Soil above shallow footing included?

[BurgoEng]

I have always accounted for the weight of soil above the top of footing when designing simple, isolated shallow footings. Typically, a footing will be in the neighborhood of 4'x4'x1'thk and about 3ft below grade, so that there would then be 2ft of soil above the top of footing. I have always done this and never really thought not too.

But now, I have a situation where for reasons I won't go into, I need to have my footing at roughly 8ft below grade, such that I'll have about 7ft of soil to account for. This causes a tremendous increased load onto the footing for bearing capacity check (about 110pcf=7ft = 800psf) which will put me over my bearing capacity of 2000psf.

Should I be including soil weight now, and/or in the future?

 

[civilperson]

Did the 2000 psf allowable come from a geotech report? Read it closely to see if that value is NET or total. The weight of soil always must be included, sometimes the geotech has done it for you in giving a NET increase in soil bearing.

 

[BurgoEng]

I realize after doing a bit of reading that my typical calc accounts for a cone of soil above the footing. I've been using this same spreadsheet for 5 about years as a standard calc within the company, so i didnt pick up on the cone right away.

Our geotech info is always a bit lacking. Typically it just says Allowable capacity is xxx psf.

Would the "net" capacity be loading applied above and beyond soil weight and a "gross" capacity be including soil?

 

[CarlB]

My experience is that the allowable bearing provided in a report is usually "net", if not explicitly stated.

See thread507-95219 for lots of discussion.

 

[msucog]

i agree--net is usually stated. let's say you dig out 10' of soil in an area 10sf and pour back 1 ft of concrete and replace the 9 ft of soil. has anything changed? not really...only the new load applied from the column load (above) itself. this of course assumes that the soil was there long enough before that fill induced and secondary settlement had played out from the soil by itself (and yes, i realize concrete weighs more than dirt--let's just say "in round figures" since you're roughly looking at 30pcf increase for the concrete and i doubt most are good enough to split hairs with the variabilities of soil for simple scenarios). i would say that you would not necessarily need to count the soil above. it is however a little safety factor for those "unknowns" that happen from time to time (usually from the contractor end--and i can say that as an engineer becuase i was once a contractor and have seen some disturbing things on jobsites). that's my opinion anyway...
i just glanced at the link from carlb and it has just about every point of view you can think of regarding the subject, so my opinion may be repetitive.

 

[fattdad]

There is no need to account for the weight of the soil above the foundation element when determining the allowable structural reaction at the bearing surface.

f-d

¡papá gordo ain’t no madre flaca!

 

[Lion06]

Well, I apparently learned how to design foundations wrong. I learned in school, and have been designing since then, to account for the weight of the soil when sizing the footing. I do not account for it when designing the rebar for the footing.

msucog-
your reasoning makes sense. Unless I get CLEAR information from the geotech as to whether his value means I can neglect the soil above the footing, I would not neglect it.

 

[fattdad]

If you are really going to look into this refer to Meyerhof or Terzhagi and Peck for the rational for foundation bearing pressure.

Considering that there is little real difference between the density of the concrete and the soil that it's replacing the transfer of load to the foundation bearing depth originates at the ground surface. With increasing depth you actually get an increase in the net allowable bearing pressure as the rotational failure surface has more soil to move (i.e., the failure plane is longer).

Can't comment on how you were instructed, but practicing as a geotechnical engineer for the last 20 years or so, I can attest that the intention is to use the bearing pressure as net allowable.

f-d

¡papá gordo ain’t no madre flaca!

 

[Lion06]

fattdad-
I am not doubting your credentials, but I do have a question.
I was instructed correctly, but I believe it is the ULTIMATE bearing capacity that increases with depth. Obviously, if you use the same FS the net will increase as a result. That being said, however, the engineer reading the report has no way to know for what depth the "baselin" allowable bearing pressure is for. It would be unwise to increase it with depth not knowing that.

 

[BigH]

A reminder ALL:
Allowable (net/gross) Capacity is used when determining pressures with respect to shear failure/movement.
AllowablePressure is used when determining pressures with respect to serviceability issues (e.g., settlement).
Unless one has very soft to moderately firm clays or very loose sands, it is almost always the serviceability issues that govern so unless the allowable bearing capacity is lower than the allowable bearing pressure - don't do too many fancy bearing capacity computations.

