1503-44, I dont know what you mean by "net never changes" and "gross varies with soil depth"? Can you explain?
OP - I have never known that NET is used over GROSS when it comes to mat foundations. I have only every specified a net allowable bearing pressure, with an example formula for what is i.e. net = gross - overburden.
There is too much vocabulary when it comes to bearing capacity net, safe, working, permissible etc.
I have only ever used the term "net allowable bearing capacity" when discussing shear failure. and "net allowable bearing pressure" when discussing bearing pressure for settlement limit.
Yes, I guess there could be some confusion. I get soil bearing allowable in terms of allowable net bearing pressure. Say 4000psf. But geotech guy does not know the depth of various foundations I might select. If soil unit weight is 100pcf and my fdtn is at 5ft, then gross allowable is 4000 + 5ft x 100psf = 4500psf. If fdtn depth is 3ft, then gross is 4400psf. Gross is net + overburden stress. Overburden stress changes with depth, but net never does. Its a quote from my soil mechanics class professor.
I was trying to find a URL with the same, but could not. I did find something similar, equation same, at Uni Bristol, but didn't like how the page was written. I offered them a suggested revision.
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Following are some comments I have concerning the referenced URL.
"When excavating for a foundation, the stress at founding level is relieved by the removal of the weight of soil. The net bearing pressure (qn) is the increase in stress on the soil.
qn = q - qo
qo = g D
where D is the founding depth and g is the unit weight of the soil removed."
The above statement implies that removal of soil overburden increases the net bearing pressure, when it is actually the gross bearing pressure at depth D that can be increased by the weight of soil removed. "As net never changes", it is easy to remember the difference
qo = overburden stress
Therefore q (what I call the gross allowable stress) = qn + qo
Your equations are correct, as
qn = q - qo does equal q = qn + qo
however it is far easier to immediately see that when the equation is presented in the later form.
I also do not think this following paragraph is correct.
"The allowable bearing capacity (qa) is the maximum bearing stress that can be applied to the foundation such that it is safe against instability due to shear failure and the maximum tolerable settlement is not exceeded. "
Allowable soil bearing capacity is not the maximum stress that can be applied to the foundation. It is the maximum stress that can be applied by the combined loads of structure and foundation TO THE SOIL.
Ultimate bearing capacity changes with depth, and significantly so in cohesionless soils. Your ultimate gross bearing capacity should be based on an assumed or provided foundation width and depth. If your foundation is constructed deeper than was assumed by the geotech, and puts the failure wedge below the water table, your ultimate gross bearing capacity is going to change, and so is the net ultimate bearing capacity. See attached screenshot from FHWA.
I also separate terminology of bearing capacity and bearing pressure based on shear failure and settlement, respectively.
No disagreement there. Shape factors, actual footing dimensions, water table depth, specific layers and loading duration and many other variables might all have an influence on design.. "Net never changes" is just a simple means to remember the basic difference between those two terms that specifically answered the OP's question. Bristol's website says the same with their equation. He wasn't asking about anything else. Remember it in that context alone, or not, as you prefer.
That said, and assuming the same soil from surface to sufficiently far depth below bottom of mat to be uniform soil type, would you not normally have a "given net soil bearing capacity", a constant, to which you would apply your various shape factors and the like to arrive at a specific allowable soil bearing capacity for a particular foundation shape at a particular depth?
1503-44, my thought is that your geotechs are giving you a blanket bearing pressure, which I imagine is conservative. Hence why you can apply it to any foundation depth. (this must only apply to small strip/pad footings too?, as it would be very conservative if it was applicable to larger rafts based on settlement calcs, say 5x5m +).
The term "net never changes" must only apply to a blanket net allowable bearing pressure. As net can change with depth, the reason being depth factors, water table as mentioned above. Similarly for settlement calcs, net can change if your foundation extends deeper, along with its pressure bulb and loads a soft clay with high secondary compression. You could gather a lot of settlement in just touching a layer at the end of you pressure bulb.
Yes. Absolutely. For designing refinery type structures of all kinds, quickly and perhaps not so efficiently in use of materials and with a team of 20 fdtn engineers, so nobody argues about appropriate shape factors, etc. thats exactly what they do. That is the concept. Every day delay in in even a small refinery startup costs a minimum of $10MM gross revenue and of course, civil are the last ones to get the information and they need to be first in the field. Nothing gets erected without the foundations going in first, so its always working under extreme pressure. Offshore work is even worse, but mostly superstructure, rather than fdtns. Conservative design and simplest, fastest construction technique rules.
