Interior footing minimum depth

[LUGuy]

Are there any minimum depth requirements for spread footings under interior building spaces?

We have a single-story masonry building that will bear directly on bedrock. Rock is around 1' below the proposed slab elevation. To minimize rock excavation costs my intent is to excavate rock (which may be within inches of the floor slab) to just 12" deep and then to fill the trench so that the top of footing is equal to the top of rock.

Similarly, if the earth will be backfilled several feet against an exterior wall, and the top of floor slab is deeper than the frost depth, would the exterior wall have any minimum depth requirements?

TIA,
Miscmetals

 

[ashjun]

Hi Miscmetals,
I do not understand ur idea of flushing the top of the footing to the surface of the rock. Could you please enlighten me?
I have seen interior footings being constructed at 0.5 m depth and if there are surface rocks/boulders, the footings have sometimes been placed even at the surface - this may have to do with the entire floor slab resting on the soil-boulder matrix (?)
Regds

 

[LUGuy]

If the footing were set on top of the rock - rippable weathered basalt according to the geotechnical report - the footing would protrude through the slab in some areas. Trenching into the rock will allow the footing to be below the slab. Setting the top of footing elevation = the excavated top of rock elevation provides for a constant depth of gravel base.

As the rock extends deeper in some areas, the footing may be set directly on top of rock without the need to trench.

Thanks,
Miscmetals

 

[GeoPaveTraffic]

I know of no reason to place interior footing, i.e. footings that are not in a frost zone, to any specific depth.

 

[Focht3]

It sounds reasonable to me. What does your geotechnical consultant say?

Where is the site located?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[LUGuy]

The geotechnical report says all footings are to be 24" wide and to bear on rock. I believe they mean bearing on the weathered layer, not the bedrock which is up to another 5' deeper. I haven't confirmed that assumption yet.

The site is in New Jersey.

Thanks for the comments!

Miscmetals

 

[Focht3]

"Weathered" means different things to different people. And the design bearing pressure is an important factor, as well as variations in the 'quality' of the weathered material.

The real issue isn't the bearing pressure - it's settlement. To achieve uniform settlement, you will need uniform support. Don't second guess your geotech -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[CSEllc]

Typically, interior footings in a heated space are installed just below the slab (the top of the footing is 8" below the top of slab) so that the anchor bolts don't stick up thru the floor.

If the space is unheated, thge footintgs should extend to below frost or may bear directly on competant rock. It sounds as if your srock is easily removed so I would remove the "weathered" rock to below frost and place the footing there.

Usually, code requires tyhat thge bottom of footing elevations be placed below the finished exterior grade, assuming a heated space. If the space is unheated, I would assume the lowere of the interior slab or the exterior grade as my "finished grade" and use that as my reference elevation.