What footing changes needed for sand?
I’m new to the experience of additions and foundations, so what we hit
while digging for the new footings for our addition surprised me.
We’re adding on an addition which will have 9′ deep basement to our existing house, built in the early ’50’s, which has a standard depth basement.
So we’re digging pretty deep, past the existing home’s footings, about 4′ deeper, maybe 12′ deep total.
Clay, clay, more clay. Pure clay. Such pure clay, there aren’t even any stones/rocks/pebbles in it, just pure, pure, clay. Clay everywhere.
Until we reach the corner where the addition will join the existing house. Sand. Black sand. Looks like an ancient bank of a stream.
Dry sand, but the pocket seems deep. Just a pocket of black sand in the corner of the excavation, sort of rising up in the corner about
two feet above the floor of the excavation covering a triangular area in the corner of about 10′ along the new side wall, 6′ along the existing wall.
So again, I’m new to this. Is this anything to worry about? Are we going to have future settling problems in that corner? What happens when the drain tiles around the footings bring moisture to this currently dry pocket of sand? It is dry, just a pocket of sand capped by 10+ feet of pure clay. What would you do in this situation? And again, it’s in the corner of the excavation, where the new basement wall will butt up against the existing basement’s wall and footing. The rest of the excavation’s floor seems to be solid clay (it’s holding up under the weight of a good-sized backhoe anyway).
Our contractor is saying wider footings with rebar (and that’s the first time I heard footings aren’t regularly poured with rebar around here…I thought that was a given). I’ve asked him to add rebar to all of the footings, not just in the affected area. But will that do it? Anything we should do to the pour above the footings? To the ‘joint’ between the house and the addition? Any special concrete mix/blend?
This is in the Chicago area, so we are subject to freezing and get plenty of rain.
Thanks in advance for any help you can offer.
In a different vein: anyone have any theories on the geology of the situation? How does a black sand beach get capped by 10+ feet of clay?
Thanks again.
Replies
A: Talk to an engineer or architect- let them tell you what's required to accomodate the different material (if anything). The building department may require amended plans anyway, so why let the contractor dictate the change?
B: Rebar isn't necessarily "required" in the footings, but whether it is or it isn't, it shouldn't be a mystery- it should be clearly delineated on the plans (you did have plans drawn by an architect, right????).
C: Sounds like the sand may be material that was brought in for backfill when the house was originallly constructed. It shouldn't cause any problems for you, but have it looked at (see A above).
Bob
"(you did have plans drawn by an architect, right????).'
Uh ... why Bob?
do they have some special superhuman powers your average builder/remodeler doesn't ?
we'd better put a stop to this design/build trend while we're at it too, huh.
Jeff Buck Construction
Artistry In Carpentry
Pittsburgh Pa
it doesnt not matter if the material is sand or clay when size of footing is designed. The calculation is figure using a bearing ration of the material. sand and clay has a close bearing ratio. Now if it was muck or uncompacted soil it would have an effect. you can increase the safety factor by adding rebar or increasing the strength of the concrete.
"do they have some special superhuman powers your average builder/remodeler doesn't ?"
No....but they do have a little thing called "foundation design courses", which gives them a slight edge over the poster's GC, who obviously hasn't a clue what to do with the situation he now finds himself with. More importantly, like the soils engineer, they have something called "errors and omissions insurance" to cover them if they mistakenly sign off on the soil as being OK to proceed on- know many GC's who have that? Didn't think so.....
My comment on the architect had more to do with the soils situation and the "isn't rebar typical" question- seems to me that someone hasn't a clue. I've seen all too many times where a GC says "don't worry about having an architect draw the plans- I'll draw them, you sign them saying you drew them, and we're good to go". See, here in NJ, the only people who can draw up plans for permitting are an architect or the owner- not the GC, not a "design builder"- and he who signs them owns liability for them.
I have no problem with design-build as a contract type- in fact, we do it ourselves- but we partner with an architect and engineer for the design portion. It's the "I''ve built dozens of additions, so surely I can design them too...." mentality that's the problem.
Kinda reminds me of the old Aamco commercial- "I once fixed one-a them-there foriegn transmishions once....."
Bob
"Kinda reminds me of the old Aamco commercial- "I once fixed one-a them-there foriegn transmishions once....."'I thought that you where going to use the "Payme now or Payme Later" commercials.
>> you did have plans drawn by an architect, right
What's that got to do with his situation? Thousands of homes are built without an archi, right? You sound like you're trying to guilt trip the guy."But I say to you who hear, love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you.... and just as you want people to treat you, treat them in the same way."
"you did have plans drawn by an architect, right
What's that got to do with his situation? Thousands of homes are built without an archi, right? You sound like you're trying to guilt trip the guy."
If you read my response to Jeff above, you'll see that I'm not guilt tripping anyone. I'm trying to point out that neither he nor his GC seems to know what to do with this "mystery soil", and there's also some confusion regarding rebar requirements. All too many times, a GC takes it upon himself to play designer in situations he's not qualified to do so in (see my response again). I was simply pointing out that if an architect was involved, perhaps he could shed some light on the soil situation and the need (or lack thereof) for rebar. If not, a soils engineer certainly could.
No guilt trip meant or implied.
Bob
Hi,
I recommend consulting a bonified soil engineer. Remember that in the last ice age your area was near the southern toe of the ice cap. A soil engineer would have a good idea from goverment soil surveys and should attend your site for samples and evaluation. The cost should not be much compared to your building budget and it might just save you lots of money and heart ache.
