Footings for a new house

Bajohnson

I just started the process of building my house, with the help of my grandfather and others as needed. My grandfather as been building for a very long time, especially in the area I am building. It is all sand and clay.

We had a guy come in and dig the basement and the next day dig the footings since it is hard digging with a shovel because of the clay. That night we got the laser out to check everything and realized we went a little low since the back side of the house is a view out.

He wants to backfill with sand about a foot. We will make sure it is packed. He is very confident in this decision, but I am actually really nervous since the footings won't be on undisturbed soil. I don't have much expierence in foundations, but he does.

Wanting to see if anybody has ideas on the good or bad side of this, or other ways to going about it.

Thanks a lot.

joed

How much extra in concrete is going to cost to make the footing thicker and have them on undisturbed soil?

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Bajohnson

Have not figured it up yet. We had the footings all marked out and dug, now there is back fill over that already. Sure I could find it digging pretty easy. With your statement, would you leave a little sand in the footings since it's clay and just pack it?

joecaption

Sand does not compact.

Duckweather

Right joe. We were taught in school if you fill the foot on one spot you have to fill a foot all the way around with the same material. Identical support is very close to undisturbed.

Bajohnson

The sand is a sand/clay mixture from the pasture behind the future house. It seems like it packs very well. Are you guys saying not to use sand under flat work concrete as well? That is about all I see that happens around here.

They wet it and use a packer on it.

Duckweather

It is still best to fill an equal amount all around. You may never get 100% compaction, or compaction equal to the undisturbed soil without testing. Under the slab is different than the weight of a building on the perimeter footings. That should be compacted as usually done. Another thing many have been doing in New England is a strip footing for any lally columns, not just 2' x 2' pads. The weight on a column is concentrated much more than all the area of the wall footings. The clay should bind the sand together. Sand will not pack well because it is so loose. Just the weight of the building would compress regular sand, that is why it should be filled uniformly.

gregzoll

When they built the Kansai airport in Japan, they compacted Iron ore, before laying the foundation. Talk about a magnetic personality. As for your situation, if it is lower on that end, either lower the rest of the footings, or figure the difference of slope and go from there. I would say that it would be easier as long as they do not go too low, to just bring the rest down to the level of the other.

You may want to consult with another GC or have the inspector come out and give their two bits on what would be best, and go from there. May require changes in the plan at this point, so time to stop and figure out the next move, before you make it.

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hand drive

figure extra concrete for any overdigs in the footings. I've seen our concrete guy dig down 8' in a spot to find solid ground to pour concrete foundation footings. concrete forms are a must if getting into excessive overdigs. sand works good for dry situations but sand might undermine the footing if you do not have your foundation drain 100% accurate.

joed

Why not dig the rest to that level and then have a higher basement ceiling. Make the walls a foot higher.

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Willie T

Damn, someone thinking...... and ending up with a better build due to it.

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jcarlilesiu

Sand and clay aren't the best fill materials to build on. Sand doesn't compact and its unstable, and clay expands and contracts with wetness.

Instead, I would look for a gravel aggregate base. It compacts and drains well. Really, people should be putting down a gravel base before pouring a concrete slab in the first place. Pouring the concrete directly on dirt isn't good practice.

Your other option is to contact the concrete company and have them pour a slurry to 1 foot and let it set for a day or two before forming up the actual footing. The slurry will be a really wet, aggregate filled concrete, cheaper than you would pay for say 5000 PSI concrete.

I agree with the other post though, why not just make the basement walls a bit taller and and regrade around the walk-out basement?

Canarywood1

"The slurry will be a really wet, aggregate filled concrete,"


There's no aggregate in slurry!

ddawg16

When we were doing our footings, my guy doing the digging went down about foot too far....the architect said "DO NOT BACK FILL." The cost of the concrete is nothing compared to potential problems with uncompacted soil....

And, yes, we are on that lovely clay adobe soil....

I like the idea of a higher basement ceiling.....

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Canarywood1

Check this out,this guy's a consulting engineer.


http://www.concretenetwork.com/concr...ils_matter.htm