The hdpe pipe sizing & designations are confusing. I bought a premade 205' loop - it was 1.05" od but since i only wound up with a 100 ft hole i bought 200' of 160psi hdepo pipe and tried to fuse it to a socket weld u fitting i had bought - didn't work since the hdepo pipe had a smaller OD than the u fitting.
Turns out that pipe intended for socket welding has a controlled od and as the psi rating varies, they vary the inside diameter. But with hdepo pipe, it is intended for barb fittings and so has a controlled id with the od varying with pressure ratings. They have different Astm #s to show what they are intended for.
Then there is also CTS hdpe pipe (i think thats the correct designation) which has a considerable smaller id and od than either of the above in 3/4" designation! And that may be what you have.
Don't ask how i figured this out!
But if you are custom making your u fitying you should not have this problem.
As to grouting, i found that dry sand would slowly sink around my pipe to the bottom if the hole is full of water if you shake the u tubes. But your hole is much smaller.
There is a grout called neat cement which i think is just portland cement and water - i know it is very fluid before it sets but don't know if its pumpable thru 1/2" pipe to any depth?
Let us know where you wind up in grouting
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