Waffle-type slab foundation

[melwynnd]

Hi,

I'm new here.  I'm planning on buying the Victoria's house plans at the end of the month(payday).  I will be building to replace the old mobile home we are currently living in here in Basin, Wyoming.

Our soil is clayey for the first 24" and sand and gravel after that.  The water table is very high in summer due to irrigation, but that ends as soon as the canal quits running.  We are in a high mountain desert and get less that 7 inches of precip a year.

Two summers ago we poured a slab for a 40x50 steel shop in several sections.  We've had no cracking so far and there doesn't seem to be much frost heaving.  I want a slab foundation for our house so we can have in-floor radiant heat and to store solar gain.

Could someone tell me a bit more about the waffle slab foundation?  I've never heard of it until I found this website.  I especially like the idea of using the old tires.

Thanks

Sherry

[glenn kangiser]

That is what Jonesy used in Australia.  I haven't seen it here but it might be.  I think some of the others are working on an insulated slab.  That may be good for the radiant heating and storage also.

I remember trucking through Wyoming - It was so cold smoke was coming off the highway

[jonsey/downunder]

Hi Sherry,
Waffle slab foundation.
Ok here we go, the waffle pod system has been in use here in Australia and New Zealand for quite some time. It is also known as rib raft, or waffle slab. It is reputed to be an ideal system for areas subject to ground swell of certain clays, or the subsequent settling of reclaimed areas, when positive loading is imposed. What you have, is a raft slab, 300mm thick with the void formers reducing the amount of concrete required. The slab is just like a waffle, if viewed from underneath.
In most cases, the voids in the slab are formed with polystyrene blocks. These blocks are about a meter square and are set out so they form 100mm ribs through the slab. The perimeter beam is about 300mm wide. There are also systems that use environmentally friendly recycled cardboard. In addition, as you have seen in my case, you can use old car tires. The cardboard system is slightly different, in that it is used as a temporary support platform for the concrete placement until the structural slab sets and can support itself. The cardboard material placed under the structural concrete, gradually absorbs ground moisture and loses its strength and subsequently disperses after the concrete has set, creating a space into which soil can expand without causing damage to the building. Some people believe the cardboard to be superior to the polystyrene because, in reactive soil areas, the soil expansion can still transfer the load to the structure above.

The thing with reactive clay is the constant cycles of wet and dry, this causes the clay to expand and contract. This constant movement is the main problem with most footings. The trick is to try and stabilize these cycles, i.e.: either wet or dry. In most cases dry is the option although I vaguely remember someone telling me of a case where the soil was kept damp. Most folk in this area have a concrete path around the perimeter of the house to keep the water away from the slab. With the slab for my laundry I have not bothered keeping the moisture away and will probably have garden right up to the slab edge. The small amount of movement will not be a problem as the slab is not large and the connection to the main house is designed to be flexible.

At some stage, I intend to build the Victoria cottage for my Daughter and will use the waffle slab as the foundation. I consider it the best option for this area. Because the footprint of the cottage is small, there should be no problems with the slab cracking.  

As far as cost, the waffle slab will save on the amount of concrete used and by using tires as void formers, you save on the cost of the pods. Another plus, is you get rid of a few tires in the process.

I suggest you talk to an engineer to get the best advice for your area. You will probably find that it doesn't cost that much and will be worth it in the long run. I think you will find that most engineers over there will know of this system and will be able to advise you as to the suitability for your area.
Whatever system you use there is always an alternative and the cheapest is not always the best. Foundation problems do not occur immediately, they occur 3 to 6 years after the event and as such, your maintenance cost will be greatly affected, dependent up the appropriateness of the foundation type used. A little bit of extra thought at the design/construction stage may save you money and stress in the future.
Hope this is of some use to you and good luck with your home building.
jonesy

Here are a couple of links for you to check out.
http://www.koukourou.com.au/home/services/waffle/constructiondetails/constructiondetails.html

http://www.koukourou.com.au/home/services/waffle/wafflestep_1.html

PS, sorry about the metrics. There is a conversion table on the index page of my web site, which may be of some use to you if it's a problem.

[Amanda_931]

Jonsey--how did you fill your tires so they wouldn't be full of concrete?

The kind of packing peanuts that melt in water sounds like an idea.

The summer I spent in Mexico, I watched the girls measuring fabric by inchworming their thumb and forefinger along it.  Roughly 10 cm/100mm.  And about the same size as the official "hand" used in measuring horses, 4."   Not even "good enough for government work" but OK to get an idea what's going on.

[melwynnd]

Glenn,

We actually live in the "bannana belt" of Wyoming.  We've also had very mild winters the past couple of years.  However, being a high mountain desert, we get real temperature extremes.  By the way, I love your underground house.  It reminds me of the tree house on the Disney movie Swiss Family Robinson.

Jonesy,

Thank you for your advice.  We don't really have many wet-dry cycles here(perhaps that's why things don't seem to move much).  The soil is damp from the ground water all summer(not usually to the surface unless the neighbors go overboard with the irrigation) and the water drains away through the sand/gravel substrate before freeze up.  Digging our water line to the trailer turned into a much harder task than we expected(isn't everything? because the gravely stuff didn't want to stay on the walls of the trench.  

I was also considering digging a 4' grid of post holes to the sand/gravel layer as "legs" under the slab.  This may be a simpler way of doing things.  However, I thought the waffle slab sounded very interesting and perhaps cheaper.  When you're paying as you go, that is an enticement.  By the way, we have no codes here, and since the septic, gas line, and power are already in....no inspections at all.

Thanks everyone,

Sherry

[glenn kangiser]

Glad you like the cabin -

Did you read this about the light duty concrete floor we posted before.  Sounds like your pier thing but lighter- I haven't had any problems with it.  Made sense to me - I hate putting enough concrete in a floor to support a truck - I'm a little overweight but not that much

http://www.countryplans.com/bbs/messages/7250.html

Works good for an independant floor.  It doesn't depend on compacted ground and in fact creates it's own insulating air space.  Just experimental for me.  You would still need a foundation for the house.  Follow the above thread link for more info.



Glenn

[jonsey/downunder]

Amanda,
You are bang on; rule of thumb 100mm is near enough 4'' ... 25.4mm to the inch.
The tires, I filled with some of the loam dug out when levelling the pad. There was also a little bit of trenching needed to get the 300mm depth for the slab. I just pounded it the same as the earthship folk do, although not quite as tight. Once the concrete is set, a little subsidence is not a problem.
Just a thought on the cost of the foundations. I looked at a number of options when planning the house and found,  by the time I got to "walk on floor stage" there was very little difference in the price of each system. I went with the adjustable stumps because I wanted timber floors.  
Some of the options looked at where

waffle slab, polished and stained
waffle slab with mud floor on top
perimeter foundation with piles

(crawl space)
And the one I chose, adjustable stumps with timber floor.
[/list]
All these methods had to be designed for reactive clay soil. Costing of these systems was based on off the shelf products. Savings in any of the one of these methods depend on how good you are at scrounging (Dumpster diving) and your ingenuity.
jonesy.