Non-Round Sewer Pipelines – What to Do When It’s Time to Rehabilitate
October 20, 2015Different shaped pipes have been used for centuries to help facilitate low flow velocities in sewer systems. These non-round sewer pipes were traditionally used in large cities where pipelines carried sewer and stormwater together. These pipes had the capability to maintain reasonable cleanout velocities for the sewer effluents during times of dry weather, but also had the capacity to carry stormwater during significant rain events.
These non-round sewer pipes are located all over the globe and the performance of these pipelines are incredibly important to each municipality.
More specifically, many of the largest cities in the United States — such as New York, Boston, Washington D.C. and Los Angeles — have these types of sewer pipes in its systems. These pipes were traditionally constructed of unreinforced concrete and bricks or clay tiles. The majority of were handcrafted manually by arguably some of the most skilled masonry laborers to create very specific profiles to achieve the necessary hydraulic conditions for these cities.
Many of these sewer lines were constructed in the early 1900s and with continued city expansion and developments, some of these sewers do not follow the public right of ways and even cross underneath existing buildings and residential housing. Due to the corrosive nature of a sanitary sewer environment, these sewers are now approaching the end of its service life and are in need of structural rehabilitation.
With the constraints of public disturbance of digging and replacing the pipes, many city engineers are exploring new methods of restructuring with corrosion-resistant materials. The difficult challenge that each engineer faces is coming up with rehabilitation solutions that not only create a new pipeline structurally, but it also has to be able to maintain capacity of the existing sewer.
In Los Angeles, the existing sewer conditions are continuing to deteriorate. Many of the decaying sewer pipelines in its collection system are made up of non-round brick and mortar pipes. The City is rehabilitating a number of non-circular sewers using methods, which are common in the industry, but using pipe materials that are new to the City.
As part of the design of a recently awarded rehabilitation project, the City specified the sliplining construction method using fiberglass pipes (Reinforced Polymer Mortar Pipe). The choice to use a slipline installation method was due to the existing flow conditions that eliminated that possibility of bypassing pumping. Los Angeles has used the slipline method of rehabilitation for its sewers for decades with round fiberglass pipes. It is recently where the City has started to use non-round fiberglass pipes for these sewer rehabilitation projects.
For this non-circular sewer rehabilitation project, the pipes are manufactured using a filament wound manufacturing method. The product is manufactured by winding resin saturated continuous fiberglass filaments around a mandrel along with the addition of chopped glass fibers and silica sand. The end result is a very dense laminate that maximizes the contribution from all three basic raw materials. The continuous glass fibers along with the chopped glass fibers are incorporated for high hoop strength and axial reinforcement. The sand component is used to provide increased stiffness to resist external loading from the soil above the pipe and ground water pressures. This unique manufacturing equipment allows for the application of special liner resins which can be modified depending on the performance requirements of the project.
For manufacturing, the mandrel is custom fabricated to match the profile of the existing non-round sewer pipe. Using this manufacturing method, the pipe can be fabricated in practically any profile needed to match the shape of the existing sewer pipe. The profile of the mandrel is sized to allow a gap between the host pipe and the new fiberglass slipline pipe. Once the pipe is installed, this space is then filled with cellular grout through injection ports installed in the wall of the pipes.
The non-round fiberglass pipe are designed and manufactured in accordance with ASTM D3262, which is the standard specification of fiberglass sewer pipe. This standard requires that the pipe is corrosion resistant to combat the corrosive environment of an active sanitary sewer system. In addition, the pipes are designed to handle the axial loading conditions that are anticipated during the installation process. For slipline installations, the pipes are typically pushed into place in one continuous string of pipe. This requires that the pipe has the necessary allowable pushing forces to install the pipe inside the existing sewer pipe. One unique aspect on the exterior surface of the pipe is the “slides” or guides that are located on the lower quadrants of the pipe. The purpose of the “slides” is to help prevent rotation of the pipe material during the installation process.
The non-circular pipe is designed with a bell and spigot joints for connecting each segment. This gasketed joint system provides flexibility and sealing to prevent exfiltration of the sewer and infiltration of groundwater into the system. The bell of the joint system is also flush with the outside wall of the pipe to create a uniform pipe barrel from pipe segment to pipe segment.
The pipe manufactured for this project is Amiren non-circular fiberglass supplied by the Thompson Pipe Group. A total of 1,700 lf of the non-circular fiberglass slipline pipe has been delivered to the jobsite in 9-ft section lengths. Installation of the material is planned to begin this month and it is anticipated that all of the material will be installed over the next couple months. By using this product, the City is able to retain the original cross section and minimize the loss of hydraulic capacity while effectively rehabilitating the existing sewer line to prevent future failures.
Jeffrey LeBlanc, P.E. is director of engineering at Thompson Pipe Group, Zachary, La. Since 2004, LeBlanc has worked in the design, manufacture and installation of pipelines for water, sewer and hydropower utilities.