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is 12 feet from grade bottom of spread footing ok? any constructibility issues?

is 12 feet from grade bottom of spread footing ok? any constructibility issues?

is 12 feet from grade bottom of spread footing ok? any constructibility issues?

(OP)
i have 6'x6' spread footing. i need to miss a pipe so i had to got 12 feet below grade. it made me think that this is too deep below grade and maybe difficult in construction like setting up the rebar, etc.. but i'm not too experienced in construction maybe this is ok.

RE: is 12 feet from grade bottom of spread footing ok? any constructibility issues?

The excavation will have to be shored for the safety of the workers. Otherwise, for just an axial load, it should be OK.

RE: is 12 feet from grade bottom of spread footing ok? any constructibility issues?

Difficult constructability! Must be shored as hokie66 noted. Limited room for hand excavation....too small to get equipment down there.
Have you considered drilled piers and either a cap on the pier or multiple smaller piers with grade beams?

RE: is 12 feet from grade bottom of spread footing ok? any constructibility issues?

Is it just for a single footing? Could you precast it and scoot it under the pipe?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com

RE: is 12 feet from grade bottom of spread footing ok? any constructibility issues?

1) Check with the civil consultant. Depending on maintenance concerns etc, it may well be possible to leave your footing at a higher elevation than the pipe and eliminate the concern altogether.

2) Other than excavation cost, I see no reason why this couldn't be an unshored open excavation. In my neck of the woods, the slope between footings required by geotechnical is usually shallower than that required by OSHA and constructability anyhow. So, depending on proximity, neighboring footing might have to be lowered as well.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: is 12 feet from grade bottom of spread footing ok? any constructibility issues?

Just curious, but would differential settlement be of a concern here? Will the soil at bearing elevation of -12'-0" behave differently than the soil at say -3'-8"? How forgiving is the project that you are working on if this "deep" spread footing doesn't settle as much as the adjacent footings?

EIT

RE: is 12 feet from grade bottom of spread footing ok? any constructibility issues?

Check the water table at the site. Depending on your location this could be an issue.

RE: is 12 feet from grade bottom of spread footing ok? any constructibility issues?

I would consult a geotechincal engineer.

RE: is 12 feet from grade bottom of spread footing ok? any constructibility issues?

Excavation straight down is common in bridge construction. This is often performed with a crane using a "clamshell bucket", see the image below. A good crane operator can dig a surprisingly neat hole with a firm bottom. A foot, or so, of crushed stone at the bottom will provide a good working base. Twelve feet deep is not a big deal. Of course, as mentioned above, shoring will likely be needed. Depending of jobsite conditions, water table, and soils, a sloped excavation may be faster and/or more cost effective than shoring.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

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