Pipe stiffness is likely the most commonly referenced and least understood performance parameter in the pipe industry.

When most people think of pipes or try to describe pipes, they first describe it by the material it’s manufactured with (i.e. concrete, HDPE, steel, PVC, etc.).  They may also describe it by some physical attributes such as by it’s joint type (i.e. bell-spigot, welded, flanged) or whether it’s a solid wall (SDR) or profile wall.

However, none of those items would be considered properties that would describe the pipes performance.  Quantitative descriptions of performance properties are harder to identify.  If the pipe is designed for pressurized usage, then the pressure rating would qualify as one of those terms.  However, most pipes don’t have a pressure rating.

That’s likely why pipe stiffness has been latched onto by many engineers as a property that is intended to describe the performance of a pipe.  And pipe stiffness does describe the performance of a pipe – sort of. 

Pipe stiffness measures the load required to deflect an unsupported pipe a distance equal to 5% of its diameter at a specific temperature and at a prescribed rate of loading.  Period.  Pipes with high stiffnesses can resist deformations during the handling and installation of the pipes, but they have little impact on pipe performance once the pipe is installed.

Understanding the pipe stiffness of a pipe is good information to know, but because it’s one of the few quantitative performance measures available, the relevance of the measured stiffness is sometimes overstated.  For instance, some people believe that a pipe with a higher pipe stiffness can carry more load than another pipe with a lower pipe stiffness.  This is not true.  There are plenty of examples of pipes with lower measured pipe stiffnesses that can carry higher cover depths than similar pipes that have a higher pipe stiffness.  Corrugated steel pipes can support fill depths much deeper than plastic pipes, but their pipe stiffnesses are frequently similar to the plastic pipes.

There is also some thought that a pipe with a higher pipe stiffness will perform better over time as compared to a pipe with a lower measured pipe stiffness.  This is technically true, but the degree of difference in performance is so slight that it is essentially negligible in most cases.  Highlighting the limited impact that pipe stiffness has on long term performance of pipelines was the topic of an article I recently wrote for Informed Infrastructure.  In the article, I provide quantitative evidence of the limited impact that pipe stiffness has on long term pipe performance.

Pipe stiffness is a meaningful property for pipes.  A pipe needs to have enough stiffness to resist the transportation, handling and installation of the pipe without deforming excessively.  However, pipe stiffness has little impact when describing the strength of the pipe in carrying soil loads or on the long term performance of the pipe.

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Author Biography

Darrell Sanders, P.E., is chief engineer for Contech Engineered Solutions. He holds a BS degree in Civil Engineering from the University of Cincinnati and an MBA from the University of Dayton. He holds a Professional Engineering license in several states. Sanders is a member of several industry committees, including NCSPA, AASTHO, ASTM, and CSA. Darrell can be contacted at dsanders@conteches.com.

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Thursday, March 16, 2017 11:49 AM
Pipe stiffness is an important indicator of a pipe's brute strength. That is, how does it perform without a soil envelope? In that sense, stiffness is an indicator of its dependence on the quality of the soil around it. Flexible pipes often rely on the soil to supply as much as 90% of is structural performance. I crawled through miles of ADS' N-12 pipe years ago and saw first-hand how an 'easy-to-install' pipe performed shortly after installation.

That's why our firm designs a high-quality, high stiffness soil envelope around larger-diameter plastic pipes, and we inspect the installations full-time.

I started my career at Armco, and spent 8 years in the concrete pipe industry. Ironically, we do not allow concrete pipe (my decision) for sanitary sewers because we've had too many hydrogen sulfide failures. Toward the end of my tenure in the concrete industry, I developed a presentation tilted "Plastic pipe will work--let me show you how." That presentation form the basis of our current plastic pipe specs.
Friday, March 24, 2017 4:48 PM

Thank you for your comment. It sounds like you approach the topic of structural backfill in the proper manner. It’s interesting that you used the term ‘high stiffness’ when describing the soil envelope you specify around large diameter flexible pipes, because that is the primary characteristic we’re trying to achieve when we describe the backfill requirements for pipe installations. Typically, we describe the backfill through its physical characteristics such as gradation, moisture content, Atterberg properties and required compaction level. However, what we’re really doing is describing a backfill envelope that, when in place around the pipe, will provide enough stiffness to resist the overburden loads to which it will be subjected during its service life. At the end of the day, it’s a soil modulus and bearing capacity that we’re trying to achieve in the structural backfill zone. Once that is achieved, the ring stiffness of the pipe has little to do with the pipe performance.
Thursday, February 1, 2018 9:35 AM
Darrell, I've seen what is tantamount to an all-out attack on synthetic pipelines being waged by the concrete pipe industry, using the California wildfires and fire resistance criteria as the basis of their attacks. Being from Florida, where in my area we receive almost 67-inches of rainfall a year vs. perhaps 12-inches a year as I think is case in Southern California, I had to lamblast the author. But what would a major manufacturer the likes of Contech have to say publicly about pipes that the accusers say "melt during Califirnia wilfires"?
Tuesday, February 6, 2018 12:17 PM
While it is certainly true that high temperatures impact thermoplastic pipe materials (HDPE, PP, PVC) more than others (RCP, CSP), that alone doesn’t constitute a reason to select one pipe material over another. As you point out, there are many other factors that should be considered. Among the most important of these would be the potential likelihood of a major wildfire event. Unfortunately, we’ve seen some of these occur across portions of the western US over the past few years, but that doesn’t mean that the same potential exists in other areas.

Secondly, I don’t know that I’ve seen much in the way of legitimate reporting of the actual damage or extent of the damage to buried culverts as a result of the wildfires. I’m aware of at least one press release from the ACPA that pertains to culvert damage due to a wildfire in west Texas from a few years ago, but I think the bias in a report such as that is self-evident. I would be far more interested to know the reports of damage to pipelines from a more independent source that can put the real impact of the actual issue into a clearer focus. Are culverts actually burning in a manner that causes roadway collapses or is the damage largely limited just to the exposed ends of the pipes? How many of the lines require repair versus just sustaining some superficial damage? I haven’t seen anything that provides that level of detail on the subject.

Finally, even if this were deemed to be an issue that an agency wanted to address, I believe there would be other approaches that could be taken to help mitigate the potential impact of a wildfire event. It would make sense to believe that reducing the exposure of any protruding ends of a culvert or pipeline would help protect it from the potential impact of a wildfire event. That means that non-flammable end treatments such as steel end sections or concrete or steel headwalls could be used in lieu of protruding pipe ends.

I hope this discussion of the topic is helpful. Thanks again for your question.

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