Let’s face it, no one likes to talk about sewers — let alone its contents. But avoiding the talk has cost U.S. municipalities billions of dollars each year in the maintenance and repair of aging systems ill-suited to move our mess. While clay, concrete, and even Orangeburg piping systems have served us well in the past, they are starting to age and require replacement to meet demand in modern communities. With a range of cost-effective installation methods and high performance, the market conditions are perfect for polyethylene (PE) pipe to take over.
What’s holding us back? Some misconceptions about the availability and performance of PE pipe. This article attempts to address some common misconceptions about PE sewer pipes and explain why PE pipes are a superior choice for modern, cost-effective infrastructure.
Common misconceptions about PE pipes
Most misconceptions about PE pipes stems from the poor performance of existing PE systems that were not correctly installed due to inexperience with the material. While other systems may be more straightforward, the design and installation of PE pipes require a bit more finesse as design considerations like system economics and local conditions affect how the pipe behaves under service conditions.
For example, a low DR PE pipe of 7.3 will have pipe stiffness of about 1600 psi and will have higher resistance to deflection. This pipe will be capable of handling load, requiring less support from the soil and embedment. On the other hand, a higher DR pipe of 32.5 will have a pipe stiffness of about 12.5 psi and will be less resistant to deflection. The higher DR PE pipe depends more on the quality of the embedment.
In other words, the wide range of flexibility in PE pipes means different installation requirements for different DR’s may be necessary to achieve successful and economical installations (1). Different installation requirements also explain another common misconception about PE pipes: that depth of the cover matters and must be a certain value in every case. In reality, the depth of cover for PE pipes varies and can be between 2.5 feet and 16 feet (at SDR equal to or less than 26). But smaller diameter pipes can be installed at very shallow depths without much change to pipe functionality or performance.
Another misconception is that PE pipe is prone to heavy deflection under traffic. While PE pipe is known to be very flexible, the surrounding soil actually provides the significant portion of deflection resistance as with standard pipe materials. In these cases, load force is transferred from the pipe to the surrounding soil. For this reason, it is important to pay close attention to the local soil, the embedment soil, and the placement of the embedment soil to provide support beneath and on the sides of the pipe. As the pipe flexes, the surrounding embedment soil provides support that can yield to the changes in the pipe while also maintaining the system’s integrity. This pipe feature is especially important in areas prone to earthquakes.
While steel pipes offer some deflection and can also provide a measure of “earthquake resistance,” PE pipes are rated to deflections of up to 7.5% (compared with 2% in steel pipes, 2). Concrete pipes are rigid and are not capable of deflection before failing.
PE pipes for modern infrastructure
PE pipes are especially well-suited for infrastructure due to their involvement in a wide array of applications and their cost effectiveness. For instance, pipe joint seals in old water and sewer systems are starting to break down, causing leaks and water loss that costs municipalities millions each year. In contrast to these older systems, PE pipes are capable of having fused joints that do not leak, eliminating infiltration and inflow problems. The process uses heat fusion to form a leak-free joint that is as strong as the pipe. Due to the low leakage rates (for PE pipes, the “allowable water leakage” rate is zero), PE pipes possess a long lifecycle (3).
In addition to leak resistance, PE pipes are impervious to most chemical and biological agents. The pipe will not corrode, tuberculate, or support biological growth, and is resistant to aggressive soils (low pH and high concentrations of chlorine and sulfates). Additionally, the pipe demonstrates abrasion resistance, making it well suited in sewer applications that involve the transportation of solids.
Finally, PE pipe systems can be constructed using cost-effective installation techniques such as Horizontal Directional Drilling, Pipe Bursting, Sliplining, Plow and Plant, and Submerged or Floating Pipe. These techniques can save considerable time and money in most applications. The pipe’s flexibility, low density, and light weight speeds up installations and reduces equipment requirements.
Sourcing large-diameter PE pipes for large-scale sewer applications
For decades, engineers in the United States only had access to large diameter pipes made of other materials. Today, you can use some of the world’s largest PE pipes in your water and wastewater applications. AGRU America manufactures a PE piping system that is available in sizes of up to OD 2500 mm (98.4 in) and made from PE 100 and PE 4710. Pipes of these sizes are for perfect for high-volume flow applications, offering superior long-term hydraulic properties and operational cost savings. Other benefits include a high resistance to corrosion, wear and tear, and UV radiation.
- “Underground Installation of PE Piping.” PPI. Accessed September 30, 2018. https://plasticpipe.org/pdf/chapter07.pdf.
- F. Gozarchi, “Comparison of deflection measurement methods of large diameter steel pipes with control low strength material.” Accessed October 8, 2018. https://rc.library.uta.edu/uta-ir/bitstream/handle/10106/24487/FarrokhiGozarchi_uta_2502M_12736.pdf?sequence=1&isAllowed=y.
- “Sewer Force Main Benefits – HDPE Pipe.” Accessed September 30, 2018. https://plasticpipe.org/municipal_pipe/mid-sewer-force-main.html.