Technical Note 813-TN
PE Pressure Water Piping Systems
Mechanical Restraint and Poisson Effects
Introduction
When non-PE pipe and components are installed in the same pressure pipeline with PE pipe, or when PE pipe is connected to unrestrained joint piping such as bell and spigot joint PVC or ductile iron, unrestrained joints in the transition area should be protected against pullout disjoining.
When pipes made from ductile materials are pressurized, the diameter expands slightly and the length decreases in accordance with the Poisson ratio of the material. With unrestrained bell and spigot joined lengths, the effect is limited to the individual pipe lengths, but with fully restrained pipes such as fusion-joined PE pipe, the effect is cumulative over the entire restrained length of pipe. When fusion-joined PE pipe is connected to unrestrained mechanical couplings or bell and spigot joint PVC or ductile iron piping, Poisson effect pipe shortening can cause pullout disjoining of unrestrained joints where the PE pipe transitions to the unrestrained non-PE pipe. To prevent Poisson effect pullout disjoining in the transition area, provide protection by installing external joint restraints at unrestrained bell and spigot joints, or by installing an in-line anchor in the HDPE pipeline, or by a combination of both techniques.
Conventional thrust blocks at directional fittings are not effective against Poisson effect pullout because conventional thrust blocks are intended to resist pressure and fluid flow thrust forces that would push the fitting off the end of the pipe. Thrust blocks cannot counteract forces that would pull the pipe end out of the joint. Snaking pipe in the trench is generally not effective and is not recommended.
The Poisson Effect
When a tensile stress is applied to a material, the material elongates in the direction of the applied stress, and contracts perpendicular to the direction of the applied stress. This relationship, called the Poisson effect, is a natural response to applied stress that occurs with all materials, but is particularly apparent with ductile materials. For example, when a metal bar is pulled in a tensile test, it stretches out and necks down on the sides. The Poisson effect occurs naturally in all materials including iron, steel, copper, concrete, glass and plastics. When pipes such as polyethylene, PVC and metal pipes are pressurized, the diameter will expand slightly, and due to the Poisson effect, the pipe will shorten in length.
A pipe section with fully restrained joints such as a long string of butt-fused HDPE pipe will transmit Poisson forces from length to length through the restrained joints along the pipe string. Restrained joints include fusions, bolted flange connections, MJ adapter connections or other restrained mechanical connections. If an unrestrained bell and spigot or mechanical joint is in-line with the restrained section, the cumulative Poisson effect shortening may cause in-line unrestrained joints or connections to be pulled apart. Therefore, unrestrained joints or mechanical connections that are in-line with fully restrained HDPE pipe must be either restrained or otherwise protected against pullout disjoining.
Connection Restraint Techniques for Pressure Water Systems
In pressure water systems where DriscoPlex™ HDPE pipe is connected to non-HDPE components such as belled PVC or ductile iron pipe, belled fittings and components, and mechanical joint bell pipe, fittings and components, the connection must be restrained to prevent Poisson force joint pullout.
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