Credit: Photo: HOBAS Pipe USA
In a recent San Antonio sliplining installation, the flush exterior joint and the high-strength, thin-walled pipe allowed for optimal flow.
After years of watching contractors install centrifugally cast, fiberglass-reinforced, polymer mortar (CCFRPM) pipes in trenchless projects, the San Antonio Water System (SAWS) decided to do some installing themselves.
A public utility owned by the city of San Antonio, SAWS is having its crews perform more utility maintenance, relying less on contractors. “We saw an opportunity to save time and money and we capitalized on it,” said Gordon Mahan, SAWS manager of distribution and collection. “We had been considering expanding our work projects to include doing live sliplining ourselves. We had the equipment and the manpower to do it.”
A group from SAWS visited the HOBAS Pipe USA factory in Houston to watch pipe production and get a better understanding of the product and its use. They also visited a sliplining project in a neighboring city. Although many feet of the pipe have been installed in the San Antonio area, this was the first SAWS attempt at sliplining. “For this project, we needed a proven product; we didn't consider products without a proven history,” said Dennis Laskowski, engineer II.Structurally Sound
After the collapse of a 48-inch outfall, SAWS crews put their knowledge to the test. Initially, the failure location was thought to be in an isolated area that included a siphon. Crews made emergency repairs, but a thorough study determined additional areas south of the collapse also needed repair, though not on an emergency basis. “We had a contract to install cured-in-place pipes in the deteriorated areas of the siphon but wanted a more cost-effective approach to the rest of the line repairs,” said Mahan. “We decided to take our time and do it right, and do it ourselves.”
Laskowski, one of those responsible for the design, voiced concern: “The failing line was under a roadway adjacent to a cold storage plant where 18-wheelers travel constantly. We were concerned about the structural condition of the pipe due to these heavy traffic loads and the condition of the pipe, which could cause an immediate pipe failure. Sliplining the existing pipe would solve the structural issues, prevent further pipe corrosion, and ensure leak-free joints. It would also reduce the amount of street cutting and downtime the cold storage plant would suffer if the pipe were to be open cut.
“Another reason we considered CCFRPM pipe is the capacity of the line,” said Laskowski. “With the hydraulic characteristics, we wouldn't see any flow reductions from the decreased diameter.” A 42-inch diameter pipe was evaluated hydraulically and met the design needs.
SAWS issued an invitation to bid/ proposal for 2666 feet of 42-inch, 82-psi stiffness class, 20-foot joints of bell-spigot pipe. Because of the agency's positive experiences with the manufacturer, the pipe's ability to meet the design criteria, and confidence in the product, they chose to take bids only on that material.Ease of Installation
SAWS crews began by scanning the existing line with remote-controlled cameras to assess conditions and look for joint offsets or misalignments that might hamper sliplining. The first push was upstream; it appeared relatively straight and had been sufficiently cleaned. “Throughout the duration of the project, we were in touch with HOBAS field service group,” said Mahan. “They provided us with a lot of information and guidance.”
“Initially, the installation of the pipe itself didn't set any records for pace, but it was consistent and non-eventful,” said Omar Carrasco, HOBAS field service technician. “On day one, they installed 10 pipes. By the end of the first run, pipe installation rates were up to five pipes an hour, with the first 1000-foot run completed in only three days. The next run was inserted from the same pit, but downstream. There was one point, close to the end of the second push, where I thought they might have some difficulty, but [SAWS crews] used sand bags in the pipe to divert water to the annular space, lifting the liner pipe over the obstruction.”
Carrasco credits the crew's preparation and knowledge—and the pipe's attributes—for the rapid installation. He cited the consistent outside and inside diameter, and the pipe's dense wall and high compressive strength, which allows for high jacking capacity.