Marcel Roelfsema is a technical advisor at Wavin in The Netherlands. He is a specialist in the technical and behavioural aspects of plastic (PP, PE, PVC) sewer materials and components such as pipes, inspection chambers, manholes, and separator systems.

Would a plastic sewer system be at risk of floating when the groundwater level rises? Pipe flotation calculations show why not.

“In our municipality, quite a large part of the sewer infrastructure needs to be replaced soon. We are open to using plastic, but are not completely sure that it would stay in place due to the low pipe weight. Wouldn’t a plastic sewer system be at risk of floating when the groundwater level rises? I mean, is the pipe or manhole weight sufficient to withstand flotation, or is the weight of the layer of soil that covers it essential?”

The hypothetical quote above illustrates the sort of question we often hear when talking to municipalities about new sewer systems. Marcel Roelfsema, technical advisor at Wavin, explains why one doesn’t have to worry about pipe flotation. “When we’re initially designing storm or foul water sewer systems – which consist mainly of pipes, manholes and inspection chambers as well as attenuation tanks or oil separators – we always take the floating issue into account. First of all, it is important to know that a plastic sewer system isn’t just loose in the ground, it’s always connected to other pipes, inspection chambers or manholes for example. That helps to keep it in place.

On top of that it is important to know that for an installed and backfilled system to actually ‘float’ – or exhibit upward movement – the upward forces must overcome the weight (counterpressure) of the system. The weight of a system is determined by not only the weight of the sewer system itself but also by the weight of the material surrounding it, especially the layer of soil covering the pipe (usually between 50 cm and several metres deep), and the road surface or pavement. 

Don't forget the shear angle 

The downward force on a pipe (the fore that counteracts floating) depends on a number of variables, inclusing the shear angle α.

While the specific weight of the layer of soil covering the pipe (or a layer of gravel, coarse stone and sand) contributes significantly to the stability and fixation of sewer systems in the ground, the shear angle (α) of the soil is also crucial for the long-term flotation-preventive counterpressure to the system. The shear angle ensures that any upward-directed flotation forces are spread along the width under this specific ‘shear’ angle. In this way, the effective weight of the cover layer increases as it is not only the weight of the soil vertically above the system but a wider projected soil layer that contributes to the counterforce. The shear angle for most aggregate soils is between 30o and 35o, but in my pipe flotation calculations for sewer systems, I assume 20o in order to build in extra safety and to allow for uncertainties. 

More FACTS to boost your knowledge on this topic: How to calculate pipe flotation...

Prevention is better than cure

Here at Wavin, we are aware that extreme conditions can prevail as a result of climate change etc. Excessive levels of water can change everything under the surface – such as a steep rise in ground water levels, soil erosion, subsidence and other factors. We are trying to get ahead of the game by setting up a preventive and sound storm water management. This allows extreme amounts of rainwater to be drained in the best possible way.”

More FACTS to boost your knowledge on this topic: How to avoid flooding in urban areas?