While making plans to expand the San Diego International Airport with the construction of a new 10-gate terminal, engineers for the San Diego County Regional Airport Authority identified nearly 1,700 ft of a 96-in. sanitary sewer trunk main that runs 25 ft directly below the site of the planned terminal. While structurally adequate for existing conditions, the 20-year-old concrete pipe was not originally designed to handle the loading of an airport terminal or the aircraft that would be landing on the runways and apron adjacent to it.
Consequently, the authority was tasked with the challenge of increasing the load-bearing capacity of this critical pipe without digging it up while also maximizing the pipe’s flow capacity. In order to achieve its goals, the authority considered three trenchless options for the project including sliplining, traditional cured-in-place pipe (CIPP) and composite-reinforced CIPP.
To accommodate current and projected flow requirements, the pipe rehabilitation thickness was specified not to exceed 1.26 in., which quickly ruled out the use of sliplining technology as it would have reduced the pipe thickness by nearly 12 in. Traditional CIPP technology was also eliminated as an option on this project because it would require a minimum liner thickness of 2.07 in. at the designed load-bearing capacity. With the fiber-reinforced CIPP liner, however, it was possible to achieve the prescribed load-bearing capacity and reduce traditional CIPP thickness by approximately 40 percent to 1.26 in., thereby meeting the airport authority’s flow and strength criteria.
Charles King Co., the general contractor for the project, brought in Insituform as the lining subcontractor. Insituform pioneered the traditional CIPP felt and resin approach, and recently introduced its iPlus Composite CIPP process, which increases strength by integrating reinforcing fibers into tube construction.
While this approach offered the structural and capacity solutions required, there was one hitch: fiber-reinforced CIPP had never before been installed in a pipe exceeding 60 in. in diameter or a length longer than 200 ft with the water inversion method.
Located on the site of a former landfill, the 96-in. diameter sewer is owned by the City of San Diego (a separate entity from the San Diego County Regional Airport Authority), which uses it both for storage during storm events and to carry sanitary sewer flow to a pump station before directing the flow to the Point Loma Wastewater Treatment Plant. Because there were no properly located manholes at the site, a single access point was constructed from which two CIPP installations in opposite directions were staged: one 300 ft in length and the other 1,393 ft in length. Wastewater also had to be temporarily diverted to and from a parallel 114-in. diameter line through badly deteriorated diversion structures, requiring continual dewatering.
Additionally, because the construction site was located within a quarter of a mile of San Diego Bay, workers were required to pump millions of gallons of groundwater a day that was atop the 25-ft deep pipe. Crews worked around the clock for approximately four days in 12-hour shifts to complete each installation, vacuum-impregnating the fiber-reinforced tube with resin at the site before installing it via a conveyor into the 8-ft diameter pipe. It took more than 260,000 lbs of resin – or 5.8 full tankers – to impregnate the 1,700-ft-long fiber-reinforced CIPP tube. This is 40 percent less than would be required with traditional CIPP technology.
“This projectwas very difficult from a logistics standpoint,” said Brett Gillespie, AreaManager for Insituform. “All the tubes had to be brought in from our home officeand required special DOT permits to get it here.”
Additionally, staging the delivery of the resin was a challenge in itself. Coordinating the cleaning of the host pipe and bracing for the diversion structures were also challenging because of the size and depth of the pipe.”
To date, this project represents the largest of its kind in both diameter and single installation length ever attempted. It also marks the first time the technology has been installed using an “over the hole” wet out process.
Additionally, the first 300-ft installation was a “blind shot” with no visible termination point due to its proximity to an existing runway. More than 638,000 gal of water were then required to cure the finished pipe, using four 500-hp boiler trucks. With the project site visible from the runway, other unique field conditions at the project site included soil contaminants and high ground water.
Crews successfully installed the iPlus Composite CIPP, working with the airport authority and the general contractor to ensure that the challenges of this project were addressed and the airport’s expectations were met. “We were very fortunate to have this technology available for use in the structural reinforcement of the City’s pipe,” said Iraj Ghaemi, Director of Facilities Department, San Diego International Airport.
The project was completed and tested in September 2009, and the sewer line was returned to service according to plan. The project demonstrates a new structural rehabilitation and/or reinforcement option for owners of large-diameter pipes.
“We were forced to deal with one issue after another,” Gillespie said. “Every time we turned around we were faced with a new challenge. If there hadn’t been a spirit of partnering on the job between the contractors, designer and owner, it could not have been done. We had a committed group of professionals working together to complete a unique and difficult job.”