Drywell Freezing

[EIT9667]

I'm looking for a good explanation on why open structure drywells do not freeze during the winter.

I work in the northeast where typically it is assumed frost depth is around 4'. For stormwater infiltration basins we are required to install drywells (typically concrete drywell structures surrounded by coarse stone, not trenches filled with stone) to allow for winter infiltration. Requirements state drywells must extend below 4' to allow for infiltration in winter conditions. I would have assumed that because there is cold air in the drywell that there is the potential that the soil at the bottom of the drywell would freeze, the same as soil on the surface. I know i have observed catchbasin sumps frozen, but these are typically shallower than frost line. Is it simply that there isn't air circulating, with constant heat from soil that allows the air in the drywell to remain slightly warmer and not freeze? This is assuming that the grate is exposed to air and not covered in snow.

 

[SwinnyGG]

The soil at the bottom of the drywell is connected to an earth-sized thermal reservoir.

 

[oldestguy]

First off, that "frost depth" is only a rough idea of what you may have at any particular place. Site factors that influence depth of freezing temperatures and the effects are many. Moisture content of soil, insulation on top, frost heaving susceptibility, wind, snow depth are involved. Of course weather conditions also are variable, some winters are harsh and others easy. Lately it seems they are easier, at least for this old guy. A detailed examination of your installations may surprise you. For instance, I point to a case in LaCrosse, WI before Christmas, a gas explosion in a house was caused by a ruptured gas line in the street. I measured below freezing temps in sandy soil at 8 feet depth. Water lines there have frozen at 10 feet depth, yet building codes require footings at 4 feet. No heave in clean sand..