Bearing Capacity of Clays given SPT
Bearing Capacity of Clays given SPT
(OP)
Pals
How do you determine Bearing Capacity of a Clay Layer given only SPT values? I can not find that in any textbook. N is correlated to bearing capacities of sand.
Dont tell me to ask a geotech(He/she did not care to report that at depths I am going to put my foundation, and usually in our area, once a report is published, geotech keeps away from further consulting).
respects
IJR
How do you determine Bearing Capacity of a Clay Layer given only SPT values? I can not find that in any textbook. N is correlated to bearing capacities of sand.
Dont tell me to ask a geotech(He/she did not care to report that at depths I am going to put my foundation, and usually in our area, once a report is published, geotech keeps away from further consulting).
respects
IJR





RE: Bearing Capacity of Clays given SPT
Allowable bearing pressure is often determined by settlement criteria. So, if you are dealing with an unsaturated clay, if the natural moisture content is well below the liquid limit and if the blow counts (N-value) is pretty good (along with undrained shear strength - i.e., pocket penetrometer values), you can likely look at elastic compression, forecast immediate compression and evaluate the rational bearing pressure.
Do you have pocket penetrometer or tor-vane data? We don't have much to go on from the OP.
I'd ask the geotech. I mean if s/he prepared the report assuming at-grade construction and you then want to go with a basement (or the other way around), the geotechnical engineer should be consulted. And, yes you may need to pay a few more hours of service. It's better for the project record to pay the supplemental consulting fee than get advice from this forum (that is if you proceed with modifying the design).
f-d
¡papá gordo ain’t no madre flaca!
RE: Bearing Capacity of Clays given SPT
Such correlation grossly holds for the edometric modulus used to figure out settlements but in such a case it is 600 kPa Ed to the blow (for low plasticity indexes, 400 for high plasticity).
All above to be construed as a gross estimate.
RE: Bearing Capacity of Clays given SPT
RE: Bearing Capacity of Clays given SPT
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RE: Bearing Capacity of Clays given SPT
Good one. Right over the head...
RE: Bearing Capacity of Clays given SPT
He gave me c = 2 to 5 N (kPa), where N is the SPT
Thank you for your time
respects
IJR
RE: Bearing Capacity of Clays given SPT
He kept pretty much on the cautious side
RE: Bearing Capacity of Clays given SPT
What should it be then , some text says c=10 N (kPa) where N is the SPT. Is that what you mean?
respects
IJR
RE: Bearing Capacity of Clays given SPT
Once you have an "estimate" of Su, then 2 x Su is allowable bearing capacity of the soil for all intents and purposes using a safety factor in the order of 3. Of course, in almost all clay deposits, you need the allowable bearing pressure which, in my view, is the serviceability pressure that you use for a specified or presumed limit on the settlement/distortion. Warning: use of such correlations of N and Su must only be made by experienced geotechnical engineers - as we all know that the SPT N value is a poor cousin to a proper determination of the undrained shear strength (say by field vane test - many will go PiezoCone or others but these, too, while more "reliable" are also fraught with correlation spread). One of the best things is to develop, for a particular locale and a particular deposit, an area specific correlation of N and Su - you might get something like the attached graph for a site I had in India (again, to be used with caution!)
In Singapore, they seem to use 5N (for kPa). I asked some there once if this was "corrected" or not - and they did not know!
In response to the OR's question . . .
RE: Bearing Capacity of Clays given SPT