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).

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Eng. Onorio Francesco Salvatore

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