Footing in wet, high clay content soil

[joenorm]

Hello All,

I am working on a perimeter foundation with a footer that is 16 inch wide and 6 inches tall. The stemwall height is 2 feet. This is a single story residence with a one-room second story.

The footer is sitting on undisturbed, dense clay in western Washington State. I have heard of no reports of expansive clay in my area.

My question is about water content of the clay and how it can affect the foundation. During the winter I have seen that water will move under the footer during heavy rain as there is a bit of lateral water movement though the slope.

The clay goes from rock hard when dry to having a squishy top layer when wet. I am wondering if this "wetting" will affect the foundation over time?

How does "dampness" or "wetness" affect the bearing capacity of soils?

Thanks

 

[oldestguy]

Sounds like pretty suitable support. Without test data no specific bearing capacity can be stated. All clay likes to hold water yet it is the shear strngth that we are concerned with,usually. I'd contact a local geotech firm if you are really concerned. If possible be sure all standing water is taken out of footing trenches when placing concrete. You don't want to soup up fresh concrete, since it can severely weaken the final hardened concrete product. On your question about water content effect on clay type soil, usually the higher the water content, the weaker the shear strength and also the higher the potential for settled footings sitting on it. Clean sand is not usually weakened by high water content. These are general statements, usually applicable to your situation.

 

[joenorm]

Thanks for the reply. The foundation is already poured and half a house is sitting on top of it. The site was excavated in the spring, I noticed a little seeping ground water then. Now that it is raining more I am seeing the hillsides true colors.

I understand you need a sample to say for sure, but generally speaking is it OK for footings to sit on wet ground? This must happen all the time all over the world? It's just my house now so it concerns me a little.

It's not a flood or anything, but when it rains hard there is water intrusion under the footing on one side of the house, the uphill side. I imagine this will only get worse as the winter rolls on.

It's only in the low septs that it collects. If I had gravel in the crawl filled all the way up to the tops of the footing I would never see the water. Its because the crawl floor is the same elevation as the batting of the footing.

So is this a concern? I don't want my house to sink into liquified clay, but I may be overreacting.

thanks again

 

[oldestguy]

One thing you might do to ease any concern is do an elevation survey of the house. It is simply done without surveying instrument. Take a garden hose and get hose connection attachments for both ends to then extend the length of hose about 3 feet with clear plastic hose on each end. Fill with water and remove all bubbles. Stand the clear hoses side by side to be sure the water level in each is the same. Now you have what a surveyor would call a level instrument in effect. Get two yard sticks. With a helper on the ground floor. have one stand at your reference place, such as where a floor is supported by a foundation that is relatively free of possible "bad" or changing elevation (sinking?). Then start an elevation measuring survey of that floor elevation. At the reference place (surveyor would call it a bench mark) read the height of the water (bottom of the surface) above the bench mark. Also then read the height of water at the other end at one place on the floor. Record both. Do the same at all places to "measure elevation". Both may change due to the position of the hose, yet both water levels are at equal elevations. number or letter each place. Always then you have a reference of elevations with respect to the bench mark. Next month do the same, etc. This way you an get an on-going record of the house floor elevation. My opining, based on your comments is that running reword will always show the same elevation differences, if any, from the bench mark.

 

[joenorm]

I have a laser so i can do an elevation record with that. The soil in question is not pure clay, it has a bit of sand and gravel mixed in. It is absolutely rock hard without standing water on top of it. It does get a little squishy on top when water sits on it for awhile, presumably because it brings more of its "fines" to the surface which are pretty gooey.

Again, I may be overreaction. People have told me I should have filled the crawl with drain rock up to the tops of the footings. Thats just not what people do around here so my excavation contractor would have told me that was a waste, even if I would have suggested it. But if I would have, I would not even be seeing this water I am seeing. The footing would be in wet soil over the winter but it would be out of sight, out of mind.

I own the house directly next door to this lot and live in it. The whole lot, including bottom of crawl is almost pure sand. Strange how things can very site to site. I was really caught off guard by the clay.

Thanks for the responses. What a cool forum this is!

 

[oldestguy]

With your laser you will have several set ups, maybe. What is its accuracy?

 

[joenorm]

It's a nice PLS, pretty accurate.

 

[joenorm]

So, seeing a small amount of water on the surface of the crawl floor is not necessarily a huge red flag?

It only seeps under on the uphill, wet side of building. I have dug and installed a "ditch" style drain to intercept the water flow but it has not stopped 100% of it.

 

[oldestguy]

Hey: When I was growing up I lived in a house (old house). Under the house in the crawl space was standing water, The side hill backyard had a spring house with a depressed"container". Years earlier that was the source of household drinking and cooking water. With ventilation under the house all the wood floors, etc. had no effect from that, etc. So I say, it is a common thing and we live with them.

 

[joenorm]

Oldestguy-

Thanks for taking the time to respond to my inquiries. It is challenging trying to find reliable sources these days.

-Joe