At Wavin, flotation calculations apply wide safety margins
The soil weight, more than the pipe weight, prevents plastic products from floating.
The specific weights of plastics are relatively low, which makes plastic products easy to handle and allows a simple and fast installation. But according to Archimedes’ Law, every body that is (partially) immersed in water is subject to an upward force, equaling the weight of the volume of water that it displaces. In order for the plastic product to stay in place, this upward force must be cancelled out by an equal counterforce.
Flotation calculations show that for below ground plastic sewer systems, the major part of this counterforce is due to the weight of the layer of soil that is on top of the system (or the surrounding soil in case of manholes). The weight of the product itself represents only a small part of the whole counterforce. Because the cover layer has a relatively high specific weight, the counter pressure is easily several times that of the upward force on the system – even in installations at shallow depths.
It is important to know that soil materials transfer the external forces they are subject to (gravity, pressure from the paving) at an angle. This is caused by the mutual shear angle of the sand particles. Thanks to this property of soils, a small horizontal rim at the bottom of a plastic manhole is sufficient to fix its position. This rim bears the load of a cone-shaped volume of soil, preventing it from floating in a situation of high groundwater levels.
The average shear angle (α) of most backfill soils is close to 33o. In the flotation calculations that Wavin performs, for example for sewers close to the surface, we assume a shear angle of 20o . This simulates a ‘worst case scenario’ (see diagram 1) and guarantees a very robust solution. Additionally, all elements of a sewer system are interconnected, creating another degree of below ground fixation. In short, no floating will occur in a soil-embedded sewer system under normal circumstances: it will last for a minimum of 100 years.
Diagram 1. A narrow rim at the bottom of a manhole is subject to the weight of a cone-shaped volume of soil.
More FACTS to boost your knowledge on this topic: Complete specifications of Wavin's Tegra manhole.
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