WHAT BENDS, DOESN’T BREAK

Flexibility is the key strength of plastic pipe systems

We generally see flexibility as a good thing, whether we’re talking about working patterns or joint mobility. When it comes to municipal sewers however, there’s often a preference for rigid pipes and systems. Generally, they’re seen as the tried and tested solution which offer robust resistance to traffic pressure, earth movement and damage. Flexible pipes, on the other hand, are largely seen as unproven and prone to deformation under pressure. But new studies of underground pipes reveal a different picture. Flexibility, it seems, is actually a hidden strength of plastic pipe systems, as XXX of TEPPFA explains…

The ‘Great Sewer Debate’ rages about the relevant merits of different materials. In one corner, there are rigid systems such as concrete, clay and cast-iron which point to strong structural strength and years of proven service. In the other there are flexible systems such as PVC-U, PP and PE, which highlight ease of installation, tightness and resistance to corrosion. While these factors explain the steady rise in popularity of flexible systems, doubts still linger. The reason? It seems to be simply that flexible systems are…well, flexible.

It’s a fact that flexible pipes bend – or deform – when they’re buried underground. It’s also a fact that rigid systems don’t. To many people, this is evidence that plastic simply isn’t up to the job – but evidence now exists to the contrary.

A recent independently verified study by TEPPFA compared the incidence of faults in over 1800km of buried pipes – both rigid and flexible. This revealed that the average number of defects in flexible sewers is 80% lower than that of rigid sewers – highlighting the fact that rigid pipes are unable to accommodate shifts in soil or obstacles in the earth. Flexible pipes, on the other hand, can more easily readjust shape without breaking.

However, it seems that, among sewer engineers, there remains a lack of understanding about how flexible pipes behave in real life, and that many are still concerned about deformation and its effects on performance and longevity. This prompted TEPPFA to undertake further research into the design of buried pipe systems, setting out to provide some valuable insight to help sewer engineers make informed choices.

Jointly sponsored by TEPPFA and Association of Plastics Manufacturers in Europe (APME), the research studied the deflection of buried pipes under different conditions. A 315mm solid wall pipe was chosen for the research because it represented a typical pipe application and allowed a wider range of pressure and non-pressure tests to be applied – including the ‘ring test’ and Internal Hydrostatic Pressure testing. To provide real-life insight, the pipes were buried on two different sites: one at the foot of a hilly area with sand and small amounts of silt, the other in a coastal location with a clay based soil, so that the differences between granular and cohesive soils could be compared. Finally, a variety of back-filling methods were applied, ranging from ‘poor’ to ‘good’ in order to assess what impact the quality of installation would have on deflection.

The results – independently validated by industry application experts – were informative. Firstly, even where poor installation practices were deployed, no failure or stability problems were recorded during the 4 years of the study. Secondly, the research demonstrates that deflections occur soon after the soil has settled, and once the maximum has been achieved, there is little further deflection throughout the service life of the pipe. Thirdly, that the depth of cover, and even level of traffic load, have no significant impact, while the key factor determining deformation is not the stiffness of the pipe, but rather the quality of the installation. Finally, even with poor installation, the expected deformation is well below nationally accepted levels and so there is a built-in safety factor with flexible pipes.

This research was consistent with other similar studies performed in operational sewers throughout Europe. For example, research in Sweden by Lars Eric Janson (Plastic pipes for sewage supply and water disposal) shows that PE and PVC pipes can be subjected to significant strain without risk of cracking. In Germany, Hoechst has undertaken 40 year tests of PE pipes without sign of cracks. And in a previous TEPPFA study, flexible pipes with an in-service life of up to 38 years were excavated from sites across Europe, and none displayed any loss of stabilisation.

All of these different research studies point to the same conclusion – that the deformation in flexible systems has no significant bearing on pipe performance. Equally, that the unique properties of plastic pipe systems, not least their natural strainability, means that they are more capable of resisting earth movement. Far from being a weakness therefore, flexibility is a key factor in the long-term effectiveness of plastic pipe systems.

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