The design engineer should provide as much information as possible about the characteristic of the pipeline, the type of soil, superimposed loads and constraints, so that the pipe manufacturers can design the most suitable pipe.
The following data must always be specified:
• Reference code and standards
• Design and operating pressure
• required ring stiffness
• Transient pressure/vacuum
• Flow rate and velocity
• Type of fluid
• Type of native soil and backfill
• Burial depth min/max
• Height of the water table
• Surface loads, static or dynamic
• Support span (if above ground)
• The type of joint.

GRP pipes, when installed underground, are subject to ovalization. This must be limited to 3% of the pipe diameter.

The manufacturer shall be always asked to justify the proposed type of pipe by means of a design report based on a recognized standard.
The design of underground GRP pipes shall always be based on strain.
The strain shall be calculated as the combination of the strain due to pressure with that caused by the external loads.
The calculated combined strain shall not exceed 25% of the strain at weep of the pipe (safety factor of 4).
The specified ring stiffness shall never be less than 1500 Pa in case of stable soils with the water table below the bottom of pipe while it should be 2500 Pa in the other cases.
Stiffness higher than 2500 Pa does not bring any benefit; in fact the ovalization of the pipe is controlled by the passive reaction of the backfill rather than the rigidity of the pipe itself.
Moreover the strain due to ovality is directly proportional to the wall thickness so the strains are higher when the pipe is thicker.

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