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Re: GSBN: Rubble Trench Foundations (was Cemento (combined responses))



Graeme -

I've used rubble trench foundations on small buildings (less than 600 sq.ft.
or 60 sq.m) but would use a similar system on larger buildings, and probably
2 story buildings, including in high seismic zones (like California or
northern Pakistan).

But certain issues would need to be looked at carefully, especially relative
to "holdowns".  That is, what the uplift loads are, what the holdown system
is, and how it is anchored to the foundation system.  The issue of the above
ground bond beam sliding on top of the rubble trench also needs to be
considered.  There is a fair amount of friction there, depending on dead
loads, the contact area, and the irregularity of the contact surface, but it
needs to be evaluated relative to the design lateral loads of an earthquake.

The issues I just described probably don't need to be considered if a
building is not subject to earthquakes, although severe wind loads could
impose similar or greater forces, especially uplift if there are wide
overhangs (which many SB buildings have) and the structural details are such
that those uplift loads get transferred through the holdown system and into
the foundation before the roof gets blown off.  Darcey Donovan and I have
been using holdown systems that engage the entire rubble trench (and its
associated weight) but if you engage only the above ground bond beam there
is obviously less weight (than a full concrete foundation) to counter the
uplift loads. 

My typical rubble trench is as follows:

1. Trench to below frost depth or code minimum into undisturbed soil (12"
for 1 story, 18" for 2 story).  Width as needed structurally.
2. Line bottom and sides of trench with filter fabric to keep soil out of
the trench.
3. 2" of gravel on bottom of trench, then perforated drain pipe sloped to
outlet.
4. Fill trench to 3" below grade with rubble, stone, gravel, throw in the
kitchen sink if you want (not really, you'll need it in the kitchen).
5. Form and pour a reinforced concrete stem wall (I suppose you could call
it a grade beam) on top of the gravel.  The concrete should have the
appropriate bolts, and holdowns.

In one building I had small concrete "feet" poke down into the gravel to
help resist sliding, but I'm not sure how effective they would be.  I've
used earthen floors with these foundations, but you could pour a separate
concrete slab as well.

Regarding your concern about wet soils, that's one of the advantages of this
type of footing.  It is self draining and should keep water away if the
drainage components are done correctly.  The system can drain without a
drainage pipe by moderately sloping the bottom of the trench to a low spot
that outlets away from the building, but a drainage pipe is preferred.

I won't go into all the holdown options, but straps, threaded rod, and mesh
can all be used in various ways engaging the top plate, and the bottom
plates, concrete grade beam and/or gravel trench.


Martin Hammer



> Graeme North wrote:
 
> I hate large foundation trenches filled with more of the material that
> necessary where it is mostly used as for bulk stuffing, esp under wide straw
> walls.
>  
> So back to rubble foundation trenches Ð has anyone any idea of how these may
> be designed to work in a seismic area where the soils are often wet, and
> where we need to keep the bottom of strawbale walls well above the ground
> (say 300mm) to keep them dry?
> 
> As it happens I have been seriously asked to look into this,  this very
> week, but most engineers here are stumped -
> 
> Here is a more thoughtful response than some Ð maybe there is some
> information to be gathered from out there on this list?
> 
> I have seen rubble trenches a lot in books on eco building from the US.
> I have no experience with them. I do not know of any recent buildings
> that have rubble trench footings in this area. They have significant
> advantages in areas subject to heavy frost which is not such an issue
> here. With a rubble trench footing you generally need some kind of
> concrete footing. I generally always recommend reinforced concrete
> footings. A reinforced concrete foundation beam is required for the
> earth walls to act as shear walls.  I am a great fan of the concrete
> raft type footing and have  utilised them now in
> several earth walled buildings.
> 
> Cheers,
> 
> 
> Graeme,
> Graeme North Architects,
> New Zealand