Depth of a Shallow Foundation for Residential Homes

[AymanRaouf]

I struggle with foundation engineering books in giving a straightforward steps to determining the amount of excavation I need to do for a residential home.

Any advice on this matter? When you make a sample pit, do you do it on surface? Do you do a sample pit at depth intervals?

What soil parameters do you look into?

Once you have a characteristic along depth profile, how does the depth of foundation affect the strength based on Terzaghis equation?

I came across Rankines equation that gives an angle of repose. Are there situations where this is not acceptable?

 

[1503-44]

After removing the top layer containing organics and any previously disturbed soil, usually 6 to 12in, and groundwater is not a problem, the most important variables are frost depth, soil type and bearing strength.

Provided all are OK, the edge of a turned down slab (concrete filled trench) is often between 12 in minimum to 24in, or frost depth as a maximum. Of course it may also depend on keeping the floor level on the topography of the site and height above finished grade to facilitate proper slopes for drainage away from the slab.

Reinforcement looks better done like this one below, although this one says 24" minimum below grade. Your local building code may give some requirements as well.

Soil bearing capavity
Typically an increase in depth raises the gross soil bearing capacity by the same amount as the weight of the soil removed above.

Soil expansion coefficient and water/moisture content
Clay soils that expand with water content change may need to be excavated and replaced, or mixed with lime, or other treatment applied to reduce volume changes. Paying attention to drainage can help too, both around the building and under the slab, which is why 4" min of sand and gravel is added there.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[iandig]

Whilst you may be outside of the UK, the UK does have very clear guidance on this published by the NHBC (National House Building Council) which includes the depth of foundations, AND not limited to just considering the stiffness and bearing capacity of the soil.
There is an update to the NHBC standards done annually, with the next version going live on January 1st 2024. This has a number of updates and additional chapters, including 4.6 on earthworks (I was part of the expert panel)
If you are looking for clear guidance on how to go about the assessment on foundation depths, this would be a good place to start. The NHBC standards are written in a manner to be read by a wide audience, considering the limited technical knowledge & experience a self-builder may have all the way up to National House Builders, Consultants, Engineers etc.