The Trinity River Authority of Texas (TRA) is the largest wholesale provider of wastewater treatment services in Texas. The conservation and reclamation district provides water-related services throughout the 18,000-sq mile Trinity River basin to 60 cities and millions of residents.
In April 2012, TRA awarded a contract to rehabilitate more than 17,200 ft of 96-in. sewer pipelines in the City of Irving. This is one of the single largest projects completed to date for cured-in-place-pipe (CIPP) in the world. The project consisted of nine segments of the 96-in. diameter TRA Elm Fork Interceptor (CAC-11) that needed rehabilitation due to concrete degradation from exposure to hydrogen sulfide (H2S). The nine segments averaged approximately 2,000 ft each and were separated into 18 separate installations over the course of the project.
The CIPP portion of the project was designed to meet the criteria of ASTM-1216 in accordance with the depth, diameter and flow requirements of the system. Notice to proceed for the project was issued on May 29, 2012, with a contract completion date of spring 2014. As of June 2013, all nine segments have been completed, almost one full year ahead of the original anticipated completion date.
Four types of rehabilitation were originally specified, including reinforced CIPP, non-reinforced CIPP, and two different types of spiral wound rehabilitation. Fiber reinforced composite was selected for the project. Depths of the inversions ranged from 13 to 26 ft. With full-depth groundwater assumed for design, the resulting thickness requirement was 35 mm, approximately 40 percent thinner than traditional CIPP due to the use of fiber-reinforced felt tubing. A traditional CIPP project of this length and size would have required nearly 5 million lbs of resin. Despite this 40 percent reduction in resin, the project still required more than 3 million lbs of resin.
There was a limited window of opportunity for completion for portions of the project. While undeveloped park land allowed for continual access during the project construction period, a public golf course and two developed, heavily used park areas only allowed for a three-month window for rehabilitation work to minimize socioeconomic impact. If not completed in the restricted window, the contractor would have to wait for the same limited months the following year. In addition, pipeline cleaning in heavy recreational areas was limited to nighttime cleaning only.
There were a host of other project challenges. Access to the project involved crossing concrete hiking and bike trails, which required heavy steel plating for trail protection, as well as flag personnel when trails were in use. In addition, limitations on the maximum length of CIPP installations could not exceed 1,200 ft due to a study of past U.S. projects. This required two insertions from opposite directions for each 2,000-ft pipeline segment, with overlapping sections of spliced CIPP. Sound and visual barrier walls were also required at a height of 30 ft to keep noise below 65 decibels at all installation points in and near park and golf course areas.
Large-diameter CIPP installations present a variety of challenges, including an increase in wall thickness proportionate to diameter, more weight due to the amount of resin needed, extended cure time, large installation equipment and the need for an onsite wetout facility. The CAC-11 project is one of the largest CIPP projects ever completed, requiring 3.24 million lbs of resin delivered to the job site in 72 tanker trucks.
Manufacturing, handling, logistics, wetout and installation were all challenging due to the size of the 96-in. tubes. The use of fiber-reinforced CIPP, relatively new technology in the industry, saved nearly 1.8 million lbs of resin. Beyond the project size, major challenges include the project’s location beneath environmentally sensitive areas containing wetlands, an active flood plain with regular flooding during wet weather, and several public areas in the City of Irving, including a golf course, recreational area with playing fields and concrete trail system required to be kept in service.
Nearly half of the project was subject to constraints to protect surface usage during construction. The golf course, for instance, stipulated a three-month winter window to complete six insertions. If the time frame could not be met, the contractor was required to come back in 12 months within that same time period. Further constraints included odor control, 30-ft high visual screening, sound containment, and noise monitoring to minimize community impact.
From an environmental perspective, wetland and tree clustered areas were cordoned off from construction, resulting in strict access limits enforced with protective fencing prior to construction and temporary roads for access. The floodplain of the Elm Fork of the Trinity River, which encompasses the majority of the project, was analyzed to predict flood levels during several wet weather events. A real-time weather monitoring system was developed to reduce impacts particularly during the contractor’s non-interruptible construction, such as the five- to six-day insertion and curing process. This weather monitoring system has since been recognized by the National Hydrologic Warning Council for its innovation and was credited with early warning of two flood events during the project. In addition to external flooding potential, the pipeline was subjected to periodic wet weather flow surges internally that could not be diverted, interrupting construction and requiring coordination with system operations throughout the project.
Due to extreme concrete loss (more than 50 percent of the pipe wall) in portions of the project, designs included installation of new reinforced concrete prior to rehabilitation, a technique for adding strength with minimal CIPP wall thickness while eliminating the possibility for resin fins due to large variations in the host pipe inside diameter. At completion, the project was deemed successful at a cost of $16.5 million, nearly $3 million below engineer’s estimate and $800,000 below the low conventional CIPP bid, with several accomplishments given the linear footage and diameter, including a 40 percent reduction in resin usage, completion time nearly one year ahead of schedule, and an increase of the total U.S. footage of 96-in.-plus CIPP by over 350 percent.
Project Owner: Trinity River Authority
Engineer: RPS Espey
Contractor: Insituform Technologies LLC