Wrong concepts about plastic pipes
→ The deformation increases in deep installations and with traffic loads
→ The ring stiffness of the pipe is the factor which determines its resistance
→ The loss of the ring stiffness of the pipe with the time, reduces the capacity of enduring the load
→ It is needed a long design method to predict the structural behaviour
→ The flexible behaviour is an advantage
→ The deformation of the pipe produces loss in the capacity of load and in tightness
TEPPFA y APME (PlasticsEurope since 2005) performed a long development project focused on these arguments
Only there is a point in which it can be affirmed that conventional rigid materials for buried pipes have an advantage in comparison with plastic pipes and it is that most consultants are more used to them. Many engineers, even among the more qualified ones, will avoid to specify a system of flexible pipes, simply because they have doubts about their flexibility. They prefer what they consider a good quality, the “strength” of the rigid pipe.
However, if these engineers stop a second to think, they will realize the mistake that is hidden in this way of thinking and besides, they don’t take into account other important effects, like corrosion, incrustations and abrasion in metallic or cement pipes.
The loads that have to support the wings of an aeroplane or the rotatory wing of a helicopter to elevate in the air only can resist if they are flexible. If they were rigid, they would break before they supported themselves in the air.
The engineers specialized in structures know that a high building is flexible, although any person can climb to the top of a skyscraper during a gale and confirm how it moves. The Eiffel Tower experiments displacements up to 130 mm with strong winds and, of course it is not about the highest or the most flexible structure of the world. The highest suspension buildings of the world have a grade of flexibility that is essential for their survival.
The quality of the settling bed is essential for the function and durability of the pipes of any kind of material. Rigid pipes transmit the loads directly to the ground in the points of settlement. This increases the pressure on the ground, and depending on the stability of the kind of ground, influence of the movements and deviations of the pipe.
Installed flexible pipes follow the movements of the ground. The loads in the superior part of the pipe (weight of the filling, traffic, etc.) are transferred to the ground in all the width of the dig and not only in the contact pipe-ground, with minor movements of the pipe and ground. In a test, it was measured the movements of two sanitation pipes, a ceramic one and another plastic one, both installed in the same conditions. The deviation measured from the initial deformation, 4 years after being installed, was around a 50% higher in the rigid pipe.
Understood the supply of loads between pipe-ground and observing the movements of the joining of the buried pipes, flexible pipes are the best option for durability and functionality of the pipe systems. The reports of researches performed about the ground support and confirm these conclusions. In short:
“Flexibility = Resistance”
>>> I don’t want to say goodbye from this post without talking about the compacting of the filling material around the pipe and above all, in the cores so that the pipe works well during its useful life of about 100 years, but let us not deceive ourselves, this compacting is necessary for any pipe of any kind of material. We will talk about it in another post. If you want, you can leave a comment about this subject.