Why are shallow foundations embedded a minimum depth?

[Lomarandil]

In most geotechnical reports, and a lot of common practice, I see that shallow foundations are set to bear at an assumed minimum depth (here, 1m).

I sometimes feel that this is not strictly required, so I want to understand the reasons such a requirement might be made:

A few that I can think of:
[ul]
[li]Avoiding organic materials[/li]
[li]Frost depth[/li]
[li]Some degree of erosion protection[/li]
[li]Potentially less moisture content fluctuation[/li]
[li]Providing some minimum overburden to boost bearing capacity[/li]
[li]Incidental lateral capacity[/li]
[/ul]

Any that I'm missing?

----
just call me Lo.

 

[geotechguy1]

Frost and seasonal moisture content variation are the big ones.

It's not always strictly required - I've been involved with projects that went for shallower footings using insulation around the building to control the frost issue. Also many quasi-temporary structures like sheds or detached garages usually don't have deep footings, but it does mean they move around more seasonally and may have a shorter lifespan than the house.

 

[TigerGuy]

If you are working in an area with expansive clays, there is a depth to which the moisture will fluctuate. Below that depth, the moisture holds pretty constant. You want your foundation elements to bear in that constant moisture level to avoid the potential for shrinkage and swelling of the clay underneath.

In heated buildings, in areas with freeze thaw cycles, your interior foundations often will bear just below the slab. The heating of the building keeps frost from causing problems under the interior foundations. In unheated buildings, all foundations will likely be designed at the same depth for uniformity.

 

[Phil1934]

12" for chipmunks, future plantings

 

[Ron]

In my area it is mostly for erosion/scour.

 

[HTURKAK]

I should add also the following items ( at least common practice im my zone )

- The soil bearing stress is defined for 1.0 m depth ,

- Due to the tree roots which may cause movements and shrubs ,

- To make sure below the levels of holes or cavities which could be made by burrowing animals ..

 

[ukengineer58]

if there are trees nearby and materail is cohesive (clay) then you could get settlement if tree demands water. So you go below that level.
Dont think there is a 'standard' or normal depth. can be buried or at grade depending on the circumstance.

 

[dik]

I would think protection from shrubs is OK, but not trees.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[ukengineer58]

Dik, I was referring to the depth that the tree extracts water from. Not protection from routes. The extent of water demand can be several tens of metres away from the tree and way outside the root zone. In the UK depending on the tree type and plasticity the closer you get to the tree the deeper you need to go. Until of course your outside the influence zone.

 

[Lomarandil]

UKengineer58, is that influence zone quantified anywhere for engineering purposes? Or does it require the expertise of a horticulturalist of some sort?

----
just call me Lo.

 

[dik]

Thanks, I gathered that. The effects of the 'root ball' can have an effect on a very large area and depth. A lady a couple of blocks away has foundation problems that may be caused by the desiccation of the clay soil by tree roots. We have a highly plastic clay, often with a PI in excess of 80. Shrubs may be less likely an issue.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[ukengineer58]

Lomaranddil, in the UK (not sure where you are based) it is contained in a design guide - NHBC Part 4 chapter 4.2 building near trees. You can design justify not following it to the letter by looking at potential settlements etc and depending on what you are doing/building. this is a house building guide essentially so gives charts of depths etc. But it gives you details of when you should be concerned at least. You can download it free. But i am not sure how applicaable it is outside of the UK.

 

[dik]

I did the design of a home for an architect student (I was a student, too) about 55 years for his parents. It was 'nestled' in among several mature trees will allowance for branch movement. It was founded on friction piles. The main house area was framed using OWSJ, with a 2x6 nailer and 2x6@16 joists over. I don't know if there was provisions for roots and any drainage system. It was interesting to see it among the trees.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik