Backfill for Buried Pipe

I don't like anything but clean sand touching my pipelines, hence I do not allow concrete mixes of any kind. A flowable sand may potentially buoy up the pipe before it sets, therefore I would not use that either, certainly not without careful control during placement. I don't see corrosion as an issue as presumably the water will leave the trench eventually and subsea pipelines use FBE coatings without significant issues. For a 200 ft section of relatively small diameter pipeline, I'd let him do it, IF placed with careful control, 2 or three levels of placement, allowing water to drain out between, without any apparent uplifting buoying movement of the pipe during placement and a guarantee that the contractor will meet compaction requirements, or remove the fill and replace with suitable material, hand compacted, if any problems arise.

Independent events are seldomly independent.

i think you are confusing bedding with backfill. bedding up to springline or up to top of pipe or even sometimes 1 foot over the top is generally a good quality, free draining sand and gravel mixture. Silty or clayey material is not very suitable for bedding. Pipe can also be set in a concrete cradle or alternatively encased in a CLSM up to about 1 sack mix. Problem with encasement with flowable mixtures is that the pipe will float unless it is either ballasted or fill is poured in lifts. Over that is backfill which can generally be any clean, native soil as it has little effect on any loading on the pipe. It has more of an effect on settlement of the ground surface over the pipe so it should be well compacted.

addition of more than 1 sack of cement per yard is not generally necessary and may be detrimental since it makes it very difficult to inspect or repair a pipe encased in a high strength concrete.

sand bedding with jetting or flooding can be used, if the trench will drain out and if the contractor has the experience to do it right. It can be a suitable method of consolidating sand around the haunches of the pipe.

have never used AB for pipe bedding, sandy bedding material is generally easy to find, cheaper and easier to work with.

not sure what popcorn is. around here we have "chips" which is essentially pea gravel or pea size crushed rock and is a waste product when producing sand and gravel. It is a poor choice for bedding.

I'm with big inch. The trench sounds quite deep (why?) so reduction of risk to construction workers is worth looking at alternative techniques. The key to good backfill is good supervision whilst it's going on. First fill to mid level of pipe max, then leave it to drain and settle before next fill to top of pipe then final 300mm and compact with a whacker plate. I assume you're not talking about filling the whole trench like this so after that backfill in layers of 300mm/1ft then compact. I would only use concrete mixes if there is a lot of ground water and then never around the pipe itself.

I've seen dry backfill watered to aid compaction so this is just a variation of that so long as the mixture is fairly stiff and only has the minimum amount of water to make it flow slowly.

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Also: There's usually a good reason why everyone does it that way

This technique has been used in Australia by a state rail coprporation. AS 2566.2 covers the technique.

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It is a fairly common practice to specify imported soil for bedding and embedment. An alternative might be recycled native soil placed as flowable fill.
Flowable fill is granular soil, usually native, with enough fines that it can be ixed into a slurry. Flowable fill reduces or eliminates problems such as uneven bedding, ring deflection, non-uniform embedment support, and voids under the haunches.

Flowable fill does not have to have concrete in it.

Here is a design document:

I guess it could be interpreted the OP without further definition could conceivably be referring to some extent bedding as well as backfilling the pipelines in question basically by just pumping a sand only slurry into the trench. While it certainly does not mean that it might not somehow be done for these particular small steel pipelines (this size with I suspect quite high and dependable ring etc. strength), I see the good reference provided by bimr for controlled low strength material (I think basically "flowable fill") for backfilling steel pipelines does however contain the further guidance on page 54 , "Compressive strength of 40 psi(280 kPa) is generally adequate. A slump of 10 inches is about right for flowability. See Flowable Fill, page 62".
With regard to your question, "Zero sack is basically just water and sand right"?, if you go to page 62 you will see that "flowable fill" is supposed to contain Portland cement and/or appropriate class fly ash etc. binder as well as sand aggregate to come up with the desired properties.
In conclusion I believe wet or slurried sand by itself and without working and/or good drainage may well not have a compressive strength of 40 psi i.e. 5,760 psf. While there are many sites that provide information regarding flowable fill backfill, as I suspect you have found there are not a lot yet that specifically elaborate on standard definitions of "zero-sack" or such mixes, and my only purpose with this further response here is to make sure it is understood at least some kind of further specification and "cement" (at least pozzolanic flyash) as well as properly controlled placement of the fill as others have noted may need to be provided in the mix for good pipe laying practice/support. It should probably also be further understood a flowable fill with only flyash and sand may develop its cemented strength generally at a somewhat slower rate than one that contains Portland cement as well.