Conventional Foundations in Expansive Soils

[cbear1]

I am currently working on a project where the soils engineer has called for conventional foundations in soft, expansive clay soils. (Class CH, Blows 4-9 to 41' in depth). The foundations have been deepened to 5', the level of a fairly stable water table. The soils engineer states that conventional foundations should be fine since the water table is stable and foundations will not experience the shrink/swell otherwise anticipated. Also, he states that deep, soft soils preclude the use of friction piers. Instead the foundation engineer has designed 6' wide continuous perimeter footings to even out the bearing over soft soils. Is this a prudent measure? It seems like the water table must vary somewhat throughout the year, but the engineer seems to think that this type of clay has such low permeability that the moisture won't change much - any initial thoughts/opinions/suggestions? Thank you all so much for your help!

 

[csd72]

How does the geotech know that the water table is stable if he has only visited the site once?

 

[theCorkster]

cbear1;

Provided expansive soils moisture content do not change, then the detrimental impacts associated with shrinking and swelling should not occur. This direction relies on the fact that the current water table elevation does not change over the life of the structure. Since he's indicating the level at 5 feet, one would assume that he is setting next to either a lake or river, or in close enough proximity that the groundwater level supported by these water sources doesn't change. Is it possible that the water table can change in the future due to changes in the way these bodies of water are maintained?

The selection of the continuous foundations over pile support is plausible - the question is what is the estimated settlement of the foundations under the design loads? More information regarding the strength and consolidation characteristics of the soils to depths on the order of 25 feet would be needed to respond to this question.

 

[cbear1]

Thanks for the input - basically, the site is located in a lowlying area equidistant between a river and a slough (each about 1/2 mile away)and the soil is composed of clay with very low permeability. The soils engineer has performed investigations on several parcels in the immediate vicinity and says that the water table is constant there as well, and seems to think even if there were a bit of seasonal variability the lack of highly permeable material would keep the bottom of the foundations moist.

Consolidation should not be much of an issue since the project is actually putting a new foundation beneath a 100 year old house. The house is in the same location as it has been and is up on jacks currently - it is assumed to have consolidated to the point of not being a major design issue.

I welcome any additional comments anyone may have.

Thank you!

 

[fattdad]

Pretty standard design. If the ground water table is at 5 ft or so, there will be sufficient capillarity to maintain a pretty consistent moisture content irrespective of whether the water table goes up or down a few feet. More importantly is to keep the footing away from the upper few feet where summer drying will occur.

We typically would lower footings to 4 ft if in an area of expansive silt/clay.

f-d

¡papá gordo ain’t no madre flaca!

 

[JAE]

Hmm...it all seems to make sense (that the moisture content will be fairly stable under varying weather patterns....but do you really want to pin your design on that? What if there is a prolonged dry spell? Some clays can be quite powerful under shrinking, and then subsequent swelling once it starts raining again. It would be a mess.

 

[msucog]

the soils in mississippi i grew up around sound similar to this scenario (PI 60-90). the fix sounds along the lines of what i was taught since the groundwater levels there don't see drastic changes and what changes that do happen don't do all that much because of the very low permeability. even in the middle of a scorching hot summer, you dig down 2 feet and the stuff is just as wet as it ever was. most of the problems i saw were due to footings located shallow and then experiencing the uneven shrinking/swelling changes from season to season.

 

[fattdad]

but do you really want to pin your design on that?

There's a reason why professional liability insurance for geotechnical engineers is more than for structural engineers. Beams are predicable, eh?

Making a design decision based on geology requires experience in the local area and a proper field exploration. That said, for the case that the water table is truely at 5 ft (not some seasonal perched effect), then I'd go with the recommendation to lower the footing into the s/s soil and move on with life. I would not go with a 2 ft bearing depth as there's just too much moisture variation in the near surface.

f-d

¡papá gordo ain’t no madre flaca!