When PE pipe is pressurized, it expands a little and shortens slightly due to Poisson's effect. Reductions in length may also occur due to thermal contraction. A pipe section with fully restrained joints such as a long string of butt fused PE pipe will transmit pipe shortening effects, from length to length, through the restrained joints along the pipe string. Restrained joints include butt fusions, electro-fusions, socket fusions, bolted flange connections, MJ adapter connections or other restrained mechanical connections. If an unrestrained bell and spigot or mechanical sleeve joint is in-line with the restrained section, the cumulative 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 PE pipe must be either restrained or otherwise protected against pullout. Joint separation can be prevented by installing external joint restraints (gripping devices or flex restraints) at mechanical connections, or by installing in-line anchors, or a combination of both. Additional restraint mechanisms are available to supplement the pull resistance of these types of fittings if needed.
A typical pullout prevention technique is to restrain the transition connection by butt fusing a wall anchor in the PE pipeline close to the connection and pouring a surrounding concrete anchor. Another method of anchoring this connection is to electrofuse several flex restraints to the PE pipe.
Wall Anchor Diagram