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This is a strange question, but I have a trench with a water line (HDPE poly pipe) in it from my new well to my house that I haven't yet buried. There's a cold snap coming where we'll see temps drop to around 10F for 4 nights or so. The trench is filled with water (from profuse rainfall in the autumn) right now; it's a bit more than a foot deep. My question is, does the fact that the trench is filled with water protect the pipe from freezing at all? i.e. does the trench being filled with water have a similar effect to if the pipe had been buried?

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    How long has the trench been dug? Is the soil around it frozen? What do you expect the weather to do after that?
    – isherwood
    Dec 23, 2021 at 22:20
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    Imagine for those temps the trench is deeper than four feet. If not then running water at least every 20 minutes(or continuously) 24/7 is necessary.
    – crip659
    Dec 23, 2021 at 22:26
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    @isherwood I'm in the seattle, wa area -- in a typical winter the ground usually doesn't freeze at all. I'm hoping after this "record breaking" cold snap, we'll go back to milder temps. The trench has been around about a year, but I put the water line into service this past summer.
    – atanamir
    Dec 23, 2021 at 23:21
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    on the bight side if it bursts you won't need to dig it up.
    – Jasen
    Dec 24, 2021 at 12:08
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    Unless you're looking at a very different forecast, I don't think Seattle is getting that cold. Don't forget that windchill factors don't affect things like stuff actually freezing. Keep an eye on the real temperature - not the scary headline windchill numbers.
    – J...
    Dec 24, 2021 at 14:30

3 Answers 3

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The trench should have been dug deep enough in the first place that it's permanently below the frost line, and should be at least several feet deep in any case so I'm going to assume that this is the case.

Water and ice are not as good of an insulator as soil, but for four days it will be fine. It takes a good week of hard cold for ice to grow to even a foot of thickness on open water - something very familiar to people keen to go skating or ice fishing on open water - and your trench (I sure hope) should be much deeper than that.

If you're nervous it's easy to go out there and drill a hole - you'll see the ice will only grow by inches a day at best. Get it filled as soon as you can, though - with proper soil covering the trench it won't freeze at all, even in the coldest of winters. If it was going to be a week, not four days, and another ten degrees colder you might consider rolling some bubble wrap or placing some hard foam panels over the trench to add a bit of insulation but four days of 10F is not really enough to freeze that deep. The forecast for Seattle, as of right now, is also looking more like 20F for the next few days, not 10F (windchill doesn't count here) - and with a good amount of snow. I think you'll be fine.

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    You've noted that the OP is in Seattle, and there are numerous comments that there isn't a frost line in the Seattle area as the ground rarely freezes. OP doesn't indicate how deep the trench is, but the definition of "trench" indicates that it's at least deeper than the surface frost they may get on occasion. You're first paragraph isn't all that helpful in this situation. It is, though, generally good advice.
    – FreeMan
    Dec 24, 2021 at 15:33
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    @FreeMan The point was that, even without a frost line, the required depth for buried water lines should be at least 2-3 feet, so since we don't know anything about OP's line, I'm assuming that it's at least that deep and am basing the rest of what I say on that. Maybe it's a DIY hackjob that's only buried 6" - in that case, all bets are off, but I'm saying that if it was properly installed the rest that follows should hold.
    – J...
    Dec 24, 2021 at 16:12
  • Yeah, the trench is about 2 feet deep. I was going to bury the line 18"-24" deep, which is general recommendation for this area despite having no official frost line.
    – atanamir
    Dec 24, 2021 at 20:25
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    As water freezes, it gives off a large amount of heat, while the temperature stays at the freezing point until all the water freezes. Since ice floats, it will freeze from the top down. So the more water, the better. You can also cover it with a tarp to slow cooling, especially on clear nights. Citrus growers actually spray water onto trees to protect them from freezing. The freezing point of the leaves is slightly colder than pure water, so as long a there's liquid water in the process of freezing, they're safe.
    – Bob Kerns
    Dec 25, 2021 at 18:14
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If the pipe is at the bottom of the trench, the water above will protect it from freezing to some extent. You see the same effect in a shallow pond, where the surface freezes first and the lowest depths freeze last, enabling fish to survive. This occurs because water's greatest density occurs at 4 deg C or about 40 deg F.

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  • Seen ponds of 3 feet or more freeze solid... But sadly that caused the loss of many fish.
    – Solar Mike
    Dec 23, 2021 at 22:53
  • I'm in the PNW (Seattle, WA) -- we don't have a frost line at all; in a typical winter our ground doesn't freeze. As luck would have it we get the cold winter when I have some exposed water pipe :(. Hopefully it wont' freeze completely. Would adding salt to the water help?
    – atanamir
    Dec 23, 2021 at 23:24
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    Do not add salt. Salt lowers the freezing temperature of water that has salt in it. It will not stop water from freezing in the pipe. Liquid salt water at below 28F will freeze plain water in the pipe faster. Covering the trench with wood/dirt/blankets will help more.
    – crip659
    Dec 23, 2021 at 23:55
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    Yes, salt would be a terrible idea unless you put salt in the water in the pipe. Indeed, it would make things worse, because it would bring <32F temperature down to the pipe via convection. If you keep plain water, it will develop an ice layer on top which will stop convection. Dec 24, 2021 at 5:09
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Depending on how the day and night temperatures go then that trench could freeze solid, but it may not.

One way to protect the pipe is to have water always flowing in it…

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  • Seems like I'll go with a combination of gambling (hoping the water doesn't freeze) in addition to running water when it's cold.
    – atanamir
    Dec 23, 2021 at 23:22
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    Given the new information on local climate, full freezing is rather unlikely.
    – isherwood
    Dec 24, 2021 at 14:06

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