Making or Rather Overcoming the Grade

The City of Portland maintains 991 miles of sanitary and 883 miles of combination sewers (storm and sanitary combined) with more than 75 percent of the combined 1,874 miles of sewer infrastructure composed of 12-in. diameter material or smaller.

Recognizing that the majority of the established sewers are aging and capacity-strained — nearly 50 percent are composed of 8-in. nominal material — proactive city officials have devised an aggressive routine replacement plan, as evidenced by the Portland Heights upgrade. Among the most prominent and comprehensive was a $2 billion, 20-year megaproject, coined “the big pipe,” an endeavor just completed that will control and reduce combined sewer overflows (CSOs) from entering the Willamette River and Columbia Slough. Portland started the CSO control program in 1991 with a set of cornerstone projects designed to remove storm water from combined sewers, hence improving river water quality. The program reduces CSOs to the Columbia Slough and Willamette River by 99 and 94 percent respectively.

Yet with all the sewer upgrades completed or scheduled for the future, the importance of developing contingency plans for challenging infrastructure jobs was never more evident than it was for Sherwood, Ore.-based Northwest Earthmovers Inc. (NEI) during an intricate sewer line upgrade the contractor completed recently in an affluent Portland neighborhood.  

18-in. Upgrade for Uscale 18th Street

Although residents recognized the need and resulting benefit of replacing the nearly century-old sewer line that had long been serving their neighborhood, homeowners located along a stretch of 18th Street in upscale Portland Heights also had concerns. Installation of the new sewer was specified to be installed using trenching approach precisely beneath the center of the street. But homeowners were concerned about street disruption, access to their homes, and the integrity of this well-established, tree-lined avenue that their estates — most valued in excess of a million dollars — have bordered for decades.

Dating back to the early 1900s, the sewer serving Portland Heights residents was a combined sanitary and storm sewer, now an antiquated 8-in. clay-fabricated line entangled by the root systems of maturing trees and a lot of infiltration. Cary Gaynor, a construction manager with the City of Portland Bureau of Environmental Services, documented the challenges facing the Bureau specific to the project, and how an alternative approach proposed by NEI helped minimize disruption, alleviate residents’ displeasure and streamline the installation.

“The old sewer was undersized and failing,” Gaynor says. “The design for the new sewer was specified as
open-cut in the middle of 18th Street rather than digging up the old line that was situated on the west side beneath the curb. This allowed residents to remain connected to the existing line during the installation, and later be connected to the new sewer once installation was completed. The old sewer will simply be abandoned.”

After concerns expressed by residents escalated once work began — mostly because of frustrations getting to and from their homes — officials realized that the volume of traffic in the neighborhood had been underestimated and trenching was exacerbating the problem. Consequently, a contingency plan involving an innovative trenchless method — the Axis guided boring system, developed by Vermeer and proposed by Northwest Earthmovers — was enthusiastically embraced.

“We had used the Axis system on city projects previously so we were familiar with the technology,” Gaynor says. “The guided boring system was used on a storm drain job in the downtown business district and the contractor suggested using it to help minimize street disruption. We also found the costs for using the boring system, compared to open cut, to be a wash; or perhaps slightly in the favor of the Axis system when considering the expense of hauling away spoil, asphalt and street repaving. What you can’t put a dollar amount on is the value of reducing disruption and minimizing public inconvenience.”

The Axis system is a pit-launched trenchless installation method designed to achieve pinpoint, on-grade accuracy while eliminating some of the difficult steps associated with other installation techniques. The Axis system is ideal for manhole-to-manhole installations and is capable of maintaining the strict tolerance and accuracy required for sewer and water projects. This combination combines on-grade accuracy with efficiency while helping to lessen street restoration costs.

“The very steep roadway grade, in places close to 16 percent presented a major challenge,” says NEI project manager Tim Looney. “Steel track undercarriages on equipment don’t perform all that well positioned on sloping concrete platform grades. And rubber track machines really aren’t effective when installing large diameter pipe. We needed to look at another option.”

The Axis System Integration

Work on the project began on the downstream side with trenching and installation of approximately 500 ft of 24-in. concrete pipe, followed by a stretch of 18-in. HDPE. The bulk of the 4,700-ft job consisted of installing 15- and 12-in. PVC main lines. After digging into the job on the downhill side, Looney and crew knew there had to be a better way.

“The conditions we were digging through consisted of old fill with a concrete street situated on top,” Looney says. “It was a nasty, very slow process. Trying to open trench that stuff on a 16 percent uphill grade had become our worst nightmare. Representatives with Vermeer had visited with us about trenchless options, even before we submitted the bid. But at the time we were locked in to estimating the job based on the open cut specifications. Once we got into it and explained to the city that we felt we could complete the job more efficiently using the Axis system, they were quick to agree.”

Hence, a five-block stretch — upward of 1,250 ft  — was installed using a Vermeer laser-guided boring system in five separate shots of approximately 250 ft each on a steep uphill incline. Ground conditions beneath the surface were composed primarily of clay, with pockets of very soft silt. NEI allocated a day to dig launch pits, with each positioned near a manhole on the lower/downhill end of the route, and a half to full day for equipment setup and positioning the Axis system components.

Boring would then commence on Day Three while another crew began excavating the receiving pit on the uphill side of the path. Barring setup, most of the individual bores were completed within one day. NEI selected a 2 Bar clay/shark combo cutter to complete the pilot bores, and a 19-in. 3 Bar clay/shark combo cutter with bolt-on coupler to ream and upsize. Backreaming and pullback of the 20-ft interlocking sections of 16-in. sewer pipe occurred simultaneously at depths averaging 13 to 14 ft, most on a 14 to 16 percent grade.
“Usually we would start drilling out of the completed launch pit by the end of the setup day,” Looney says. “In the meantime, the base crew would move up to the next/receiving bore pit, and dig that while the Axis system was boring. The next day was dedicated to boring. If our timing was in sync, the pilot bore would break through at about the same time the receiving pit was being completed. Then we’d turn right around and pull back. It was a two- to three-day process for an entire block, I’d estimate up to three times faster than with open-cut in those conditions. It worked out really well.”

Happy Neighbors  

The City of Portland was in constant communication with the Portland Heights residents affected by the project — both prior to work beginning and providing updates throughout — so all knew up front what was about to take place and what they could expect once work began. The City had been forthright about the complexity of the job and potential personal inconveniences, and even went so far as to describe the installation approach, providing detailed descriptions of activity, equipment and challenges given the steep terrain and nasty conditions. And once work began, they were also informed about alternative methods being considered to open-cut.

“The city’s public relations department did an excellent job communicating details of the project,” Looney says. “Once trenching began, had they not been informed, the degree of displeasure would likely have been much greater. But because residents were kept apprised throughout, they felt like the City was listening and really cared about their concerns. Many of the residents are retired so they watched the progress with keen interest. The ones that had to live through the open trench stuff down below were more than delighted when the alternative method was employed. That was an important component to the success of the job.”

Randy Happel is a features writer for Two Rivers Marketing, Des Moines, Iowa.