While welded/fused joint hdpe pipe is in effect self-restrained, as is a ductile iron system with restrained joints, it is quite different from the standpoint of potential movements. Pulled sections of hdpe pipe sometimes exhibit substantial end movement (in effect a sort of “recoil” or memory behavior) for some time e.g. after some HDD pulling installation. Additionally, Poissons ratio of plastics are generally higher than metals (resulting in more shortening of the pipe string under pressure?), as is the coefficient of thermal expansion (and perhaps more importantly in some practical applications, contraction?). As can be seen from ratio(ing) of comparative values shown on page 17-24 at
the coefficient of contraction of hdpe piping in response to thermal changes/cycling also can be roughly 15-20 times what it is of steel or ductile iron piping (or for that matter relative to reinforced concrete structures/rock encasement etc.) All of this makes for significant potential for subsequent end (and/or sideways if not strapped down or backfilled) movement of hdpe, and for that matter also may create issues involving any transverse connections to the piping (such as services etc.) in the area. I thus believe at least some hdpe manufacturers claim they can provide structures (similar at least in principle to welded-on thrust collars used for steel and ductile iron piping?), that would ostensibly allow for independent anchorage of their piping vs such movements e.g. by the Contractor with transverse concrete thrust walls.
I would think alternatively asking e.g. ductile iron pipe manufacturers to increase the cost in their piping quotations by providing extra thrust collars or restrained joints purely to control the movements inherent with the specification and adjacent use of their competitor’s (hdpe) piping could conceivably involve questions of engineering ethics or fairness; however, in any case it would at least be responsible/courteous if this is done to advise the ductile iron folks exactly how and with what magnitude of forces etc. the polyethylene piping system will be pulling or pushing on their piping/joints. There are apparently other perhaps non-obvious issues that could be potentially involved with connecting to, repairing, or cutting into hdpe pipe systems, e.g. as explained by a large coupling manufacturer at
Finally, while I suspect there has probably been some utility to buried flanged connections of hdpe e.g. to metal or ductile iron piping as I believe was one means suggested earlier in this thread, there could conceivably also be some sort of issues with this as well as I heard many years ago third-hand of some problems at such connections involving the hdpe. I hope this information aids however in better understanding of this issue.