winter 1993 i lived through 30 consecutive days at 25 below zero. I saw pipes freezing during the week after the AIR temperature had risen to more reasonable winter temperatures.
Explaining that might help understand what is at stake here. Heat and cold are the same thing, each one expressed as the negative number of the other. A huge mass of frozen ground is easier to think about if you imagine it is a huge mass of boiling hot lava. Its heat (or the negative number equivalent) will radiate heat (or opposite equivalent) to the other objects and to the ground surrounding it. Even after air temperatures change (lower or higher), the mass has its own energy; it has thermal inertia, and nothing can stop inertia.
In this case the twenty-below temperature mass had become big and substantial enough to suck the remaining heat energy out of the water in the buried pipes and cause them to freeze long after air temperatures had come back up to about the freezing point. We know that ground water feels cold to us, but it does still have heat energy in it; we only consider it to be cold relative to another benchmark comparison point. It has heat energy in it that its temperature indicates. When below-freezing cold "energy" (or anti-energy?) penetrates down to the level of the buried pipes, the temperature difference between that mass of "cold" and the water in the pipes becomes so high that the water loses its heat energy below freezing point and it freezes. This explains why it is good to run the water in the taps as then that about-to-freeze water gets pulled out and new water slightly warmer comes through the pipe up to the place where cold mass is pulling heat energy out of the water. The longer it sits there the more heat it loses, so instead of leaving it there to freeze you remove it and let warmer water take its place. Calling it "warmer" doesn't mean it feels warm to you, but it is indeed warmer than the water that was just about to freeze.
Insulating a pipe enables that water to keep its heat energy longer and not transfer it to a surrounding mass that absorbs it. It won't freeze as early, as soon. That can make all the difference; it can be significant. Throughout the entire universe, that function is all insulation can do: slow down heat and cold transfer. In day to day life, in buildings, in clothing, in winter sleeping bags, wherever, we use insulation to slow down transfer too, that is, to slow down the transfer of heat energy to colder mass. We often say "prevent heat loss" but we don't mean it as "stop entirely", so ultimately the word "prevent" is inaccurate when used as "prevent cold from penetrating" and other similar expressions. Insulation doesn't prevent heat transfer from occurring; it slows the process. That's the law. Like the law of gravity, it just is. The Law of Retarding an Unstoppable Process.
Insulation may also prevent damp or wet earth from coming into contact with the pipe; that can also add a lot of insulating properties all by itself. Anything that lets ground water drain away from the pipe surroundings is a very good thing, since water is a conductor of heat and cold, not an insulator like trapped air. Dry sand is a far better insulator than damp sand, which conducts / transfers heat/cold. Any continuous layer of any non-heat-conducting material can act as an insulator, no matter whether the material is sold as an insulator or not. Example: PVC pipe as a sheathing can insulate the water bearing pipe; on the understanding that no water collect / remain stagnant in the PVC pipe.
If you have to run the water pipe under a driveway or a country road you need to go a lot deeper than otherwise since the ground is compacted there and not insulated by fluffy snow in winter.
Summary: Insulation works. When it stays dry, it keeps working.
Recommendation: (1.) Closed cell foam would be my choice, any thickness, laid on top of (2.) a PVC sheathing around the water supply pipe, something like a French drain with holes in it, and (3.) dry sand and gravel, any kind that will stay dry and not get water logged, placed so as to stay dry and not get water logged.
david
