18 Oct Buried Pipes – Rigid vs Flexible
Plastic pipes are flexible and although they can support certain external loads by themselves, their real behaviour derives from their deformation as they take on the lateral passive pressure of the ground that surrounds them. As rigid pipes can’t be deformed, they have to be able to fully support the loads themselves, meaning the possibility of a break is higher.
Flexibility is considered a physical weakness and priority is often given to rigid pipes which are always more robust and rigid than the surrounding ground, concentrating the loading in the pipes. With movement of the ground this loading has the tendency to increase. Flexible pipes don’t have to support the extra loading. They deflect and transfer the load.
In the case of flexible pipes both the pipe and the ground surrounding it absorb the loading. This means that with good ground compression the depth of the line is almost unlimited. A flexible pipe can absorb deformations by settlement of the ground without affecting its tightness, however, in a rigid pipe, the system can’t be deformed because it could break.
Many engineers have experience in the utilisation of rigid pipes and still think that all buried pipes must be sufficiently robust enough to support all the loading of the pipe installation. This is true (and a problem) for rigid systems. Plastic pipes use a different approach. They deliberately have an inferior rigidity to the surrounding ground, with the aim of passing loading to the ground and creating support from the fill.