NET or GROSS bearing capacity on mat foundations
NET or GROSS bearing capacity on mat foundations
(OP)
Why do we use NET instead of GROSS bearing capacity on mat foundations?
Thanks in advance
Thanks in advance
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NET or GROSS bearing capacity on mat foundations
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NET or GROSS bearing capacity on mat foundationsNET or GROSS bearing capacity on mat foundations(OP)
Why do we use NET instead of GROSS bearing capacity on mat foundations?
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RE: NET or GROSS bearing capacity on mat foundations
Gross varies with soil depth.
RE: NET or GROSS bearing capacity on mat foundations
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.
RE: NET or GROSS bearing capacity on mat foundations
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.
http://environment.uwe.ac.uk/geocal/foundations/fo...
"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.
RE: NET or GROSS bearing capacity on mat foundations
I also separate terminology of bearing capacity and bearing pressure based on shear failure and settlement, respectively.
RE: NET or GROSS bearing capacity on mat foundations
RE: NET or GROSS bearing capacity on mat foundations
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?
RE: NET or GROSS bearing capacity on mat foundations
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.
RE: NET or GROSS bearing capacity on mat foundations