Why do Water Mains break in freezing weather?

A nearby town had a water main break yesterday, and I just happened to be there when it broke. It was quite impressive! It began under a huge pile of snow from a parking lot, and that 8 foot pile of snow just melted into the flowing water, and vanished. Another guy who was watching it, (who had called the fire dept. about it), jokingly said "that's a great way to get rid of snow". Anyhow, I watched the city guys dig up the pipe, from my car, and heard one of them say "it split in half".
Anyhow, it appears that pipe breaks tend to occur much more often when the temperature drops far below zero, (which it had just done over the last several days).
But I cant understand why this happens...... These pipes are below the frost line, so why should they be affected by severe cold at the surface. When they dug it up, I could see that pipe was about 6 or 7 feet down.
The only thing I can think of, is that the fire hydrants are lifted by the frozen soil at the surface, and that stresses the pipes down below.
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On Wednesday, February 25, 2015 at 1:37:40 PM UTC-5, snipped-for-privacy@spamblocked.com wrote:

IDK, could be that kind of effect. The other day on the news they were talking about "frost quakes". I'd never heard that term before. But apparently in some areas of the country, police were getting 911 calls from people who heard loud booms. The news said it's frost quakes, where earth saturated with water freezes until it suddenle seperates, cracking the earth apart with a bang. Could be some kind of effect similar to that? Or another factor could just be that the water in the pipes winds up being much colder than usual, because it's coming from above ground storage tanks, etc. The colder steel gets, the more brittle. Plus it would cause the pipe to contract latteraly and if it can't move, maybe it breaks.
We had a big nat gas explosion here in NJ, 20 mins from me, yesterday. Took out a house and sent a bunch of people to the hospital, two critical. No determination yet what caused it.
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They break when it gets cold, they also break when it warms up. It's the earth moving that does it, not a direct effect on the pipe usually.
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The reason the water and other pipes break, even above freezing temperature s, is due to shrinkage. Imagine a water line 1000 feet long that ends in a T at both ends. When the pipe shrinks due to the cold temperatures, the T' s at each end keep the center pipe under extreme tension. When the tension gets too much, something gives, like the center pipe.
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snipped-for-privacy@spamblocked.com wrote:

Hi, It is as simple as pipe being buried to shallow. Up here frost line is 6 feet. Pipes are buried at about 8 feet or deeper. Another reasons are corrosion, ground moving causing pipe crack, etc.
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On 02/25/2015 12:32 PM, snipped-for-privacy@spamblocked.com wrote:

Not so much the hydrants themselves that move but the ground above/around the pipe...

breaks include:

...
I disagree w/ the respondent who blames tension as a primary cause...
--


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As the weather turns cold, water main breaks become more common. This is due to the expansion and contraction of the pipe material, weakening it. Pipe corrosion, soil conditions, age and ground movement can also cause a water main break, creating unexpected problems for customers and motorists.
DC Water averages between 400 and 500 water main breaks per year and most occur in the winter months. For this reason, the Authority schedules more stand-by crews in the winter months and has cross-trained sewer repair workers to make water main repairs.
http://www.dcwater.com/wastewater/watermain_break.cfm
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