And here, for another nice explanation for lay folks like me:

http://www.newton.dep.anl.gov/askasci/eng99/eng99532.htm

*"The diagrams show that at -20C, solid ice is ALWAYS the stable phase of

water, so pressure or not, you are guaranteed to have solid ice at -20C.

The “bulk modulus” of ice is about 8.8E9 pascals. You can find that number

on the Internet. “Bulk modulus” is a term that describes how stiff a solid

is. Styrofoam has a small bulk modulus. Rocks have large bulk modulus.

If you completely freeze the ice it expands 9%. If you try to SQUEEZE the

ice back down to the original size, you would need to push with a pressure

of about 790 megapascals of force. (8,800,000,000*0.09) That is about

114,000 pounds per square inch, and is a simple estimate of the pressure

that ice could exert when it freezes, under ideal conditions.

Now, this estimate is not actually correct because in the process of freezing

under pressure, these high pressures can transform the ice into type 3 or 5,

and it is difficult to know what will exactly happen. One can find the bulk

modulus and expansion of ordinary ice, but probably only ice experts know

the bulk modulus and expansion of the other kinds of ice. So I will leave a

perfectly correct calculation up to the ice experts.

In any case, water expands strongly when it freezes, and whether it is

114,000 psi, or 100,000 psi or even 50,000 psi, it can burst pipes and

disrupt foundations."*