frost protect shallow footing utilities
I have spent a bit of time looking for some guidance in past posts on how to place utilities under a frost protected slab on grade, but have found none. I live in central Wyoming with a frost depth of about 4 feet. I will be building a fps house this summer and was wondering how i go about placing the corridor for the plumbing (waste) under the slab? Do i put it in a insulated box under the slab? do i trench and compact? at what depth should the waste pipe and water pipe and gas pipe leave the slab? also, it seems that the best thing to do to save energy is to build the insulated barrior under the slab as if it were an uninsulated building. If that is the case, is there a problem with the penetration below for the waste, water and gas?
thanks for eveyones help on this issue and my past posts.
No. 1 What is the Frost Free depth in your locality???
No. 2 Keep all you liquid services below that depth until you are inside the edge of your frost free foundation by at least that depth. Have a Factor of Safety of 2 or greater in that distance.
No. 3 Turn your pipes up and come out the ground.
No. 4 Back fill any trenches with very good draining gravel or rock for a foot or so. Put in a layer of filter cloth to keep the fines out of the backfill. Compact as required and backfill the remaining trench depth with good drainable fill compact in reasonable lifts until you get to the depth of the bottom of your frost free foundation. Put in whatever insulation you are going to use and either form your foundation or make the required trenches for the foundation.
No. 5 Place you concrete and finish the surface. Your done in a very simplified explanation.
As with all this new stuff, there's a simple answer and a more complicated one.
The simple answer is highly rooted in tradition and would suggest that the utilities should stay under the frost line until within the perimiter of the foundation, as was suggested earlier.
However, the basis of a frost protected foundation is that the frost line close to the house is higher to grade than the areas further away and that the foam allows the frost line to rise further than it would otherwise. All this you know already.
Applying the same principles to your utilites, it isn't a big stretch to see how you're perfectly safe to have water and sewer less than 4' deep if it's either close enough to the house to benefit from the shallow frostline, or under it's own insulation, again to keep the frost line shallower than it would be without intervention.
I think it was Piffin that once posted a link to an article about shallow frost protected pipes at a university. They used ridgid foam under a parking lot and monitored the frost line with simple remote thermometers. Quite effective, but no hard rules or anything like that.
If there is a section of hard rock that makes going deep expensive, we've run water and sewer at 18" below grade with no problems on the Colo/Wyo boarder using insulation to raise the frost line.
For a rule of thumb take a look at the information for unheated buildings and shallow frost protected foundations. It's probably a little conservative for plumbing.
I'd love to plant some remote temp probes under, at and directly above the ridgid foam. Our next project will have that built into the design from day one, mostly for curiosity sake, but it could be justified on the basis of making sure the system actually works.
On the basis of what's most practical, run the utilities deep until within the foundation unless you're on rock.
Best of luck
Beer was created so carpenters wouldn't rule the world.
I think from a practical POV it is better to bury the water lines deep, but sometimes I can't because of ledge.In that test, they extendded the foam panels 4' sideways. That means a water line - instead of buryiong it four feet deep, would go 18" deep, but need to have the foam 8' wide over it. That's a lot of digging.goal for us is a four foot deep frost level for footings and water supply. Electrical at 18"
Waste lines are often 10-12" down in some spots. rarely a problem there, though one of my guys just had his waste line freeze up for a few days.
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Don, jvhanna, Piffin:Thanks for all your help agian guys. I will go with the deeper lines as it is not a problem here.steve
Where are you located in Wyoming? I live in Carbon County and want to approach the building official here about frost protected shallow foundations. Would adhere to the unheated type since I want to insulate under the slab and use radiant floor heat.
That is a great comment!
There is a lot of truth to it.