This is a situation I inherited from someone who previously worked for the company I do. Have a building lot with soft soil that tends to be we too. It is in a neighborhood of about 20 lots. About 10 of them have been built on with no or minimal problems. These are starter homes with very tight budgets. This one is a spec – if that matters. My pretisessor had the footers installed on the problem lot. Normally, we only need to go 1′ deep as there is, for the most part, no frost line here in this part of NC. For these footers, when the excavator started digging he found soft soil so a soils engineer was called in and engineered footers were installed to 4′ deep consisting of about 3′ of gravel and 1′ of concrete. I would guess that an extra $2500 has been spent on the engineered footers thus far. The idea is to build a stem wall foundation, backfill with gravel (~1.5′), add insulation and plastic, etc and then a slab on top of the gravel. The slab would be supported by a 4″ lip around the 8″ thick stem walls (4″ brick veneer on outside) and by the gravel backfill. Problem is that the soils engineer recommends that the interior of the foundation be excavated out to 4′, and then filled with gravel – #67 washed stone. This much stone would be cost prohibitive – about an additional $2800, not to mention the extra money for excavation. I’d estimate $4000 total on top of the $2500 already spent on the engineered footers. The main house foundation is basically a rectangle that is 28′ x 48′.
At this point the choices are to continue with the slab floor, or build a crawl space type floor system which would also add additional expense, would mean the permit has to be redone, and the footer contractor would have to come back to install pier footers in the interior of the existing footer “ring”.
Here is the question: Could a slab be installed on top of the existing soft soil and ~1.5′ of gravel such that the slab would essentially be supported by the stem walls – ie how to make a slab rigid enough of span 28′? The house is to be a single story and will have a trussed roof with no point loads in the center of the slab – all truss loads are transferred through the exterior walls to the stem walls.
Thicker slab, more steel, and grade beams all come to mind.
Attached is a pic of how the construction would look under normal conditions. plastic film, insulation and reinforcement are omitted for simplicity’s sake.
Although I know this is BT, and everybody gets their say, I’d prefer to here from people who have actual, first hand experience with this type of thing.
Sincerely,
Edited 5/22/2005 2:46 pm ET by DIRISHINME
Replies
We have poured a suspended slab over a basement which had a 24' span in the narrow dimension. There were a couple of beams formed in the slab. The slab was 5" thick with #4 rerod 9'oc. I do not remember the precise size of the beams or the rerod in them. The slab was designed by an engineer and worked beautifully.
It wasn't cheap. I think steel joists with a slab poured on top would have been less expensive. In your case the stone would be less is my guess. In any event this isn't something that you want to wing.
Thanks for the benefit of your expierence. Re the extra expense for grade beams and thicker slab, I'm thinking, maybe $1K - $1.5k extra for engineer, extra concrete and steel, which would would be much better than the $4k for the extra excavation and ~140 tons of extra stone... At this point, I just need to get headed in some direction...
Matt
There was a article in the last 1-3 years, think that it was JLC, but might have ben FHB, about basment slabs in parts of CO. Because of expansive soils they suspend them.Don't remember a thing about the details or cost.What about a number of piers that go down to good soil and then grade beams.
Thanks Bill! I found the article; JLC 11/02. Although not a "direct hit" it has some usefull informaton... Re your second comment, I'd like to stay away from additional excavation if possible.
Matt
Edited 5/22/2005 7:49 pm ET by DIRISHINME
I think steel joists with a slab poured on top would have been less expensive.
A structural engineer once designed a reinforced slab (roof, in my case), to replace the steel bar joists I've been using. It was several times more expensive for the 30' span I needed. When I asked, he said that was why you see steel bar joists everywhere. Still have the engineering.
I'm also pretty sure that stone is the cheaper alternative for Matt, much as I favor bar joists. Sorry Matt, no experience, but I butted in anyhow. <G> If you can specify a load I'd be happy to read a bar joist table for you. Same as your local steel company. Sounds like the economics would only work with a value-added basement. PAHS Designer/Builder- Bury it!
just fill with sand instead of gravel
OK - but how is that going to compensate for the soft soil below?
Matt