Understanding How to Manage Stormwater

Oregon utility uses a variety of trenchless technologies to remedy storm flow issues and sanitary sewer overflows

Understanding How to Manage Stormwater

St. Helens Wastewater Utility worker Scott Williams points to a pipe defect on the Rausch monitor during a video inspection.

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For a city that sits on a giant rock, solving serious stormwater and sanitary sewer overflow issues requires a full range of trenchless technologies.

Under a state mandate to reduce SSOs, St. Helens, Oregon, used sliplining, cured-in-place pipe, pipe bursting and pipe ramming in a comprehensive remediation program that has reduced wet-weather flows to its treatment plant by about 80 percent and cut SSOs to less than one per year. That’s down from 2.5 per year in the past.

Located 30 miles northwest of Portland, St. Helens sits above shallow bedrock of hard basalt with a compressive strength in excess of 40,000 psi. As a result, the city’s infrastructure is quite shallow.

“The underlying rock was a major challenge,” says Sue Nelson, P.E., Public Works engineering director. “We had to come up with the best way to proceed, without breaking the bank and returning to old-fashioned methods.”

The program design was completed in 2009 and construction wrapped up in 2017. The project addressed mainlines, laterals, manholes and stormwater infrastructure. It was outlined in a technical paper at the No-Dig 2018 conference in Palm Springs, California.

St. Helens system

The city’s sewer system serves about 13,200 residents of St. Helens and 2,000 residents of the neighboring town of Columbia City, as well as several industries. Wastewater is treated in a 43-acre lagoon and disinfection system that discharges approximately 4 mgd to the Columbia River (1 million gallons of domestic flow and 3 million gallons from one industrial customer). Total capacity of the secondary lagoon is around 115 million gallons.

In 2005, a flow study found that the hydraulic capacity of the system was less than required to handle inflow and infiltration from a 1.5-year rainfall event. In addition, the study determined that I&I would contribute as much as 24 of 25 mgd in a five-year rain event and identified the basins in the city contributing the largest flows.

With that data in hand, St. Helens embarked on a program to address sanitary sewer deficiencies in both public and private sewer lines. It had these specific objectives:

  • Reduce the number of overflows.
  • Restore the capacity of the sanitary sewer system for future use.
  • Improve the existing storm sewers and install new storm sewers to eliminate flooding and convey storm flows diverted from the sanitary system.
  • Focus on the largest basin contributors to achieve maximum improvement.

“The city took a holistic approach — tackling mainlines as well as laterals,” says Brendan O’Sullivan, civil engineer with Murraysmith of Portland. “That was the key — understanding the stormwater component of the program.”

On the mainline

The mainline program was designed to rehabilitate or replace existing sanitary sewers and construct new storm sewers.

CCTV inspection revealed numerous root intrusions, separated and offset joints, and cracked pipes, and it enabled the city to concentrate on the lines and basins identified as the largest I&I contributors in the previous flow study.

“Once the needs were identified, we evaluated the various rehabilitation methods, including CIPP, pipe bursting, sliplining, as well as open-trench excavation,” Nelson says.

“We never wanted to rule out any technology until each was looked at to make sure what was best to address issues, as well as corollary concerns — especially the costs — and the impact on property owners. Many of our pipes ran through backyards and neighborhoods. We chose trenchless as a way to minimize impact on residents.”

“Trenchless was a lot more palatable,” O’Sullivan adds. “Surface restoration of opencut excavations alone could have involved 65,000 to 75,000 cubic feet (of soil), to say nothing of trying to get equipment into side yards.” The cost of excavation and remediation on private property weighed heavily in the selection process, he points out.

In the analysis, CIPP proved to be the most appropriate method, followed by size-on-size pipe bursting and opencut.

All CIPP liners were designed per ASTM F1216 for fully deteriorated conditions and were fabricated of needle-punched felt tube. They were impregnated with polyester resin, installed using air inversion, and steam cured. The thickness of the liners ranged from 4.5 to 9 mm based on the design parameters, required structural properties and geographic location. CIPP installers included Michels and Insituform Technologies.

Monitoring styrene

The public expressed some concerns about styrene during the construction, and the city implemented a testing program. Using gas meters immediately adjacent and downwind of the curing steam exhaust, the city monitored the first half-dozen CIPP liner installations. After the tests showed styrene did not exceed Environmental Protection Agency and OSHA thresholds, the monitoring was suspended.

In cases where the existing sanitary pipe was in too poor a state to allow for CIPP lining, pipe bursting or open trench were used, although each method presented its own challenges. “Some areas didn’t have the minimum footprint or ample staging areas for CIPP,” Nelson says.

Open-trench replacement of sewers located in side and backyards was difficult because access was limited due to existing structures, trees and landscaping. With pipe bursting, it was critical to analyze soil displacement forces to ensure that existing surface structures, adjacent foundations, and utilities would not be negatively impacted during the pipe bursting operations. All pipe bursting operations for the mainline program were 6-inch-diameter size- on-size replacement, using PE3408 DR17 HDPE, as no increase of capacity was deemed necessary after evaluating the system hydraulics.

