By Dr. Patricia D. Galloway, P.E.
Many engineers do not understand or appreciate the differences between reinforced concrete pipe (RCP) and high-density polyethylene (HDPE) pipe and the potential for liability when specifying each product. While RCP is a rigid structure that is designed, built and tested as a structure before it arrives at the construction site, the structure of HDPE pipe is actually built and tested in the field. Therefore, HDPE pipe is not an “approved equal” substitute for RCP.
Corrugated HDPE pipe, rather than RCP, is typically specified by design engineers as a result of assertions by HDPE pipe manufacturers regarding its lower cost and superior attributes. In some cases, HDPE may be a reasonable alternative to RCP, depending on the specific project requirements and design life. However, designers must be cognizant of all aspects and design responsibilities of using any pipe material before specifying it, because materials and their service lives may differ.
Some ASTM/AASHTO specifications place significant responsibility on the engineer regarding installation essentials to ensure service performance. Applicable sections in ASTM D 2321-04 include requirements placed on the engineer with regard to installing thermoplastic pipe. Engineers also must recognize that HDPE pipe requires an engineered installation in which the engineer must be involved in the construction activities. The engineer must take responsibility to ensure that post-installation deflection testing has been performed and documented. Because the prime structure in an HDPE pipe system is the soil envelope and the pipe-soil interaction can account for up to 90 percent of an installation’s success, it is critical that this determination (interaction between pipe, embedment material and native soil) be made by a geotechnical or soils engineer.
Dr. Patricia Galloway, P.E., is CEO of the Nielsen-Wurster Group and past president of ASCE. This text is an excerpt reprinted with permission of Galloway from an article entitled “Read between the Pipes – Concrete pipe offers safety benefits when used in storm sewer systems,” copyright Scranton Gillette Communications Inc.
