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Question DetailsAsked on 5/19/2016

How can you safely install a gravel driveway over a household water line in a cold climate?

We live in south central Alaska. The frost line can be feet deep instead of inches. Would encasing the section of line the driveway would be over be sufficient to protect it from crushing and frost? I know that compacted soil areas have a much deeper frost level than noncompacted soil.

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Have to put in a discliamer here: Note - this response is intended as general information only and does not constitute a "design" or professional advice for your specific situation.

In southcentral Alaska you are typically looking at 6-10 foot water pipe burial depth (at 1 foot below expected maximum normal penetration depth) unless right along the coast, and not even there in some areas like Kenai for instance, which gets colder winter conditions than much of the railbelt area in Southcentral. Yes a road surfacing does generally allow more frost penetration than ordinary soil - but the difference from maybe 6 inches or so of road versus that much existing normal soil, taking into account the fact that is 6 inches out of 6-10 feet and the insulating properties are not that dramatically different, is pretty inconsequential. And paved roads tend to have better insulating properties than the existing soil in many cases because they are dry, reducing the thermal conductivity. And actually, native soil that has seen ages of freeze-thaw and wetting-drying are generally pretty compact anyway - commonly near as much as the compaction done to driveway base material before paving, so the thermal effect at depth is pretty minimal.

You should not have to worry about casing it - it should be so deep it will see no noticeable load from the vehicles - even for heavy loads like highway truck or rail loads which diminish to an unnoticeable pressure within about 2-3 feet of depth. Only very concentrated loads like very heavy industrial moving equipment and high-load aircraft tires on runways are concerned with wheel loadings on pipes beyond about 2-3 feet in depth. Shallow seasonal use pipes like surface drain pipes and watering system pipes, if shallower than a foot or so under a residential drive, should be cased in a steel pipe or steel pipe used for that section. Casing, unless insulated, would not help with respect to the frost penetration depth - and if the pipe were metal or exposed to the air would hurt the situation.

Unless you are excavating significantly over the pipe to put in the drive, I would not be at all concerned. If cutting more than a half foot to foot or so (beyond that needed to put in the driveway base material before paving which would bring it back up to original grade or higher) then it might be a consideration. If significantly reducing the cover over the pipe than replacement insulation using Dow Hi-40 or Dow HI-60 (higher load capacity) insulation board is the common solution. Designs vary by specific situation, but typically use an R value for moist soil of about 0.125-0.25/inch (but can be higher to about R1) and high-quality closed-cell insulation board is about R5 per inch of thickness, so generally a 2 inch layer of insulation is roughly equal to about 4-6 feet of soil cover - but the width of the board has to extend out to the side of the pipeline on each side at least equal to the depth of the pipe to be effective (to prevent the frost depth from creeping in from the unprotected sides, so in your case say if you are in Anchorage with a 10 foot burial depth in most areas insulation located right under the road base (should be under at least 6 inches of cover) would have to extend at least 10 feet to each side of the pipe alignment under the road if required to replace excavated soil depth. Of course, if a new build then the pipeline could be protected at depth with insulation board - typically about a 3-6 foot width in the trench 4-8 inches thick (depending on cover depth) and located 6-12 inches above the pipe as a rule. A Civil Engineer can provide a more detailed design for your specific case and type of soil, but insulation is rarely needed - usually done more to preserve permafrost than to protect properly buried pipes, though sometimes is used to protect inadequately deep pipes after their first freezing occurrence.

If little or no excavation is needed (other than to get to decent soil depth but being replaced with compacted subbase and/or base material and maybe paving on top) then no real problem usually - the design burial depth accounts for assumed snow removal or snow-free conditions like roads or parking lots. And of course if your drive is built-up above ground level 6 inches or so to prevent overflow icing, so much the better.

Answered 3 years ago by LCD

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