Waterline freezing protection

[dBasement]

We are to install new manholes over an existing waterline that presently has sufficient soil frost-depth. The manhole will be vented to air that can reach -30c. Since we are now losing the insulating value of the soil cover by installing the manhole, do we now need to be concerned about exposing the pipe to freezing inside the manhole? The pipe will not have continuous pressure. If so, will insulation be an option? Keeping in mind that the manhole has to maintain adequate venting (It is to be used as a vent chamber)

 

[CarlB]

Yes the manhole will create colder conditions at the waterline than was present with soil cover.
But since air is not circulated in the manhole, the air will be fairly stagnant, and have a temperature closer to the surrounding soil temperature than the air above.
Similar situation exists in sanitary sewer manholes in cold climates, but they haven't been found to be a freezing concern, that I've ever heard of.
I've seen a detail where a "dust pan" below the manhole cover has insulation placed in it, to help slow any cooling effect.
Also, loose insulation could be placed over the exposed pipe/valve in the manhole. If there's any water flow, freezing is very unlikely.

 

[greggy]

We've used successfuly a powdered insulation material called Gilsulate (it's on the web). It's suitable for underground installation and has compressive strength. It slows down the rate of cold penetration. Their competitor Dri-Therm makes a similar product.

 

[bimr]

What part of the country are you in?

If you have a cover over the manhole, it should not freeze unless you are in the extreme north of the US. I work in the Chicago area and freezing is not a concern here.

If you are concerned, investigate the use of heat tape. Insulation will be a problem because the insulation will trap water and encourage corrosion. You don't need much energy with a closed vault, maybe 100-200 Watts.

 

[gerhardl]

I can confirm that CarlB's answer also cover experience and practical solutions from the northern part of Scandinavia.