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Old 04-08-2011, 09:22 PM   #1
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Deep (versus wide) foundation question


When we were leveling our lot, right at the depth to which we were leveling, the bulldozer hit a 2 inch thick sheet of shale under which was clay.

We've tried for weeks to find a geotechnical engineer with no luck. There just are none locally to be had and even those in Tulsa, 75 miles away, just won't even return our calls or emails. They do commercial work, not residential, and, apparently, no one wants to do residential work here.

So we talked to our structural engineer who is designing our foundation (along with the engineering on the rest of the house). His said that what is normally done here for that is to dig two foot deep, two-foot wide (basically square) foundations instead of foundations where w = d*2.

He's the engineer and has the experience in this area with our kind of soil so I'm not anxious to question it but I would like to hear from others about this idea. What do you think?

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Old 04-08-2011, 09:28 PM   #2
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Deep (versus wide) foundation question


Beyond my expertise, but it sounds like he is using the double thickness to span the unstable clay areas. Seems smart, but I really don't know.

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Old 04-08-2011, 10:57 PM   #3
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Deep (versus wide) foundation question


Well, I just met with him. Maybe he just doesn't communicate well, and he's an SE, not a geotechnical engineer, but eventually, only after I brought up soil heaving from wet clay, he pointed out that having too wide of a foundation would increase heaving. Makes perfect sense. My thought was to go to a 3 or even 4 foot wide footing which would certainly help compensate for the soil bearing capacity but, as he pointed out, would mean that the weight per square foot would be reduced to the point that heaving would be worse.

The only thing that scared me is that he seemed completely unaware of the concept of a foundation like I would be used to seeing but eventually mentioned the idea of a spread foundation, which technically is what most foundations are called, right?

We will start laying out our string and batter boards tomorrow based on the house dimensions. Hopefully I can get over all these footing questions soon so we can add in the footing strings and then start digging.
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Old 04-09-2011, 12:14 AM   #4
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Deep (versus wide) foundation question


We have some expansive and clay soils here in So Cal also and I understand that to compensate for possible expansion of soils during wet weather the footings are quite a bit deeper than wide, 2-3 times deeper.
Makes sense to me.

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Old 04-09-2011, 02:55 AM   #5
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Deep (versus wide) foundation question


Wider causes more heaving? Why? Is he saying that the weight of the building on a smaller footer gives more psi and will therefore keep the footer from lifting? I am no engineer, but my guess is that the force of the soil freezing could give a fat rat's about the weight of the house "holding it down". That is just my gut feeling, but I'd ask on a geo engineering forum, or call one and run this scenario by him/her. It does not compute here, but then other things don't sometimes, too. Dunno, but it's fishy. If in doubt, go deeper than the frost zone, no? Does your building department have a code they go by for foundation depth? Ours is 40" to the top of the footer, but then we are guaranteed of some freezing.
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Old 04-09-2011, 08:57 AM   #6
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Deep (versus wide) foundation question


Oklahoma is well known for expansive clay soils. The issue is that the clay expands and shrinks substantially due to changes in moisture content, which can occur due to long term weather changes, runoff from the house being directed away from or towards the foundation, or plantings. This is a very serious problem, which can lead to cracks in the floor, out of level walls, and foundation failure.

You really need an engineer who understands the problem, because not all clays are expansive, and you need tests to find out how much of a problem you have, if any. I guess I understand why a geotechnical engineer would not travel 75 miles on a residential job, they probably get paid $100 per hour, and few residential owners are willing to pay the $1000 or so it would cost the engineer to travel, collect the samples, run the tests, and write the report.

The best option may be to find a local structural engineer who is familiar with the problem, and knows how to design a foundation to avoid problems. I have seen prestressed and post-tensioned slabs used, houses on helical piers, houses on piles, and other techniques. There is an entire industry in your area devoted to retrofitting houses that have heaved or sunk due to improper foundation design. Much less expensive to do it right in the first place than try to fix the problem after it appears.
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Old 04-09-2011, 10:18 AM   #7
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Deep (versus wide) foundation question


Daniel, you may have a good tip there. You are right about the foundation repair industry being big here. I will call some of them and see what they say. Hopefully they will be willing to help me not need their services. It's worth a shot.

As for the geotechnical engineer, I would more than gladly pay one or two thousand to know how to do this right. The only price I got was a completely unofficial price from a Tulsa company who is actually doing a city water treatment plant just 1 mile from me. For 20 to 40 thousand, they do core samples on their commercial jobs. Our excavator has a relationship with this company and he called their chief engineer. The engineer agreed that the excavator could run down to the water treatment plant and pick up one of their engineers just to take a look and would charge us "a few" hundred dollars. But a week later and nothing.

I know the 20K to 40K is high but if my house caves in, I'll wish I had spent it. But if I have to spend it, the job is just off. I don't know where you can get an engineer for a thousand, or even two, or three. If I could, I'd pay it.
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Old 04-09-2011, 10:43 AM   #8
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Deep (versus wide) foundation question


I am building an extension to our house at the moment, and the subsoil is a low shrinkable type clay, but because there are trees nearby I had to go down over 7ft below the base of the trees. It is worth getting a soil test done, mine cost about $60. If you roll the clay in your hands and it breaks up easily it can be a good sign.
I also had to put clay boards on the inside of the trench.
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Old 04-09-2011, 03:18 PM   #9
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Deep (versus wide) foundation question


Quote:
Originally Posted by stuart45 View Post
I am building an extension to our house at the moment, and the subsoil is a low shrinkable type clay, but because there are trees nearby I had to go down over 7ft below the base of the trees. It is worth getting a soil test done, mine cost about $60. If you roll the clay in your hands and it breaks up easily it can be a good sign.
I also had to put clay boards on the inside of the trench.
Attachment 31769
Thousands of miles away, and your soil looks just like our's here in WI..........

Stu, did you just trench pour the below grade portion of the foundation? I know I saw pics of your fancy brickwork for the foundation, but didn't realize it was sitting on so much concrete footing.
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Old 04-09-2011, 03:29 PM   #10
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Deep (versus wide) foundation question


Hi Joe,
The concrete was filled to the bottom of the old foundation, which is the regs here. That left 10 brick courses to the existing floor level where the new joist hangers will sit. There was 5.5 cu metres of concrete in it, £438.
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Old 04-09-2011, 03:37 PM   #11
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Deep (versus wide) foundation question


That's kind of what I figured from the pics Stu.

Not exactly sure on the exchange rate to US dollars is though............

What is the addition for, and how far are you on it now?
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Old 04-09-2011, 03:47 PM   #12
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Deep (versus wide) foundation question


It's an extra bedroom, as it's only a 2 bed cottage. It's only up to DPC today, will put a few pics on the other thread tomorrow.
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Old 04-09-2011, 03:47 PM   #13
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Deep (versus wide) foundation question


Up to $40K ?!?!?!? Hold on...... I changed my mind. I'm sure I'm fully certified, or registered, or anything else you may need me to be to do that testing....... and I'll print up any kind of bogus documentation on my home computer printer to "prove" it.

And I'll pay for the plane ticket.

And rent my own car.

Good grief, they should hang their heads in shame for that quote.

Just my opinion, of course.
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Old 04-09-2011, 04:56 PM   #14
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Deep (versus wide) foundation question


The engineer had another interesting suggestion in regards to the drainage around the footings. He suggested trenching for the footings first, and then come back afterward and dig up for the drain. The idea is that it saves on buying/renting/building forms for the footings. What do you all think of that one?
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Old 04-10-2011, 10:49 AM   #15
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Deep (versus wide) foundation question


£438= us$ 716

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