Re: SB: Re: CLAY SOIL

[Speireag Alden <Joshua.M.Alden.91@alum.dartmouth.org>]

Sgrìobh an interested poster:

>I am planning on building a sb in Minnesota.  The frost-line is four 
>to six feet.  Please describe your foundation.  If you know of any 
>good references I would be appreciative.

     I'm posting this to the list in case anyone is interested.  For 
the sake of privacy, I've omitted the name of the person who asked me 
the question.

     Most of what I learned about rubble trenches I gleaned in little 
snippets from here and there.

     For a frost line that deep, a rubble trench may still save money, 
or it may not.  As I said before, it depends on how narrow you can 
keep the trench, and the deeper you go the harder it is to do that.

     The thing to remember is that anything which retains water will 
heave.  Soil retains water.  Sand retains water.  Pea gravel doesn't.

     So, there are four essentials for a rubble trench foundation:

         The trench has to go to the frost line, like any foundation.
         The fill must be pea gravel or coarser, so that water will drain.
         There must be a drain to daylight out of the trench, so that 
water *can* drain.
         There must be a barrier to soil infiltration, so that your 
gravel, over time, doesn't become soil with a lot of gravel in it.

     My RTF is five feet deep.  I dug it with a backhoe and levelled 
it all around, with a drain out of one corner to daylight, sloping 
gently downwards.  I laid down a bit of 1.5-inch bank-run gravel to 
get a slope of 1/4 inch per foot from the far corner to the daylight 
drain.  Then I laid 4-inch perforated drainage pipe (the corrugated 
kind) all around the trench, and out down the drain to daylight.  I 
put landscape fabric against both sides of the trench and pinned it 
in place, temporarily.  I covered the drainage pipe with some more 
1.5-inch bank-run gravel, to protect it.  My trench was wide, because 
my ground had large rocks and they disturbed everything as I dug them 
out.  So, I put the biggest ones back in, which saved a lot of gravel 
in the long run.  Then, by hand, I packed bank-run gravel around 
every rock I could find and filled the trench up to ground level.

     My trench was overkill, but it will never heave.  In the end, it 
cost a lot of time, but at the time I had very little money.  So it 
worked.  If I had it to do all over again, given my site and the 
economics of my situation at the time, I might go with an underground 
house, which obviates the need for more than a slab.  Maybe next time.

-Speireag.
-- 
To face death, live.

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