[Cob] rubble foundation

[Dean Sherwin]

Excellent & full reply to the question  !  (which is a good one)
My understanding as to why rubble foundations worked is that they 
allow water to drain away and therefore there is much less likelihood 
of water being trapped in the ground and freezing and causing the 
heave.  But perhaps that is wrong.  That would still need foundations 
to below frost line I guess.  Those stacked block walls much favored 
for landscaping/ site work just sit on gravel, the footings do not go 
deep.  Mostly the water is encouraged to drain away.  But the 
professionals say they WILL move, it does not matter because they are 
stacked not mortared and can move around.  Yes a barn is even more flexible.
Interior slabs never have footings of their own, they are best if 
detached from the foundation wall in case of differential 
movement.  When a slab is exposed to freezing weather, as in a stoop 
or porch slab, we do put edge downturns on them to about frost depth 
to minimize movement.

Dean Sherwin

At 03:00 PM 8/25/2008, you wrote:

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>Today's Topics:
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>    1. foundation question (Bob Smolen)
>    2. Re: foundation question (Shannon Dealy)
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>----------------------------------------------------------------------
>
>Message: 1
>Date: Mon, 25 Aug 2008 00:58:58 -0500
>From: "Bob Smolen" <boka at mwt.net>
>Subject: [Cob] foundation question
>To: <coblist at deatech.com>
>Message-ID: <000801c90677$ac7392a0$4440becf at 5bvdk01>
>Content-Type: text/plain;       charset="iso-8859-1"
>
>Time and again I read that a foundation must extend below the frost 
>line to avoid frost heave. I also read that a floating slab is 
>suitable for building a structure. These slabs dont always extend 
>past the frost line. The posts in the old barn on my place was 
>supported by piles of rocks placed on the surface of the earth . The 
>barn  failed after 100 years because the roof was not maintained.
>
>Being cautious and conservative, I lean toward following the common 
>advice that a foundation should extend below the frost line. That is 
>at least 4 ft. here in Wisconsin and maybe more. But, I also 
>understand that rubble foundations are acceptable for buildings.Do 
>rubble foundations have to extend below the frost line? Can someone 
>explain how extending past the frost line really prevents heave 
>compared to say a 2ft deep foundation. How is heave a problem in a 
>rubble foundation when the gravel and rubble would seem to give to 
>the movement caused by frost? Have studies actually been done to 
>prove that extending below the frost line prevents heave more than a 
>foundation above the frost line. Why was my barn able to survive 100 
>years sitting on loosely piled field rocks? Why are earth bags 
>suitable for foundations when placed at very shallow depths? Is it 
>all dependent on location? It is all a bit confusing. Is the advice 
>based on science ?
>Thanks,
>Bob
>
>------------------------------
>
>Message: 2
>Date: Mon, 25 Aug 2008 09:51:16 -0700 (PDT)
>From: Shannon Dealy <dealy at deatech.com>
>Subject: Re: [Cob] foundation question
>To: Bob Smolen <boka at mwt.net>
>Cc: coblist at deatech.com
>Message-ID: <Pine.LNX.4.62.0808250854340.31294 at localhost.deatech.com>
>Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
>
>On Mon, 25 Aug 2008, Bob Smolen wrote:
>
> > Time and again I read that a foundation must extend below the frost
> > line to avoid frost heave. I also read that a floating slab is suitable
> > for building a structure. These slabs dont always extend past the frost
> > line. The posts in the old barn on my place was supported by piles of
> > rocks placed on the surface of the earth . The barn  failed after 100
> > years because the roof was not maintained.
>[snip]
> > that rubble foundations are acceptable for buildings.Do rubble
> > foundations have to extend below the frost line? Can someone explain how
>[snip]
> > frost?  Have studies actually been done to prove that extending below
> > the frost line prevents heave more than a foundation above the frost
> > line. Why was my barn able to survive 100 years sitting on loosely piled
> > field rocks? Why are earth bags suitable for foundations when placed at
> > very shallow depths? Is it all dependent on location? It is all a bit
> > confusing. Is the advice based on science ?
>
>Hi Bob,
>
>Frost heave occurs when moisture trapped under the support for a structure
>freezes and causes the building to shift.  When water freezes, it expands
>and can destroy just about anything which traps it and can't move.  This
>one way that rocks are often broken down by nature over time, moisture
