Like all local governments, Washington State’s second-largest county must meet growth projections and anticipated regulatory changes while wisely assessing and rehabilitating its existing infrastructure.
Pierce County built its first sewer system in 1981. Since then, it's gradually expanded the system in response to dramatic growth. The county’s population has increased by 20% since 2000, while the regional economy has evolved from its traditional base of wood and paper products manufacturing to software, biotechnology, and aircraft manufacturing. In addition to supporting this expansion through new infrastructure and new technology, the Sewer Division also must replace aging system components. Thus, when a critical interceptor that serves as the primary influent to the Chambers Creek Regional Wastewater Treatment Plant was found to have significant corrosion damage, the division made it a priority to restore the asset's integrity.
In the effort to rehabilitate the 72-inch-diameter reinforced concrete pipe, we learned a valuable lesson about contractors: Engaging an experienced partner early on in the design phase saves time and money by nipping potential constructability challenges in the bud. Due to the interceptor’s condition, constructability concerns caused us to revise some original assumptions midway through the project. More information and more flexibility early on would have broadened our options from the outset, allowing us to develop strategies for addressing potential challenges.
Remedying a remediation
Our goal was to restore the 15,000-foot interceptor’s structural integrity while preserving as much of its 80 mgd hydraulic capacity as possible. Site logistics and interceptor configuration were the first two hurdles.
The interceptor was 80 feet to 100 feet belowground with limited access at existing shafts along the alignment, so temporary access shafts would have to be installed. Groundwater elevations along the alignment were very high. Flows within the pipe varied widely. A 24-inch-diameter “cunette,” or low-flow channel, at the interceptor invert created a unique interior cross-sectional area that had to be considered in any rehabilitation solution.
Initial attempts to address corrosion via a sprayed-on rehabilitation method were ineffective due to sensitivity to surface conditions and applicator skill. Following this setback, we went back to the drawing board.
Building a 3D profile of the interceptor
First, we needed as accurate a picture as possible of the damage.
A 3D profile of the interceptor was created using data collected by a remote-controlled RedZone Robotics inspection platform equipped with laser (for ovality, eccentricity, and average internal diameter) and sonar (for condition under flow and quantity of settled sediment) technology. The assessment included monitoring of hydrogen sulfide gas levels and closed-circuit television inspection using National Association of Sewer Service Companies Pipeline Assessment Certification Program standards.
The evaluation identified defects while keeping manned entry at a minimum. Further, the advanced assessment techniques allowed for detailed dimensioning of wall loss due to corrosion, which was crucial to evaluating suitable rehabilitation methods.
Rapid deterioration since the previous rehabilitation prompted a manned entry to evaluate the acidity and corrosivity of the interceptor crown and sidewalls by manual pH swabbing and a physical inspection for structural integrity. The inspection revealed wall loss of 2 inches and 3 inches in several stretches.
Weighing trenchless technology options
While the interceptor’s condition was being assessed, we conducted a desktop screening of trenchless rehabilitation technologies to identify those that would:
- maintain a hydraulic capacity of at least 80 mgd under surcharge
- provide a minimum service life of 50 years
- be performed in live flow conditions without bypass.
Cured-in-place pipe was reviewed but eliminated due to the interceptor’s depth, size, and unique shape. We looked at two alternatives in greater detail because we considered them more recent innovations:
- A segmental sliplining product manufactured by SABAS Pipeline Systems LLC, an English company
- SPR, a spiral-wound renewal process developed by Urban Infrastructure & Environmental Products Co., a division of the Japanese company Sekisui Chemical Co. Ltd.
SABAS used the integrated 3D profile generated from the laser data to manufacture a prototype section to the interceptor’s unique dimensions, including the low-flow channel. The company performed strength and joint testing overseas while we evaluated a prototype that was shipped to the treatment plant. This allowed us to see the product first-hand and identify potential procurement challenges.
SPR is applied in the field, so we couldn’t physically test a prototype section. Instead, we visited projects where the product was being installed and researched the feasibility of installation under live flows and with limited access. Potential limitations including the maximum distance for grout pumping and work crew safety considerations eliminated this option.
