rubble trench foundations

[bigmish]

I am currently designing a 24x36 1½  story timber frame home. I am considering using a rubble trench foundation. I few questions follow:
- The only source for information I can find on this method is in Fine Home Building's "Foundations and Concrete Work"; anyone know of any other books or articles on this topic?
- In this article 2 8" concrete blocks are used over the grade beam. Do you feel the point pressures on these concrete blocks are such as to require pilasters (as recommended by Benson in "The Timber Framing Book" for Concrete-Block foundations)? Or should I use a poured concrete wall on the grade beam over the rubble?
- Anyone used rubble trench foundations for timber frames before? Any thoughts, concerns?


Thanks very much for any advice, Mischa

[Don P]

Superior Walls uses a crushed stone footing. I've worked off of one recently. Still have trouble wrapping my head around the idea but it has worked fine. I think you'll find particulars in the 03 IRC codebook. I have the manual for our project somewhere around here.

[slowzuki]

Hi,

Our place (32 x 64) is built on a rubble trench.  I started from Fine Home Buildings detail as well.  I did find a few other references but it was called a water managed foundation etc.

Ours has 3 ft wide trench dug with a 22" bucket, the narrowest we could get.  The whole thing slopes to a corner drain the same with 4" drain tile.

We are different in that we used filter fabric to line the trench then used clean 1-1.5" crush compacted in 4" lifts to fill the trenches.  We then built an 18" high reinforced continuous footing on that tied to the slab.

I would change a lot of detail on the slab but the trench worked well.  We are in a wet area, the neighbours spent a lot of money drying out their basement while building as they hit 2 springs in it.  Our low drain runs during the spring and drys up come summer.

[TW]

Sorry for stealing the thread
Just out of curiosity:
What is a rubble trench fundation?
How does it work?

[scsmith42]

TW, with a rubble trench, instead of having your foundation footer resting directly on soil, instead it rests on a bed of crushed aggregate (rubble). 

This helps to drain the soil under the slab, and the aggregate will help distribute the weight across poor load bearing soil.

[TW]

Thanks for the information.
This sounds similar to a common method we use in Finland for one family houses and outbuildings.

What size is the rubble stones in the American version of rubble trench?

[exssnelt]

Typically the stones are refered to as #57 stone which are 3/4" to 1 1/2" in diameter.

[bigmish]

TW,

I was wondering if you could direct me to any sources that you may know of on this topic even if they are in Finnish (I can have my grandfather translate). ...

Here is a link to a book that contians the article I mentioned if anyone wishes to buy it:
(click) Long Link fixed (admin)

.M

[TW]

bigmish
The best i have is only in Swedish on paper and I do not have a scanner nor the skill to use one but I found something on the internet.    www.betoni.com/harkkokasikirja     It is an online handbook for block construction but it contains a few thoughts on rubble trench in the chapter "perustus"=foundations.

I am in the later part of buiding engineering studies so I think I can give some thoughts if you tell theese facts about your site.
- soil type?
- the maximum depth the soil freezes in winter?
- type of floor (joists or concrete slab or....)?
- basement?
- dry or wet place?

I am NOT able to dimension the foundation correctly for American cirkumstanses but I may have some roundabout ideas about the principles and about where some pitfalls are. I am currently working on the third rubble trench foundation I have made.

[bigmish]

TW,

Thanks for the link. I'll have my grandfather check it out next time we meet.

The details you asked are still unknowns to me at this point as I'm still in the research phase and we haven't yet purchased the property. I can answer the following:
- type of floor (joists or concrete slab or....)?: Stick framed sill placed on the blocks
- basement?: No

As an engineering student, any thoughts on this portion of my question: 2 8" concrete blocks are used over the grade beam. Do you feel the point pressures on these concrete blocks are such as to require pilasters (as recommended by Benson in "The Timber Framing Book" for Concrete-Block foundations)? Or should I use a poured concrete wall on the grade beam over the rubble?

.M

[TW]

I think you mean a self carrying wooden floor with creep space below and the joists bearing on the plinth.
I hope I got it right.

First some theory

The point loads from the posts will be great and a block wall does not distribute this weight over any greater lenght of the footing at the bottom.  The footing in itself has very little bending capacity which means that the loads on the soil will be great under the posts and almost nonexistant between the posts.

If the load bearing capacity of the subsoil is critical then a poured concrete wall is better. It can be reiforced as a continuous concrete beam. Then it will even out the point loads along the wall. If the bottom bars are in the footing and the top bars in the top of the wall they will work together as one beam if there are enough stirrups connecting them.

If the load bearing capacity of the sub soil is not critical then I suppose concrete blocks would work but I do not know the compressive strenght af the American typer of blocks. However I would use pillasters with the ones we have. That would also require a substantial wooden sill to distribute the point load at least a bit to each side.

The height of the footing must be at least equal to the width of the part of the footing that protrudes outside the wall. 

The agregate belog will distribute loads about 30 degrees (depending on the kind of aggregate)to each side.

Back to real world
The footing will ned very litle crosswise reinforcement bars. Common sizes for footings are 60X20 cm and 75x25 cm. The aggregate must be well compacted , even outside the footing. Whatever compacting method you use you will probably have to compact it layer by layer as few vibrators can compact more than 50 cm deep. The drain pipes should be well outside that 30 degree angle below the footing, that is in unloaded gravel.

How big problem is frost where you live? How deep may the ground freeze. You may have to dig down styrofoam to prewent the ground from freezing if you are not very far south.

I am unable to post pictures, otherwise I would make a sketch in word or autocad that shows the angles I write about. Can I e-mail it directly to you?

I hope this will help you at least a bit.

[bigmish]

TW,

I'm planning on building a stick-framed deck of 2x10s with the sills directly on the foundation (the blocks) with a crawl space below. Note sure what you mean by "plinth".

These illustrations would be great, thanks so much. Here is my email address (I'm putting it on several lines so spam sniffers wouldn't find it):

mischa

@

mischawilliams

.

com