ water-quality standards for the Willamette River and the Columbia Slough, says Virgil Adderley, CSO program manager with the city of Portland’s Bureau of Environmental Services. “The first phase, the cornerstone projects, included measures such as stream diversions, sewer separations and downspout disconnections for removing stormwater from the combined sewers,” Adderley says. “The other two phases were implemented in parallel over a 15-year period and involved treatment-plant expansions to handle higher wet-weather flows as well as the construction of large underground tunnels and pump stations for collecting and transporting combined sewage.”
The $464-million East Side Big Pipe project, the final component of the overall program and the single-largest public-works project in Portland's history, was headed by the joint venture Kiewit-Bilfinger Berger (KBB). The centerpiece is a nearly six-mile- long, 22-ft-dia tunnel constructed 110 ft to 150 ft below ground surface. “Any large-diameter, soft-ground tunnel located below the water table will be challenging, and that was certainly the case with this project,” says Bill Mariucci, project director for KBB. “We were dealing with very complex geology and had to solve the problem of balancing water pressure, maintaining soil stability and protecting existing surface structures."
Since the project route was located in a dense urban setting, KBB needed to excavate under buildings, bridges and other structures common to a city environment. “We used slurry tunneling technology and procured a tunnel-boring machine, manufactured specifically for this project, that could navigate the varying soil conditions occurring along the alignment without any surface disruptions,” Mariucci says. As part of the installation, KBB also built seven tunnel access shafts 50 ft to 70 ft in diameter and connected 14 existing outfalls to the tunnel system, accomplished through open-cut shallow-surface pipelines and micro-tunneling.
Over a five-year construction time period, KBB received a total of eight change orders, five of which were related to administration and the remaining three consisting of scope additions. KBB’s contract specified late completion damages, but as the project was completed on time and on budget, no penalties were incurred.