We have already seen that when a heating network is active, the pipes are moving. This therefore gives rise to a phenomenon of friction between the surface of the pipes and the material with which they are in contact. To allow the tubes to expand, it is necessary that they are surrounded by earth which allows for movement with controlled friction. For this reason, the ideal type of soil is to use sand. However, if we want to minimize abrasion and indentation, we cannot just choose any grain size, since:
– If the grain used is too large, the contact area between the pre-insulated pipe and every grain will be reduced. This causes a risk of puncturing the sheath HDPE, which could create leaks.
– If the grain used is too small, the reaction with soil moisture can create sand agglomerates which may block the movement of the tube. If the network cannot expand, this will cause excessive stress and breakages may be observed.

Therefore we must use sand with controlled composition; the grains must be of different sizes. To accomplish this we must use granulometry, a method of analyzing the statistical distribution of the size of sand grains.

In the case of heating systems, the suitable particle size is a standard mixture of sand grains measuring between 0 and 4 mm in diameter having a statistical distribution is given below by a particle size curve.

In addition, certain products assimilated to sand such as metal or mines residue , are viable provided they observe the homogeneous particle size.

Image source: Flickr (Mosieur J. [Version 9.1])