I always thought they burst at the point where the water is freezing and expanding. But in fact the damage occurs at a location between the plug of frozen water and a closed valve, because the water pressure increases dramatically as the ice plug increases in length. Pressure can go as high as 25,000 psi !!
How a Frozen Pipe Bursts
In this DIY Smarts, Ask This Old House plumbing and heating expert Richard Trethewey demonstrates how and why frozen pipes burst.
www.thisoldhouse.com