As a chem engr I’ve do have some experience with flexible underground piping. Not a lot of PE other than for a flexible conduit or water. My HDPE use was primarily for corrosion resistant tanks. All PEs are fine for drinking water if NSF approved. However, some physical properties do favor PE or HDPE for direct burial. Personally, the only PEX I’ve ever run is above ground and long utility runs normally had a fiber reinforced jacket to resist any abrasion from the mounting hardware. I did not do any residential piping. But all the PEX I have seen has been run inside a PE conduit until it was above the slab.
PE is often used as a generic term by non-engineers who are unfamiliar with the fact there is PE, but also LDPE, MDPE as well as HDPE. PEX is just a cooler way of abbreviating Cross-linked PE. Mostly used due to strength and temp range for smaller diameter cold and hot water piping and easy to form reliable mechanical connections. HD is the most highly abrasion resistant and stiff but it can be more expensive for little gain in residential usage. HDPE is often used for tanks and sheet goods like the food butchering/prep table surfaces in commercial/industrial operations since it can withstand the bleaching type disinfection wash downs.
Each grade has specific requirements for trenching, embedment and backfill. Some are just minor differences but rocky soils, gravel size, uneven loading could impact integrity of the installation. I believe for 1” to 3” pipe, no embedment or fill should be greater than 1/2” but it’s been a long time so don’t quote me on that! Existing PVC should be in a sandy embedment and rock-free backfill, so it should be fine for PE. You should consider things like if it will see compression due to tractor weight rolling over in a field? If so, re-compressing disturbed embedment with a good tamping should suffice. Standard PE will allow for cold radius bends generally no less than 20x OD, it should also unfurl or layout easier than HD. HD is harder to bend without heating from my experience. Mechanical connections are easier to form without leakage with PE. Most PE can be heat welded, but it is not for the inexperienced and I’ve seen experienced welders struggle to do field welding. If I were diy’ing a line from my well to house, I’d go standard PE.in fact my 1840s house in Maine had PE buried about 6ft down for over 50ft to the cellar. It originally clamped onto a copper pipe that was installed with new plumbing in the 80s. The PE was installed in 60s I believe. Around ‘05, the copper gave up the ghost and I installed PEX to the water heater and main cold water branch. When I replaced the well pump and tank a few yrs later, I went all PEX to the PE line to the house. We moved in ‘12 and all was good. The new owners never had any issues with the well or supply. That old PE from the well is probably still there.
