Adjacent foundation element

[autokoz]

Looking for practice is setting elevation of adjacent foundation elements. For spread and/or continuous wall footing(s) I have used the rule of thumb of 2 horizontal to 1 vertical unless soils report says something like 1 to 1 would be acceptable. Have used the same general rules for adjacent pile caps, bottom of caissons. Are there any other opinions or systems that do not follow this general practice?

 

[SlideRuleEra]

The US Army Corps of Engineers "Engineering and Design - Bearing Capacity of Soils" says that the spacing should be a minimum of 1.5 x the footing width

Foundations on footings spaced sufficiently close together to intersect adjacent shear zones may decrease bearing capacity of each foundation. Spacings between footings should be at least 1.5B, to minimize any reduction in bearing capacity.

(footing width = B)

Here is the link to the document, see page 1-14

http://www.usace.army.mil/publications/eng-manuals/em1110-1-1905/toc.htm

http://www.SlideRuleEra.net

 

[eric1037]

We usually recommend 5 horizontal to 1 vertical "steps".

 

[Blodgett]

In lieu of stepping down footings due to proximity of adjacent footings, you could consider excavating and pouring a lean conc mix, say f'c=1500 psi & unreinforced...bott of lean conc fill to comply with the 2:1 and then place your footings at the higher elevation on top of the lean concrete mix.

eric1037 - 5 horz to 1 vert...is that for bott of caissons? seems high for typ footings.

 

[eric1037]

You can place foundations closer together as long as you account for overlapping shear zones for bearing capacity concerns as well as overlapping zones of stress increase for settlement analysis.

For most cases, the 5 horizontal to 1 vertical will reduce overlapping zones. However, for large foundations, this may need to be increased.