Resistance of Crosslinked PE Pipes to Rock Impingement [PE]

Insofar as installing piping systems with traditional sand embedding is quite expensive and time-consuming, there is a current trend towards cost-effective, “trenchless” installation techniques (e.g. “ploughed-in”, directional drilling). Such buried pipes may be submitted to undesired external solicitations, namely point loading, stemming from rocks in the backfilling material. It has been reported, e.g. in Polyethylene (PE), that such a configuration induces the initiation and propagation of cracks actually from the inner side to the outer side of the wall of the pressurised pipe, near the impingement area, leading potentially to some premature pipe failures. To overcome this risk, one solution could be to use crosslinked PE (PEX) instead of PE, since it is well-known that PEX should have a much better behaviour towards slow crack growth than PE. Besides, according to the aforementioned failure mechanism, an alternative, cost-saving technical solution could be to employ multilayered pipes. A promising design is tri-layer pipes, with PEX-b internal layer, current bimodal PE100 resin as the core-layer, and PEX-b as the external layer. If we want to maintain the Minimum Required Strength (MRS) on the whole system, the PEX-b layer has to have an MRS10 rating as well. To assess PEX-b intrinsic stress cracking resistance, standard FNCT (Full Notch Creep Test) as well as innovative Point Loading measurements have been carried out. In this paper are related such laboratory experiments, performed on three comparative materials  : pure-PE100, pure-PEXb and PEXb-PE100-PEXb tri-layered pipes, all made of the same PE100 and PEX-b compound batches.