Plastic Pipes are classified by their ring stiffness. The preferred stiffness classes as described in several product standards are: SN2, SN4, SN8 and SN16, where SN is Nominal Stiffness (kN/m2). Stiffness of pipes is important if they are to withstand external loadings during installation. The higher the figure, the stiffer the pipe! After correct installation, pipe deflection remains very limited but it will continue to some extent for a while.In relation to the soil in which it is embedded, the plastic pipe behaves in a 'flexible' way.This means that further deflection in time depends of the settlement of the soil around the pipe. Basically, the pipe follows the soil movement or settlement of the backfill, as technicians call it.
This means that good installation of pipes will result in good soil settlement.Further deflection will remain limited. For flexible pipes, the soil loading is distributed and supported by the surrounding soil.Stresses and strains caused by the deflection of the pipe will occur within the pipe wall.However, the induced stresses will never exceed the allowed limit values. The thermoplastic behavior of the pipe material is such that the induced stresses are relaxing to a very low level.
It has to be noted that induced strains are far below the allowable levels. This flexible behaviour means that the pipe will not fail.It will exhibit only more deflection while keeping its function without breaking. However, rigid pipes by their very nature are not flexible and will not follow ground movements. They will bear all the ground loadings, whatever the soil settlement.This means that when a rigid pipe is subject to excessive loading, it will reach the limit for stress values more quickly and break. It can therefore be concluded that the flexibility of plastic pipes is such that it offers an extra dimension of safety.Buried Pipes need flexibility9Źródło: https://en.wikipedia.org/wiki/Plastic_pipework#Flexibility.