My two-and-a-half story house has an ordinary inverted T strip foundation (no slab). I’m digging out some of the (heavy clay) soil in the crawlspace under the house to try to get some standing room under there, and simultaneously digging on the other side of the foundation walls for a French drain. So I’m wondering, does the soil I’m digging out, the soil either side of the foundation above the level of the bottom of the footing, play any role in supporting the house?
I would imagine that when the clay under the footings gets wet it looses much of its bearing capacity and begins to act like mud, squishing out from under the weight of the footings. Does the soil I’m digging out counteract this by pressing down right beside the footing to keep that possibly squishy soil in place?
The code here in California (’97 Uniform Building Code, Table 18-1-C) requires that the bottom of my footings be 24″ below grade (“below undisturbed ground”), but then says (in footnote #2) that I can excavate the soil in the crawlspace to the depth of the top of the footing. That leaves 8″ of “overburden” above the level of the bottom of the footing on the crawlspace side and 24″ on the outside. I’m not seeing what sense this makes. The soil here never freezes, so frost heave is not an issue.
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You will be fine. Unexcavated clay will have an increased bearing capacity when it gets wet. (Unless it is super-saturated) Since clay restricts the movement of groundwater, the water also adds to the support. Soil above your bearing surface does not contribute to the support of your house. So don't worry about the 8" in the crawl space. Upheaval will only occur if the soil is not well compacted or becomes plastic due to super-saturation. Which is one of the reason you are installing an underdrain.
As for digging along the sides, you will also be fine as long as you make sure that you compact properly. If you follow code and tamp at 6 or 8 inch lifts, there should be no problems.
The only thing you need to watch for is buried top soil. If you dig and find some nice, black rich soil... Keep digging.
D&C, P.E.
D&C:
Thanks for your response to my question about how clay soil supports a foundation. You said a couple of things that puzzle me, so I’m wondering if I can get a clarification. You say that water "adds to the support." Is that right? Everything I’ve read about soils engineering seems to be predicated on the notion that water diminishes bearing capicity, and certainly the clay seems a lot stiffer when it’s dry.
Also you say, "As for digging along the sides, you will also be fine as long as you make sure that you compact properly." It’s not clear to me what I would be compacting. I’ll just be removing soil, not adding any. But your saying this seems to suggest that the soil outside the foundation wall does somehow help the soil under the footings bear the load of the house, even though earlier you say, "Soil above your bearing surface does not contribute to the support of your house."
And by the way, I was going to install the drainage system no deeper than the bottom of the footing, to avoid disturbing soil that might be necessary to bearing the load of the house. But I don’t see how that’s going to protect the soil beneath the footing from super saturation if water rises from below during the wettest part of the year.
Thanks a lot for your help.
--Jack
Jack. Sorry, I have an engineer's disease that prevents me from talking clearly. ;-) When you are talking engineering properties of soil you can be discussing it in different ways. With bearing capacity, your concern is with voids. As an extreme example, AASHTO No.1 is generally accepted to have a 0.40 void ratio. (40% is not stone) Which is why it's great for underground storage of water. This also applies to clay. Although I don't know the ratio off the top of my head, even well compacted clay has voids which fill with water. In disturbed earth this can be a problem for two reasons. First, water expands when it is frozen. This is powerful enough to move houses, which is why code requires you to dig so far underground. Second is that disturbed earth will slowly leach out it's water content. In addition to normal settling, the weight of the structure will push out water over time that compaction equipment does not. For undisturbed earth, the ground water in the voids of the soil is uniform. It's everywhere! On top of that, the soil has been compressed over generations. Chances are that there is next to air. As water is not compressible and it has place to go, this increases the bearing capacity of the soil.When you mentioned the drain, I mis-understood and thought that you were digging below the footing. If you were, then you would need to compact your stone that you would fill around the drain pipe with. Soil above your bearing surface does not contribute your bearing capacity. The soil immeatiatly next to your footing does. Not much, but it does contribute. As you are not disturbing it, don't worry.Finally. Super-saturation. Seasonal highwater will not cause this. condition unless you are on the edge of an large hill or cliff. Even then it is a rare event. At your depth, with proper grading around the house, you will be fine.
Dazed and Confused is my son's favorite movie! But seriously Mr P.Eng, while your attention is on this thread, I have a soils question regarding clay.
I am a concrete contractor in Ontario Canada. An foundation I poured years ago gave me some grief as follows. Upon completion of the excavation the clay ( undisturbed) dried out and turtle backed . My opinion is, .. it shrank.
We filled the interior of the foundation to the top level of the strip footings with clear stone, and installed perimeter draintile/weeper system.We also placed interior drain tile, all connected to a plastic sump box.
The house framed quickly, and we placed a 3 in basement floor slab, as per code.
Several months later, I was contacted by the owner to do remedial work, repairing cracks in the basement floor. Upon examination with a straightedge at right angles to the cracks, uplift of 1/2 in over 4ft appeared.
As the weather had turned to fall, and we had experienced much rainfall, I explained to the owner that the soil condition was the culprit as I believed the clay had expanded.
Do you think my assumption reasonable?
I hate to do this, but I think that the answer is both yes and no. If the clay dried out, it did shrink slightly and would then swell when it got wet. However I does surprise me that it lifted that much since you had your stone in place. It makes me wonder about the loading on the soil prior to excavation. How far did you dig down? Is there a natural watertable that is close to the surface? Was this area a soil stockpile prior to excavation? How long was the pit open prior to placing the concrete?One of the tricks when constructing large buildings and skyscrapers is to dig deeper into the ground. The deeper you go, the more the soil is loaded (compacted) by the soil and water above, which allows you to install a smaller footing. This is very important in the cities where you are hemmed in by roads, alleys and other buildings that you cannot touch. Once soil is unloaded, it starts to expand. Like a sponge when you ring the water out. As soon as you let go, it pops backing into it's original shape. If your bearing soil was heavily loaded by a high ground water table or a soil stockpile, it may have just expanded.
Thanks for the reply.
The excavation was 5 to 6 ft deep. Undisturbed clay, formerly cornfield, waterfront, set back 150 ft. The excavation took lace in July, our hottest season. Open approx 14 days before placing any concrete. Noticable fissuring (turtleback) of clay at time of footing pour. Excavation done with straight edge bucket. Clay cut like cheddar cheese.
Floor crackingin late Sept or October after heavy rains and..... drilling of artesian, overflowing well..which I beleive contributed heavily to excess groundwater.
Thanks again for the expert opinion. Cheers.
Henry,
>>And by the way, I was going to install the drainage system no deeper than the bottom of the footing,
Place the drain system on top of the footing. The drains are not to keep water out of the soil (impossible) but to keep hydrostatic pressure away from basement walls. You will also need a drainage plane next to the walls, typically as high as 1' below grade, down to the drain.
One concern is the need of a french drain system right up against your foundation. The results of any problems from doing this have huge impact. I would consult an engineer prior to doing this. Soil in most places moves 2-4 inches due to lunar cycles. I live in Texas and they usually use a minimum footing. Most houses setting on clay have shifting problems.