 

[fattdad]

While that reminder is correct it doesn't change whether you would include the weight of the soil prism above the foundation. Presumably, there would be no settlement of the subsoils owing to the replacement of soil that was already there. That said, I agree with your point.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

to clarify my statement, soil above retaining wall footings or "new" fill on top of the footing might need to be counted since it is above and beyond what was previously there. and there again, that's probably tied more to fill induced settlement than to bearing capacity.

for your footing, let's say a 4'x4' works at 2000psf so the load would be on the order of 32k. divide 32k by 1100psf (to account for the soil above) and it works out to be a 5.5'x5.5' footing. under this quick and dirty scenario, the footing is not that much bigger. is there really a cost difference for the owner if you provide a slightly bigger footing? it's a little extra insurance (for you and the owner) and depending on the actual scenario, the soil might need to be included. as the designer, only you can make that call...good luck.

 

[fattdad]

Retaining wall footings are a different animal (conventional retaining wall design - not SRW). While bearing pressure is critical to the design, the overall size is often determined by the overturning forces. As such, the portion of the footing with the greatest soil load is likely to have the lowest bearing stress and also the greatest overburden load. These are really the type of discussions that benefit from a whiteboard, chalkboard or calc pad. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[dik]

The bearing capacity of the soil is based on compressive strength at a given level. This level already has an overburden of soil on it and the compressive strength at that level has the same overburden... you might want to include the weight of the foundation less the weight of the soil displaced... the footing program I wrote accounts for the latter as an option... generally not done.

Dik

 

[BurgoEng]

So it seems like the idea is to pretty much do whatever feels right? I normally always included the soil weight, and for a footing that's only 4 ft below grade, it hasnt been a big deal, but for a footing 8 below, the soil cone is huge and adds much load. However, if given a bearing capacity of only 2000psf, at some point, by the numbers, the soil wouldn't even be able to hold itself, which I guess is when engineering judgement comes in.

 

[fattdad]

The idea is pretty much to ignore the weight of the soil above the foundation - it's not to "just do what feels right". Hopefully, where engineering judgement comes in is at the beginning of every project.

I guess I'm not sure what your parting post is intending. Myself and others are attempting to provide advice to your inquiry. Feel free to review the many references that are available on this topic or ask the geotechnical engineering that provided foundation recommendations for further comment.

f-d

¡papá gordo ain’t no madre flaca!

 

[BurgoEng]

f-d,
I didnt meail to intend anything from my last post. It just seemed like some are saying to include it, some are saying to include it as it would somewhat act like additional safety factoring and others had said they dont use it as the geotech is giving a value that is for the "new" loads.

 

[fattdad]

I guess a "safety factor" that renders a spread footing unusable below some magic depth is just too far-out for me to grasp.

Hopefully the insight from this group has helped.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

I think you need to go back to the source, and get with the geotechs that prepared the report to see what the 2000 psf refers to.

Trying to restate and augment some of what was said before:

If it is ultimate capacity (with an appropriate safety factor), then it is dependent on depth of embedment, width of footing, shear strength (not compressive strength), etc. See Terzaghi and Peck or any intro soils book for details. I hate it when bearing capacities are provided without any context of depth of footing, width, etc. They are meaningless without the context.

If it is based on limiting settlements (serviceability), it would almost certainly be net pressure. There, too, the footing width comes into play.

In either case, the weight of the soil above the footing contributes to the load on the footing, and it should be accounted for. What's clear from this discussion is that we are not all singing off the same sheet music about what that means. If we are talking about net pressure, then the weight is accounted for implicitly; if it's gross, pressure, then that weight has to be added on, though the cone approach mentioned by BurgoEng may not be the way to do it; I'm not sure where it comes from, or if its application is appropriate here. Fattdad's last comment is correct, and goes to the net-vs-gross issue - unless there is a weak layer at depth, there is really no way that the gross ultimate capacity could decrease or fail to increase with embedment.

 

[steve1]

I've enjoyed this post as it has caused me to think about soil bearing capacity in different ways. Here's another opinion.

We excavate a trench into an existing "undisturbed" soil mass and place at the botttom, on a prepared base, a rigid object such as a reinforced concrete footing. We then backfill, in compacted lifts, a new mass of soil that completely fills the excavation. The question is then what is the bearing pressure under the rigid object? It seems to me that the new soil mass loads the rigid object, but not the soil adjacent it. That is, there is a failure plane along the line of the original excavation that will cause the backfill material to move independent of the original "undisturbed" soil. Therefore the backfill weight (based on the geometry of the excavation) is added to the structural load on the footing.

Just a thought.