[Cob] Marlin's rubble trench

[Marlin Nissen]

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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