Good luck for a solid foundation,
Virginbuild
OK, I read your message, and based on my own doubts/concerns, I called up a soil engineer and made an appointment for Monday.
Meanwhile, the village came out to do an inspection, noticed the sand, and told the GC that he'd need a soil engineer's report before he could proceed. The GC calls me late Friday afternoon.
Did I look good or what? "the soil engineer will be out first thing monday morning" I say after he tells me what's needed and offers to try to find an engineer for me.
On a sort of related note: I've got the Tauton Press' Foundation book, and I've read it through. One thing that seems to be glossed over and missing is cure time for footings. How long should a footing cure before the basement walls are formed and poured on it? I would think the footing is the most critical to cure properly. 28 days in Chicago in June? Just looking for some independent input.
Thanks
Hi again Raokman,
sorry I got back to you so late, darned night shift.
I understand that under good temperature and moisture control, concrete should have about 70 percent of it's strength in about 7 days. If you pour generous footings with rebar I should think any expert would feel comfortable pouring a wall for residential dwelling after 7 days rest for the footing. The pressure per square inch of a 10 or 12 foot wall should work out to less than the 70 percent cure strength of the footing. Check the weight calculation for a lineal foot of 10 or 12 foot wall and work the weight into the square inches of bearing surface and you have the pressure per sq inch you will exert upon the footing. Here are two links you can go to for reference.
http://www.uwtc.washington.edu/courses/231/documents/StudentexampleRR.pdf
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http://concreteproducts.com/mag/concrete_curing_highperformance_concrete/
Good building!
Virginbuild
I think it can beshown that the curing time of footers is a non-issue. The first of your nice links shown that concrete achieves a strengh of about 500PSI at the end of 24hrs. That works out to 72,00 pounds per square FOOT (fig #1 in your link).
Now concrete weighs about 150lbs per cubic foot wet or dry. Therefore a 10 ft high foundation exerts a pressure of about 1,500 PSF on the footer, which is well below the footers load bearing capacity of 72,00 PSF at the end of day one. Since I have never seen foundation forms erected in less than a couple days, by the time anyone is ready to pour foundation walls, the footers are more than strong enough.
The 70% of full cure is the number engineers use to allow the concrete structure to used as intended. In this case, when the concrete is 70% curred, it can safely carry the house, funiture, occupants and the baby grand piano in the corner.
Chris
Its all that clay that would scare me! Sand Good! Clay Bad!
I'll have to disagree with your blanket statement about clay being bad. Around here, light grey mucky clay is bad. Dark red clay is a good thing and, generally, will easily yeild 2000 PSF of bearing. You are right that sand will yeild more bearing capisity, up to about 3000 PSF, but who needs that much unless you are building a 5 story building ;-). When I have a foundation dug and have all red clay, I'm very happy.
Read what Brownbag says. He has it right.
As to the origional poster's Q, a $5 bet says that the soil engineer will say throw some rebar in it and have a nice day, although I'll have to admit I've never seen black sand except in costal areas. Matt
The problem with clay is generally it's expansiveness. Most clays expand when wet, contract when dry. They also, of course, expand when frozen. Though most soil types do this to a degree, certain types of clay are exceptionally bad and can move up and down several inches depending on the amount of moisture in the soil.
I'm aware of the problems with clay in certain parts of the country. On the other hand, the red clay we have here in NC is great for building houses on though. As I said, to say that clay is bad is just not true in all cases. Matt
just want to add one point from a geotechincal point of view. The red clay that you talk of is really not clay. we have the same soil. Its primary family is sand, so it would be classified as a red clay sand. which is an excellent building material. a true clay when wet will stick to your feet and you will grow as you walk. no water will pass throught it. It will swell and move foundations.
Now yor describing the hill I live on."But I say to you who hear, love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you.... and just as you want people to treat you, treat them in the same way."
Spoken like a true front range foundation man!"But I say to you who hear, love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you.... and just as you want people to treat you, treat them in the same way."
Gracias!
Just to close out the thread:
The soil engineer came out today and looked things over, poked and prodded.
The final/official report will be issued tomorrow, but after talking with him today, it seems he'll be recommending(?) that the sand be dug out to a depth of 2 feet, the space filled with 18" of 2" diameter gravel (limestone) capped with 6" of 3/4" gravel. All gravel fill to be firmly tamped down. No wider footings, but rebar all around. He had no concerns about the load bearing capability of the clay, just about the "differential loading characteristics" of the two materials.
Thanks for the info on the footings. Still seems a little odd to me all the graphs and discussion about cure times in the Taunton book on foundations, but no discussion/information regarding the footings themselves. This footing WILL be holding up the concrete walls of an extra deep basement, and the one-story addition above will be bricked. Hopefully, as was mentioned above, the logistics of things will delay the load placed on the footings long enough for them to cure properly.
Thanks
I used to build in rockford, and the norm was 16x8 footers and the next day form up walls and pour. Our foundation had one run of steel close to the top.(which is more then anybody else was using.)Dont worry about curing time, because the footer is 4000lb concrete and will not be stressed. Being in the soil it will cure slower and be stronger. I have been building in Tenneessee for last ten years, and our soils can change every ten feet. So we over build our footers 24" x 12", two runs of rebar and ladders every 4'. Please do not confront your GC but work with him, and communicate or this project will be going south.