[Cob] Marlin's rubble trench

[Peter Ellis]

It seems to me that the material in that trench isn't chosen just for its 
loadbearing character - in fact, might even not be primarily for loadbearing 
qualities.  I would expect that if it's not going to be a monolithic 
waterproof block (say poured concrete with a waterproofing coating) that it 
is very important for water to flow through it readily.

No matter how well the material can withstand compression, water freezing 
and thawing within it *will* break it down.

I would go along with Marlin on this one.

Peter


>From: Marlin Nissen <marlin_nissen at yahoo.com>
>To: Mary Lou McFarland <louiethefifth at hotmail.com>, Coblist at deatech.com
>Subject: Re: [Cob] Marlin's rubble trench
>Date: Tue, 1 Nov 2005 08:21:38 -0800 (PST)
>
>We used 'river stone' or washed stone - 1 , 1 1/2
>inches usually...it's actually from glacial drop
>around here.
>
>I have read (and it made sense based upon experience)
>that crushed limestone (finds etc.) packs down and
>actually becomes a type of lime/mud. If you drive on
>country roads (and I know you do, you're probably
>happy that it's not just dirt roads where you live!)
>you see the limestone and fines break down  into finer
>and finer particles on the gravel roads.
>
>I assume underground that small limestone particles
>pack down into this same fine mud. While that may be
>'weight bearing' for awhile it also concentrates
>water, possibly heaves and can become a mud flow
>instead of foundation. Eventually, even if it's
>decades, a foundation of crushed limestone/fines seems
>like it would become indistinguishable from the mud
>around the foundation. That's what road base becomes
>as it breaks down if you dig into it with backhoe.
>I've seen the underlayers of a gravel road and it
>didn't look like a good foundation to me. Potholes,
>ripples, trenches, heave cracks .....
>
>UNcrushable glacial washed stones will never (in our
>short lifetimes or human span) breakdown, drains water
>around them very well and supports and distributes
>weight very well. We even put landscape fabric around
>the sides of the  trench to try to keep all
>organic/compactible matter out of the rubble trench
>itself. On a house I'd try bentonite or another
>barrier as a skirt going out from the foundation to
>make it dry and better insulated as well.
>
>BTW, a sandy/fines subfloor seems very different as
>it's not expposed to outside water (different drainage
>and temp shifts) so it's main function is to allow
>floor blocks to be layed or a thin layer of cob on top
>of it. When it's exposed to mud it becomes ONE with
>the mud. Sand still seems better then limestone as it
>too doesn't really ever breakdown and is excellent of
>nestling something into it like concrete or
>flagstones.
>
>Marlin
>
>--- Mary Lou McFarland <louiethefifth at hotmail.com>
>wrote:
>
> > Marlin you mentioned in your post that you like the
> > washed gravel over the
> > crushed limestone.  Just wondering why you had that
> > preference.  Hadn't
> > thought about it before but had assumed that I would
> > use the crushed stuff
> > because of it's greater stability  when taking on
> > weight or impact like
> > floor or arena base or shoulders on secondary
> > highways.  For clarification,
> > when you say crushed limestone, I am assuming that
> > you mean what we call
> > base gravel around here and it has all the fines in
> > it and that is what
> > gives it it's strength.  Also wondered what size you
> > usually go with.
> >
> >
> >
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