Public outreach

“City staff was sensitive to the problems of accessing public utilities through private property,” Nelson says. “We didn’t have a fully developed public information officer position at the time, but we posted information on the website and sent notices in advance to customers.”

She says the piping contractors played a key role. “Once the contractor was hired, we put out additional notices including door hangers, explaining who the contractor was and who the contact person accessing the property would be.”

The city also published maps of the construction areas and held neighborhood meetings to address specific problems, including the large number of laterals made of Orangeburg pipe material (see sidebar).

Pipe ramming

Pipe ramming technology was the choice for the next phase of the mainline program: replacing two undersized storm culverts, one 24 inches and one 36 inches in diameter. The new culvert had to navigate 50 feet of roadway fill containing numerous public and private utilities and an RV park whose year-round residents couldn’t be displaced.

Based on geotechnical and surface conditions and diameter of the proposed culvert, the project was designed with upper and lower pipe ram segments and two sections of open-trench installation.

For the first ram section, engineers selected a 66-inch-diameter steel pipeline with a wall thickness of 1 inch and an S-90 Hydro-Ram hydraulic hammer from IHC IQIP capable of imparting 90 kNm of energy per blow.

Within the first 40 feet, the grade began to drop due to soil consolidation. “Fortunately, the design had built-in flexibility to adjust the slope of the pipe without negatively impacting the maximum flows needed to be conveyed through the pipeline,” O’Sullivan says.

However, approximately 165 feet into the pipe ram and 36 feet below existing grade, a differing condition in the form of a basalt rock outcrop was encountered and the final section was completed using open-trench excavation.

The 170-foot-long second ram under the RV park was completed using a Torus Hammer manufactured by TT Technologies. “This ram was also challenging to complete,” O’Sullivan says. “A rock ledge was discovered that required excavation above the steel pipe so it could be lifted over the ledge as the ram continued pushing the pipe forward.”

A 96-inch-diameter manhole with a 3-foot sump for sediment management was installed once the pipe ram was completed. Open trench was used to complete the remaining pipe to the new outfall structure on the Columbia River.

Good investment

Total cost for the project, including the preliminary flow studies as well as numerous manhole repairs, was approximately $11 million. City funds, the American Recovery and Reinvestment Act of 2009, and the Oregon Department of Environmental Quality Clean Water State Revolving Fund paid for the project.

The results have been worth it.

“The rehabilitation of approximately 12 miles of the city’s 57 miles of sewer lines, 10,000 linear feet of storm sewer lines, and about 800 sewer laterals has resulted in an 80 percent reduction in sanitary flows pumped to the treatment plant during rain events,” Nelson and O’Sullivan report in their paper.

“And the number of SSOs has been drastically reduced. The city experienced an average of 2.5 SSOs each year between 1995 and 2009. Between 2010 and 2017, that number dropped to less than 1 per year. The capacity issues have been resolved and there have been no reported overflows of the storm sewer in the upstream basins.”

“We’ve seen a significant reduction in wet-weather flows at the treatment plant,” Nelson says. “They used to run as high as 24-25 mgd. We don’t see anywhere near that now.”

Making lateral connections

The St. Helens sanitary sewer project also addressed sewer laterals to private property, with the result that over 2,000 (about 60 percent) of the city’s laterals were inspected and about 800 were rehabilitated.

The city’s approach was unique, in that the focus was on the older sections of the city, and used CCTV inspections and clear, “layman’s terms” communications to document poor lateral conditions to property owners.

“All CCTV inspection footage had to be uninterrupted from start to finish and begin with a video of the structure frontage,” reports Sue Nelson and Brendan O’Sullivan in their technical paper at the No-Dig 2018 conference. “This made it very difficult for owners to dispute the results of the CCTV inspection.”

With the inspection video in hand, the city ranked laterals based on types of deficiencies and allowed the city to recommend repair or not, based on clear evidence.

“We held neighborhood meetings to discuss specific problems,” Nelson says. “One section of the city had Orangeburg pipe laterals, which would have disintegrated upon connection with new laterals on the city side. (The pipe is bituminous fiber, installed after WWII as a less costly connection.)

“Homeowners were informed that they would have to pay for replacement of the section on their side. We offered payment options to lower income folks, letting them pay the cost back over time on their water and sewer bills.”

The city also supplied property owners with a lateral repair map showing the identified deficiencies, the necessary plumbing permit, a copy of the applicable plumbing code, and a brochure of frequently asked questions. The communications protocol and open-door approach paid off; St. Helens achieved a 94 percent owner repair rate.

On the city side, about 2,000 feet of laterals under streets and sidewalks were repaired — a few using robotics and ultralow viscosity chemical grouting at the connection, with the remainder completed via open trench.


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