Material properties required for shallowly buried pipes

Brabant Water has developed the concept the Tertiaire Net van de Toekomst (TNT) (Tertiary Network of the Future). According to this vision, the tertiary network will be installed differently in the future, in part by burying the pipes two-spades deep (60 cm). This shallower installation depth saves time and money. It also means the pipes have to meet other requirements in terms of thermal insulation and strength. In this project we researched what material properties the pipes needed in this context, and whether they could be realised in practice.

Thermal insulation and mechanical requirements studied more closely

We established the thermal insulation needed for the pipe material so as to reduce the chance of freezing or of overly high temperatures in the pipes at different ground temperatures (0.25 ºC and 28 ºC). The temperature of the ground around the pipe is the main determinant of the temperature of the drinking water. At the depth of 60 cm, the 25 ºC threshold is exceeded during a long period of time and the frost boundary is close by. Insulation can only delay the heat transmission; when we take the residence times in the network into account, we arrive at higher insulation values than those of standard materials.

We then studied whether these required insulation values could be realised in practice by adding a PUR layer around a PVC pipe. As an alternative, we looked at the possibilities of keeping the ground temperatures from rising so much in the summer, for example, by planting shrubs and thus increasing the evaporation. We also studied the frost penetration in the ground and its relation to pipe material degradation. And we formulated requirements regarding the materials’ strength, and assessed whether PVC and PE pipes meet them.

Frost damage not avoidable

At a depth of 60 cm, ground temperatures will regularly exceed 25 ^oC; this will occur even more frequently under future climate scenarios. There are means available to partially limit the warming of the drinking water, for example, by insulating pipes with PUR; but the pipes’ diameter will be greatly increased and there is no way of guaranteeing the 25 ºC limit. Moreover, the shallower installation means that the influence of external loading will be greater. The strength requirements can be met in PE pipes; for PVC pipes this is only the case when there is no traffic loading. In addition, at 60 cm, the risk of pipe freezing is very real: something that no available pipe material can resist.