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djlozar (Civil/Environmental) (OP)
4 Sep 07 19:10
In Northwest Montana we bury our water mains 6.5' below ground surface.  However, when we can't achieve that depth we insulate the pipe w/ 2" thick blueboard a minimum of 2' either side of the pipe outer wall.

The rule of the thumb is 2" of blueboard insulation equates to 1' of pipe bury.  We've been using this for along time, but now I would like to back it up.

Does anyone have studies to support this?  Any other thoughts?
dicksewerrat (Civil/Environmental)
4 Sep 07 20:41
Is this 6.5 ft in roads? Doesn't seem like enough cover to me.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

CarlB (Civil/Environmental)
5 Sep 07 2:49
I've seen a few studies/articles etc. For frost penetration reduction the "rule of thumb" I've heard most often is 1" insulation replaces 1' of soil cover.  Models show insulation is more effective than that - if installed properly.  Over a pipe it probably needs to be wider than 4' wide to replace a few feet of cover, and depends how far it's placed above the pipe.  Is more effective if insulation extends vertically along side of pipe as well, in an inverted "u" shape.  Try a search for "frost shield".  I'll check tomorrow for what I may have squirreled away.
Helpful Member!  dcasto (Chemical)
6 Sep 07 0:03
To me the solution is going to be found by experience in a specific area and application.   Look at both ends of the spectrum here.  The 6.5' of cover isn't cover, its the frost line depth.  The point where the air temperature adsorbs the heat from the ground (center of the earth sort of) and the temperature is never lower than 32 F below that point.  If you bury the line any higher, the temperature of the ground will be less than 32F.  If you bury a tank with water in it less than frost line, it will freeze eventually.  If you lay a line at ground level with water flowing, the water has heat energy that transfers very slowly to the air and it won't freeze.

So the answer to how much insulation is a function of water use.  If you had a line with a constant demand flow at a high velocity, you would need no insulation.  If you had a line that ran 15 minutes every month, you may not be able to get enough insulation.

From experience in my field, we found that our lines buried below frostline didn't freeze EXCEPT at dirt road crossings.  It appears that the truck traffic compacts the soil and makes the heat transfer "faster" higher and the frostline is really slightly deeper.
dcasto (Chemical)
6 Sep 07 0:25
just saw this thread too.   thread391-195283: Cooling time of fluids
BigInch (Petroleum)
6 Sep 07 4:41
frost line is higher.

The subgrade of a road should be more compact, thus containing less insulating air space and it is also likely that there is less water contained under the road, due to typical roadway drainage provisions, thus reducing the heat capacity of the subgrade material to roughly 1/2 that of any wet soil in the surrounding terrain.

Do you know the insulation value of your "blue board"?

http://virtualpipeline.spaces.msn.com

CarlB (Civil/Environmental)
6 Sep 07 5:32
In addition to what Big Inch pointed out, areas outside of roads often have insulating snow cover, and some vegetation and organic material, none of which are present at roads.  As kicked around before at Eng-tips, there is a widespread (mis) perception that vehicular traffic "drives" frost deeper within roads.  As far as I know there is no frost penetration model that might explain that phenomenon, but the other factors mentioned have a basis in heat transfer principles.
BigInch (Petroleum)
6 Sep 07 6:38
I think its just the generally drier subgrade that makes it difficult to see frost until you get way down below into the possibly more damp areas and, if you actually measured the temperature, you find that the road material is slightly colder.  I can see from where the confusion might derive. Originally frost heaving of the soil was thought to result from the expansion of freezing water.  The latest is that growing ice crystals draw water from the surrounding soil and develop into ice lenses.  As a road freezes from the surface downwards, a negative pressure develops at the advancing frost line, that would tend to draw water up from below.  Probably would be more so with a low quality silty subgrade that would normally attract and hold water more so than a sand-gravel well compacted and well drained sub-base.  

My personal observations tend to confirm that process as, during the begining of winter, I always seem to notice many days of intermitant ice warnings for road travel before there is a snow cover on the surrounding ground.  Although later on, especially with a bright sunny day and on a black-topped roadway of a busy highway, it might seem to be the other way around.  Black would adsorb much more heat than the surrounding white snow, which after a number of days would tend to raise the frost line below the road above that of the surroundings.  Thinking about spring, I can't be so sure.  Probably, like most things, timing is critical.  Every process is dynamic in respect to one time frame or another.  What you see depends on when and how long you look.

http://virtualpipeline.spaces.msn.com

civilman72 (Civil/Environmental)
6 Sep 07 15:22
Where I live we get about 400 inches of snow a year and snow maintenance is a daily and weekly occurence.  Here's the general opinion of the guys that do snow maintenance and water/sewer district engineers in my area:

1. FACT: frost depth is greater in areas that are plowed.  Snow cover in itself acts as insulation.  So, areas where snow is plowed off of the surface (mostly for roads) always has deeper frost levels than areas where the snow is piled.  It really has little to do the subgrade materials or lack of moisture in the roadbase.  Case in point, if you leave a road unplowed all winter and let the snow build up on it just like the areas on the side of the road, you will not see a significant difference in frost levels.

To answer original poster, we bury water and sewer at least 8' deep.  We also insulate with 2" blueboard (1" for every 1' of less cover), but I have yet to see any studies to support this, except that when it has been installed it has solved previous freezing problems.
 
dcasto (Chemical)
6 Sep 07 15:24
The roads I speak of are oilfield, just a blade to push the rocks out of the way in the high deserts of Wyoming.  No ground cover, the snow doesn't stay around long as it gets blown away at night.  At 7500 feet its cold.  I think BigInch has a resonable explaination for type roads I've experienced the freezing phenomena.  

Frost line in a area takes into account the ground cover, snow, wind and such.
DarthSoilsGuy (Geotechnical)
7 Sep 07 14:38
while we're throwing out numbers:
work in Southeast Vermont
our standard detail

water
has 5.5' min
4' min insulated

sewer/drainage
5' paved areas
4' lawn areas
2' min insulated.

we specify 2" rigid polystyrene

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