Conventional Foundations in Expansive Soils
Conventional Foundations in Expansive Soils
(OP)
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!





RE: Conventional Foundations in Expansive Soils
RE: Conventional Foundations in Expansive Soils
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.
RE: Conventional Foundations in Expansive Soils
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!
RE: Conventional Foundations in Expansive Soils
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!
RE: Conventional Foundations in Expansive Soils
RE: Conventional Foundations in Expansive Soils
RE: Conventional Foundations in Expansive Soils
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!