John Kurdziel, Michael Pluimer
Papers # 2018 Las-Vegas
Corrugated HDPE pipes have been successfully used for decades for sewers, culverts and land drainage applications and are advantageous due to their durability and service life in a variety of installation conditions. Pipes used for highway and storm sewer applications typically require stricter material standards than pipes used for agricultural and other land drainage applications since the service life demand for highway and storm sewer applications is generally greater than that for agricultural applications. The primary service-life limiting factor for corrugated HDPE pipes is stress cracking via the slow-crackgrowth mechanism. Since stress cracking only occurs in tension, it is important to limit the magnitude of tensile stress in the pipe wall. For highway and storm drainage applications, this is done by installing the pipes with well-compacted structural backfill materials and conducting post-installation inspections to ensure deflections are limited to 5% or less. For agricultural and storm drainage applications, tensile stresses can be limited by using shaped trenches that conform to the radius of the pipe and by ensuring the pipe is free from rocks and large clumps of soil that may cause local impingements.
Contrary to most adopted perceptions on the use of recycled HDPE materials in pipe, it is not necessarily the high-end applications – such as sanitary sewers, storm sewers and highway culverts – that require the most engineering scrutiny. Pipes for these applications are typically placed in extremely well-monitored installations with excellent backfill, which results in relatively low stresses on the pipe wall. Land drainage applications, on the other hand, are often placed in poor soils with little or no field inspection. Although this may not be of much concern in a farm field with shallow installation of 100-mm (4-inch) pipe, it is quite a different situation with agricultural trunk lines that can include up to 1500-mm (60- inch) and are often installed in the same manner and with the same backfill materials as small-diameter farm field drainage pipe. The pipe wall stresses in these applications can greatly exceed those in sanitary and storm sewer applications. Since high tensile stresses in the pipe wall can lead to stress cracking in corrugated HDPE pipes, it is important to design the pipes with materials that are resistant to slow crack growth initiation and propagation, both for pipes manufactured with virgin and recycled materials. This paper examines the types and magnitude of stresses associated with various installations and loading conditions and provides recommendations for material performance properties for these pipes, both regards to mechanical properties as well as stress crack resistance. Minimum performance requirements for the un-notched constant ligament stress (UCLS) test are provided for pipes manufactured with recycled materials based on the desired service life and service conditions for the pipes. It also offers suggestions for the required testing protocols to achieve and validate these materials.