Can you Increase Bearing Capacity with Depth

[jwhitmer]

I am working on sizing out some footings on a project and I am wondering if there is a way to increase your bearing capacity the further down you put the footing.

I feel like I have heard the IBC or IRC has something about this.

I am looking to see if I can go 12" deeper into the soil for a pier footing if I can get my bearing capacity to increase.

 

[skeletron]

Probably a question you can squeeze an answer out of the geotechnical engineer on the project. Typically they give you bearing values based on standard depth and footing widths. Deviations from those parameters is allowed, but they would know how that affects the bearing value. Usually it is all settlement driven.

It is intuitive that in a homogeneous layer of soil, the bearing should (or probably would) tend to increase by a margin as you go deeper. This is also reflected by the depth factors (Hansen, 1970) that are used for the modified Meyerhoff bearing equation. There can be more factors in play, and that is why the question is better for the geotech.

 

[JLNJ]

Sometimes, there is a soft layer which you want to stay away from.

I doubt that a geotechnical engineer would condone any sort of automatic additional capacity just be going deeper.

 

[MTNClimber]

Allowable bearing capacities provided by the geotechnical engineer are 99.9% controlled by settlement, not the ultimate bearing capacity divided by a factor of safety.

So 99.9% of the time the answer is no, it won't make a difference if there is a surcharge from burying the footing deeper (unless you don't care about settlement).

 

[Tomfh]

The ground often gets harder and better the deeper you dig (try digging some footings by hand - you'll see), so yes the deeper you put the footing the better it tend to perform, but as noted above the geotechnical consultants wont generally just increase the allowable capacity because you went a tad deeper.

 

[WARose]

Yes....IF you were removing the soil above the footing. That's the concept of a so-called "raft" foundation.

But just 12" into the ground.....I have a hard time seeing any increase from that.

 

[EireChch]

Tomfh - the reason for the increase in capacity is due to the increase in overburden stress which stops the formation of failure surfaces.

Although, if there is an increase in strength then that will obviously help.

 

[azcats]

Didn't older versions of the code have and 'increase for depth' provision in the prescriptive design values? I've seen some more recent geotech reports that will vary allowable bearing pressure based on footing size.

Here's a snippet from the '97 UBC. I haven't seen this in any recent code.

 

[JoshPlumSE]

A lot of geotech reports that I've seen have an increase in allowable bearing pressure equal to the weight of soil removed.

 

[Ron]

Yes...in general. The more overburden soil you have around your footing, the more difficulty the soil stressed by the footing will have in "bulging" upward in a classical bearing capacity failure. There are exceptions....If the bearing capacity is related strictly to settlement in highly stratified soils or softer clayey soils, then the overburden loading would have less effect, other than to increase the lateral stress on the soil below the footing which can strengthen it but not as much.

 

[BigH]

@Ron - if settlement, bearing pressure (not bearing capacity) . . . at least in my view

 

[Ron]

@BigH.....yep! Caught me!