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RE: Frozen soil = disturbed earth?

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I should say the soil under the slab actually settled in spots and there were
voids between the slab and soil.  They had to "mud jack" the slab to fix it.

-----Original Message-----
From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)usace.army.mil] 
Sent: Friday, March 20, 2009 2:19 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Frozen soil = disturbed earth?

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  seaint(--nospam--at)seaint.org at 15:21:36 on 20 Mar 2009.
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I live in Alaska and had a friend put in a 4' foundation wall which was going
to have platform floor and crawlspace.   He did not put the platform floor on
and backfilled and let it sit all winter and the wall cracked up.  The soil
expanded and pushed inward on the wall.  I have also seen foundation basement
walls kick in during the summer that did not have the platform floor built on
top.

I have seen a slab-on-grade that that settled badly after being placed on
frozen soil when the soil thawed.  You cannot get 95% compaction when placing
frozen soil because the moisture has expanded 9% in granular soil and because
there are ice chunks in silt or clay.  When it thaws and the water drains out
the ground settles under the weight of the slab and building contents.  The
same thing would happen to footings placed on frozen soil.

Builders in Alaska frequently let foundations sit out all winter while they
are  platform framing the unheated building on top without problems.  The
footings are placed on thawed soil first.  My theory is when the ground
freezes during the winter the foundation walls span between hard spots and
the wall settles when the building is heated and the the hard spots turn
soft.  I think the foundation walls are normally not backfilled until the
ground thaws however and the foundations are normally heavily reinforced.  

I haven't yet seen a problem here with foundation walls cracking up that sat
over the winter unless they were backfilled, but like I said the local
foundation walls and footings are heavily reinforced and the foundation walls
are normally 4 feet tall.  



-----Original Message-----
From: Jim Wilson [mailto:wilsonengineers(--nospam--at)yahoo.com]
Sent: Friday, March 20, 2009 5:26 AM
To: seaint(--nospam--at)seaint.org
Subject: Frozen soil = disturbed earth?

Is previously frozen soil considered disturbed earth?  Perhaps there isn't a
definitive answer to this question, but in general terms, if there is
evidence that the soil has frozen and heaved, is it thereafter unusable for
placement of a footing?
 
The easy CYA engineering answer is to require a soils engineer, do testing,
compaction, etc.  But sometimes the easy answer is not the right answer.
 
For example, one numbskull called me this winter with cracks in his
foundation wall.  Sure enough, he placed new concrete walls and footings in
December, left them exposed and then we had sub-zero temps for about two
months straight.  Come February, the walls had several cracks.  No wonder.
But after the frost subsided, the cracks closed and didn't look like much of
anything.  Is there an argument that the foundation is okay (notwithstanding
analysis of the cracks) because it returned to its original condition?  Soils
in our area are typically sandy gravel with decent bearing capacity.
 
Jim Wilson, PE
Stroudsburg, PA

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