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?

 

[fattdad]

Regarding Steve1's problem statement (and considering ultimate bearing capacity), the two backfill planes would develop shear resistance were movement to occur. This shear strength would resist the movement of the foundation and contribute to the overall resistance at depth. It just isn't the case that the vertical excavations would slide without frictional resistance.

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!