The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Buried pipelines I – rigid versus flexible pipes and pipelines

Written By: Francesco Salvatore Onorio - May• 19•12

There are many materials on the pipelines market. Each material has pros and cons that a structural engineer need to evaluate. A pipe must have strength and/or stiffness.

Strenth is the ability to resist stress, stiffness is ability to resist deflection. Stresses and deflections are caused by soil loads, live loads, differential settlements, internal pressure, longitudinal bending and so forth.
Relating to these features, pipes could be divided in rigid or flexible. Are flexible pipe those that can deflect at least 2 percent without structural distress. Pipes that do not meet this criterion are usually considered to be rigid.

The reality could be a little different: pipes could be neither flexible nor rigid, but, for calculation purposes, they fall in the first or second category. This is the code approach, worldwide.
Let’s do some examples: steel and plastic pipes are generally flexible, while concrete and clay pipes are rigid.
Structural checks are related to strength or stiffness according to the the pipe category. In rigid pipes we need strength to resist wall stresses due to external and internal loads, for flexible pipes we need stiffness in order to resist ring deflection and possible buckling.

But how can we classify a buried pipeline as rigid or flexible? The behaviour of a buried pipeline will depend very much on how its stiffness compares with the stiffness of the native soil in which it is to be buried. Although traditionally “rigid” materials are thought of as being concrete, clay and asbestos cement, and “flexible” materials are thought of as being plastics, the differentiation is not as simple as this.
The response of the pipes under load will be largely dependent on the behaviour of the native soil. If the pipes are of medium stiffness (say 20000 MN/m²) and buried in a stiff soil (such as a dense gravel of Es soil modulus = 150 MN/m²) then the pipes will exhibit predominantly “flexible” behaviour, so they will tend to deflect on loading.
If, however, the same 20000 MN/m² stiffness pipes are buried in a soft soil (such as a very soft clay of Es = 5 MN/m²), then the pipes will exhibit predominantly “rigid” behaviour, so they will tend to settle into their foundation on loading and we need to check stresses.

The first description of the behaviour of buried flexible pipelines under loads was given by Clarke in 1897 (“The Distortion of Rivetted Pipe by Backfilling“, Proc. ASCE, 1897).
Most of the research work in the next 40 years was on the behaviour and design of rigid pipelines in the cross-sectional direction, most famously by Marston, Spangler (“Underground Conduits: An Appraisal of Modern Research“, Trans. ASCE 113, 1948) and Schlick.
The first analysis of flexible pipe behaviour taking account of both soil and pipe stiffness was published by Lazard in 1935 (“Ouvrages Circulaires Placée en Terre”, Travaux 33, 1935).

For guidance, reports and more you can contact the author at:

onorio@strutturista.com

Eng. Onorio Francesco Salvatore

You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

5 Comments


  1. […] post I wish to present you a software for buried pipes / buried pipelines classification. In “Buried pipelines I – rigid versus flexible pipes and pipelines” we talked about rigid and flexible pipes and we’ve seen how different are this two […]


  2. I have read so many articles concerning the blogger lovers except this
    paragraph is in fact a nice article, keep it up.


  3. Hello I am so glad I found your weblog, I really found you by
    mistake, while I was searching on Bing for something else, Anyhow I
    am here now and would just like to say thanks a lot
    for a fantastic post and a all round enjoyable blog (I also love the
    theme/design), I don’t have time to look over it all at
    the minute but I have saved it and also added in your
    RSS feeds, so when I have time I will be back to read a great
    deal more, Please do keep up the superb b.


  4. Hi there! This post couldn’t be written any better!
    Going through this article reminds me of my previous roommate!
    He constantly kept talking about this. I’ll forward this
    article to him. Pretty sure he will have a great read.

    Many thanks for sharing!


  5. Magnificent beat ! I wish to apprentice while you amend your site, how can i subscribe
    for a blog website? The account aided me a applicable deal.
    I have been a little bit acquainted of this
    your broadcast provided brilliant clear concept

Leave a Reply

Your email address will not be published. Required fields are marked *