>(from rain, humidity, flooding, etc.) gets trapped in a tiny (even
>microscopic crack) in the rock, then when the weather gets cold it
>freezes, expanding the water which forces the crack open wider.
>Eventually with enough cycles of moisture filling the crack and freezing,
>the crack is pushed wider and wider until the rock breaks.  In the same
>manner, moisture trapped in the soil under a building can freeze and
>expand, lifting up the building.  Simply lifting the building would not
>necessarily be a problem, however, it is not uniform, some parts of the
>area supporting the structure may not have sufficient moisture trapped
>under it to cause expansion when it freezes, and different areas may
>freeze at different times or some areas may not freeze at all.  This
>results in different forces acting on different areas of the building, so
>one part of your foundation may be holding up a much larger section of the
>structure than it was designed for, while another one next to it may not
>be supporting the structure at all.
>
>The maximum frost depth for an area is the worst case depth at which the
>ground freezes, so if your foundation extends below that level, the soil
>it is resting on will not freeze and expand, so frost heave won't occur.
>With regard to the rubble trench, the rubble is compacted and a building
>is sitting on top of it, so the rubble is no more free to move than a
>solid foundation, and it still needs to extend below maximum frost depth.
>Speculating here: I would expect that were frost heave to occur in a
>rubble trench, the rock would be able to shift laterally somewhat and
>better distribute the load so that the effect might not be as severe as
>with a cement and steel foundation.
>
>With regard to your barn, it is important to remember that the freezing of
>the ground comes from air temperatures dropping, and a building over an
>area is going to heavily insulate the ground underneath it and will also
>tend to keep the ground dryer (since it's not getting rained on), so posts
>set into the ground inside a barn well away from the walls are unlikely to
>experience frost heave in all but the coldest climates, the primary
>concern is at the perimeter of the building where the ground will freeze,
>so generally the foundation will extend down to maximum frost depth at the
>perimeter, possibly with insulation used around the edges to keep freezing
>cold from penetrating beyound the perimeter to the core.
>
>Getting back to why your barn lasted so long, aside from the inner
>supports probably not experiencing heave, old barns generally have ALOT of
>give to them, so they can shift quite a bit without much in the way of
>structural issues.  A modern house is much more rigid and most people
>wouldn't find cracks in their interior wall surfaces acceptable, where a
>barn doesn't even have finished interior walls.
>
>As far as earthbags being acceptable at very shallow depths, I'm not aware
>that they are, though when used as foundations, they are filled with rock
>not earth, and are therefore, more like a rubble trench, so they may as I
>speculated above shift somewhat laterally when heave occurs, lessening the
>effect.
>
>With regard to floating slab foundations, if they are in fact not
>extending down below frost depth at the perimeter, I would speculate that
>they simply build the slab strong enough that it won't crack when the
>effects of frost heave cause one part of the foundation to take up more of
>the load than other areas.  I don't know since I haven't worked with a
>"floating slab", but that would be my guess.
>
>And yes, frost heave is a well documented and researched problem.
>
>One thing to keep in mind if you see something new / unusual being done
>like earth bags at shallow depth, it may be somewhat experimental in
>nature, and may or may not have problems down the road.  Also, the fact
>that something is being done does not necessarily mean it is "acceptable"
>since many areas don't have codes, and even in those that do, many
>alternative buildings get built without review or inspections by local
>building departments.
>
>FWIW.
>
>Shannon C. Dealy      |               DeaTech Research Inc.
>dealy at deatech.com     |          - Custom Software Development -
>Phone: (800) 467-5820 |          - Natural Building Instruction -
>     or: (541) 929-4089 |                  www.deatech.com
>
>
>
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>End of Coblist Digest, Vol 6, Issue 116
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Dean Sherwin CPE
Certified Professional Estimator
LEED Accredited Professional
CONSTRUCTION COST MANAGEMENT
3, Cherry Street
PO Box 11
Media, PA 19063-0011
(610)892 8860
fax (610) 892 7862
costman at verizon.net