Soil above shallow footing included?
Soil above shallow footing included?
(OP)
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?
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?





RE: Soil above shallow footing included?
RE: Soil above shallow footing included?
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?
RE: Soil above shallow footing included?
See thread507-95219 for lots of discussion.
RE: Soil above shallow footing included?
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.
RE: Soil above shallow footing included?
f-d
¡papá gordo ain’t no madre flaca!
RE: Soil above shallow footing included?
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.
RE: Soil above shallow footing included?
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!
RE: Soil above shallow footing included?
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.
RE: Soil above shallow footing included?
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.
RE: Soil above shallow footing included?
f-d
¡papá gordo ain’t no madre flaca!
RE: Soil above shallow footing included?
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.
RE: Soil above shallow footing included?
f-d
¡papá gordo ain’t no madre flaca!
RE: Soil above shallow footing included?
Dik
RE: Soil above shallow footing included?
RE: Soil above shallow footing included?
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!
RE: Soil above shallow footing included?
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.
RE: Soil above shallow footing included?
Hopefully the insight from this group has helped.
f-d
¡papá gordo ain’t no madre flaca!
RE: Soil above shallow footing included?
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.
RE: Soil above shallow footing included?
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.
RE: Soil above shallow footing included?
dgillette brings up the correct point by stating what is assumed all along - that we are citing examples based on homogeneous conditions.
One more thing. Looking at incremental construction (i.e., removing soil, placing concrete, letting concrete cure, loading the concrete with backfill and then loading the foundaiton with the structure is just not the way foundation recommendations are developed. More typical, the geotechnical engineer will look at the stratigraphy (i.e., soil types, soil strength and compressibility), and then assign a bearing depth and a bearing capacity. Were the incremental factors critical, there would be more reference to these steps in the specifications and design. It just doesn't work that way. I've done this work (not that anybody is asking for my CV) from the Northwest to the Northeast - in Central America and the mid-Atlantic; in no instance have I - or have I seen - a case where the bearing pressure was cited as "gross". It's always considered the net - what can be delivered to the completed foundation at the ground level.
Here's where I see the bigger problem: geotechnical engineers that assign bearing pressures in too fine of increments. In a layered geology to imply that the bearing pressure at 2 ft is greater than the bearing pressure at 3 ft, which is less than then the bearing pressure at 5 ft is often the source of greater confusion. This implies that you can isolate the shearing stresses within a very localized interval of soil below the foundation. Often this is the result of an untrained field technician blinded by the results of a DCP (a tool that I just don't like for this very reason).
Sorry to ramble. I got on a roll and typing is easy - ha.
f-d
¡papá gordo ain’t no madre flaca!