In the end, we selected segmental sliplining with fiberglass reinforced pipe (FRP). In addition to SABAS, approved manufacturers included the Water Transmission Group of Ameron International Corp. in Rancho Cucamonga, Calif., a subsidiary of National Oilwell Varco; Channeline International in the United Arab Emirates; and Houston-based Hobas Pipe USA.
The condition assessment and detailed evaluation of trenchless technologies allowed us to optimize the phasing of the work. Contract documents were prepared with a base bid to rehabilitate 2,500 linear feet (LF) that exhibited the most significant corrosion. Additional bid provisions were also included to rehabilitate up to 5,500 LF.
As the project progressed, hydraulic modeling showed the cunette was no longer required for cleaning velocities or hydraulic capacity. The record drawings showed it had been cast integral with the interceptor, so removing it was impractical.
The FRP cross section selected for rehabilitation was optimized to provide a minimum of 19 square feet of cross sectional area, and mimic the cross section of the existing interceptor with the exception of the lower part where the cunette was eliminated.
Finding the right contractor
Because the interceptor was critical, we decided to require that the general contractor or the main rehabilitation subcontractor demonstrate experience in the construction and/or rehabilitation of tunnels with depths of 20 feet and greater; a minimum of 5,000 feet of rehabilitation of large-diameter (≥ 48 inches) pipelines with and without flow bypass; and present proof of financial solvency and insurance.
The project superintendent was also required to meet minimum requirements such as experience overseeing the rehabilitation of deep (> 20 feet) large-diameter pipelines (≥ 48 inches), a minimum 5,000 feet of sliplining in the last five years, and experience with installations in live flow conditions.
Ironically, the company that built the interceptor ended up rehabilitating it. Seattle-based Frank Coluccio Construction Co. (FCCC) met the qualification requirements and submitted the lowest responsible bid. FCCC chose SABAS as its FRP manufacturer. The FRP design submittal provided an ovoid-shaped cross section with skids on the bottom to protect the pipe from damage during sliplining.
Overcoming unforeseen construction challenges
Two temporary access shafts were constructed, one using a soldier pile shoring system and the other using a liner plate shoring system. Excavation and construction of both went well.
Due to overhead utilities and other field conditions, the contractor decided early to install the second shaft 250 feet downstream of the end of the project limit. This presented an opportunity that ultimately required a change order.
During the design phase, we decided against adding a third access shaft because we didn’t know how many temporary access shafts the contractor that won the bid would install or where they’d be. Thus, at that point, there was no permanent access manhole to accommodate future rehabilitation phases.
We decided it would be beneficial to add a saddle manhole over the interceptor for future access at the second shaft. A change order was prepared to compensate FCCC for providing a 120-inch precast concrete manhole with a cast-in-place, high-strength hatch.
When FCCC’s crews got inside the interceptor tunnel at the first shaft, they used two sections of production FRP to mandrel the interceptor by pulling them through the interceptor using a cable winch set up at the drop structure. The goal was to verify clearances and ensure FRP sections could be installed.
During this exercise, crews encountered several offset and lip joints at the cunette that hindered movement of the FRP test sections. The lips varied from 3/8 inch to about 2 inches. Crews also noted that interceptor geometry as well as the random nature of the cunette shelves’ misalignment and mismatching (clocking) would make it very difficult to join the FRP pipe sections.
An aboveground bypass system was installed so crews could prepare the joints and remove the cunette shelves. The requirements for FRP liner skids was waived in favor of hand-carrying the FRP sections to prevent damage.
The unexpected need for a bypass system is one example of how allowing greater flexibility during the design process improves project delivery.
One constraint on rehabilitation method selection was that flows must remain inside the tunnel during the work. Therefore, any technology that required an external bypass to create dry or low-flow conditions was automatically ruled out. Because interceptor flows had to be bypassed to create a suitable installation environment for the FRP liner, the project would have benefitted if the no-bypass requirement hadn’t been set at the outset. As such, engaging an experienced contractor early in the design phase helps identify and address potential constructability challenges and minimize construction risks.
While there were lessons learned along the way, owner, designer, and contractor worked together to overcome unexpected obstacles and adjust the project plan as needed. An innovative and lasting rehabilitation was delivered.