Stiffness is the property of pipes that defines their resistance to deformation under exterior loads, mainly vertical loads associated with buried pipe conditions. In non-pressure applications, this is a basic property to ensure the right performance of the installation.
Stiffness depends mainly on two factors: E modulus, which is a physical property of every material, and the geometry of the profile of the pipe.
The behaviour of the pipe, and the deformation it reaches over time, is not only dependant on stiffness, but on soil and installation conditions. A higher stiffness gives a better performance, but the following graph shows the behaviour and influence of both stiffness and installation conditions.
Under a qualified installation, stiffness is not so relevant and all stiffness classes are suitable to be used.
Some organisations for plastic pipes have proposed or developed methods to rapidly evaluate the behaviour of buried pipes.
Figure 1: Design graph for determining the pipe deflections immediately after installation and after settlement of the soil
The diagram proposed by Teppfa (The European Plastic Pipes and Fittings Association) clearly shows that SN4 or SN8 piping systems present very good behaviour when the level of workmanship is good or at least standard.
On the vertical axis, the pipe deflection is shown and on the horizontal the pipe stiffness classes. For each installation group an area is given in which the deflection after installation is expected. The upper edge of the area represents the maximum deflection to be expected. The lower edge of the area shows the average deflection to be expected.
The graph shows the deflections immediately after installation. It does not include the effect of traffic load, depth of cover or groundwater.
The soil will further compact in the course of time. This further compaction is caused by the weight of the soil, the percolation of rain and ground water through the soil and by traffic load.
In order to obtain the final deflection including the effect of traffic, one shall add a consolidation value to the initial deflection. These consolidation values are listed in the table in the graph. Hence the final deflection becomes:
Final deflection = Initial deflection + C f
PVC4Pipes recommends SN 8 as a standard pipe that covers a depth of burial ranging from 0.6 to 6 metres in most soil conditions. Other depths of burial can be employed for PVC pipes (up to 15m) but, for these depths, specific conditions have to be studied on a case by case basis.
Many other calculation methods are listed in different European countries, but all of them agree on the wide versatility of PVC systems.