Polyethylene Restraint Theory
End Restraint of Polyethylene Pipes in both pressure and non-pressure applications is essential. Unlike other pipe materials such as PVC or Ductile Iron, PE is an 'End to End' system meaning the various forces (stress) are distributed in theory to the 'ends' of the pipeline. In reality, a large part of the stress is absorbed into the ground due to soil pressure acting on the pipeline, however, unlike other pipe materials that have rubber ring sockets and require anchor blocks, the local forces in PE are designed to be re-distributed and balanced throughout the pipeline, due to the continuous nature of the jointing system.
PE is a viscoelastic material, meaning it exhibits both viscous and elastic characteristics when undergoing deformation or stress. Due to the viscous characteristic, the ends of a polyethylene pipe must be physically anchored to prevent them from moving. Anchoring pipe to pipe is achieved by fusion (electrofusion or butt fusion) at each joint or by mechanical connection such as a flanged Adaptor, threaded joint or a Type 1 Joint. These joint methods maintain the pipelines mechanical continuity, over which the stresses are transferred along the length pipeline.
Stress in a PE pipeline is typically generated by one or more of the following operating conditions:
- Internal operating pressure (creates loads on the ends of the pipes and at changes in direction)
- External ground loading (soil weight & traffic loads try to ovalise the pipe)
- Thermal change (Pipe installed at 50º, cools to ground temp 15º is trying to shorten itself 6.5mm for every meter of pipe installed)
- Ground settlement (reclaimed land settlement - a slow extension of the pipeline between fixed points)
- Ground movement (landslides - significant tension between fixed points due to the pipe pulling through the soil)
When PE pipe undergoes stress from one or more of the events above and the pipe is maintained in that condition (the pipe ends are sufficiently anchored), the stress developed in the pipe wall decays gradually with time. The decrease in stress under constant strain is called 'stress relaxation'. Given enough time, the stress level between the two fixed points approaches equilibrium along the pipeline, this state is maintained as long as the continuity of the joints and pipe anchoring is maintained.
However, once that equilibrium is changed, (an anchored end is released for example) the pipe will gradually attempt to return to its original manufactured dimensions. The extent of the pipes recovery will depend upon the magnitude of the applied stress, the length of time over which the initial stress was applied and the properties of the PE.
For example: If a valve is removed from a PE pipeline, the end to end continuity has been broken, so the two separate PE pipes will attempt to each find a new equilibrium. The PE attempts to revert to its original manufactured condition and this typically involves shortening of the pipe at the free end (in rare cases may be lengthening). A contractor may return to the valve location several days later, only to find the unbolted PE ends have moved several meters apart, making installation of a new valve impossible.
Stresses such as thermal force, translate into axial movement until the resistance of the backfill (soil friction) is equal to the sum of the stress being applied. Because the external surface of PE is very smooth, this force can be transferred many pipe lengths, depending on the soil load. Due to relaxation, acceptable levels of stress are designed to remain in a PE pipeline for its entire operating life. This makes PE systems idea for reclaimed land installations where stretching of the pipe through long term ground settlement is expected to occur. Such stretching is perfectly suited to PE systems as long as the pipe ends are anchored where they enter and exit structures in order to withstand the expected change in length due to the settlement.
Restraint must be designed to exceed the strength of the pipe in tension, such as a PE puddle flange must be either anchored in a concrete structure or restrained by steel clamp on a suitably designed frame. (Read here why anchoring of PE cannot be achieved through the concrete encasement of PE pipes or fittings). For this reason, Puddle flanges must be injection moulded or machined from a single homogeneous piece of PE, without welds in the puddle portion (long spigots butt welded onto the puddle are acceptable, just like any other pipe to pipe joint)
The best practice is to anchor a PE pipe at both the entry and exit of a chamber or structure. This creates fixed points and prevents the pipe or connected equipment (valves) from moving. PE Puddle flanges are 1.25x stronger than the pipe when in tension, so designers should consider the sum of all forces which may be applied, by the PE pipe to the structure where the puddle flange is embedded. (Click here for a table of forces and recommended concrete cover in a structure with PE puddle flanges).
Alternate methods of restraint are sometimes required where there is no opportunity to fuse a puddle flange into the pipeline, such as during pipe rehabilitation where a PE pipe is installed inside a host pipe. Often the method of installation into a host pipe creates significant stresses, as these relax, a pipe end that is not restrained is likely to pull back inside the host pipe.
For such applications Electrofusion FixBlocs are fused to the pipes external surface in a quantity and spacing sufficient to achieve the desired restraint, typically 40kN per fixBloc.
- Where maximum tensile stress could be exerted by the pipe on the fixed structure (For ground settlement or potential landslide applications) the Puddle Flange must be integrated with steel reinforcing during concrete construction. Grouting a puddle into a hole formed in a concrete structure or using a corbel behind the puddle will not transfer the full tensile forces from ground movement into the concrete structure.
- Mill-Pro Puddle flanges have tapers leading up to the puddle, these tapers reduce stress on the puddle in tension and should always be covered by the concrete to ensure sufficient embedment depth has been achieved.
- Electrofusion couplers connecting to a Puddle should not encroach into the minimum concrete cover required as encroachment reduces the embedment length.
- The puddle flange centre section must be a single piece, injection moulded or machined from a single hollow bar (without welds) in order to distribute the full tensile force.
- In sizes, ≥ DN/OD 800 puddle flanges are typically factory butt welded to a length of pipe to reduce on-site joining costs.
For more technical information on embedment of a puddle flange, expected forces and use in reclaimed ground contact us.