Bearing Capacity of Clays given SPT 5

[IJR]

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

 

[fattdad]

If I understand correctly, the geotechnical engineer gave you some allowable bearing pressure for the proposed type of construction, based on a geotechnical field program and laboratory testing. The geotechnical engineer considered some bearing depth and bearing stratum that is different from the depth and stratum that will realize the direct bearing load. Is this a difference of a foot or so or is this the difference between proposed construction without a basement and with a basement?

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!

 

[Mccoy]

IJR, in addition to fattdad's sensible analysis, I'd say that given only SPT values in a clay layer you might input the SPT value in correlations which yield Su (cohesion undrained) and use such a value as an input in the bearing capacity formula for undrained conditions. The Nspt-Su correlation is fraught with uncertainties (statistical variability and epistemic uncertaity) but it is there, the basic correlation being about 6 kPa Su to the blow.
Such correlation grossly holds for the edometric modulus used to figure out settlements but in such a case it is 600 kPa E_d to the blow (for low plasticity indexes, 400 for high plasticity).

All above to be construed as a gross estimate.

 

[motorbiketocrank]

I'm the geotech on a project where the structural report discusses the requirements for a steel framed building. Now we need to change it to wood framing. How do I design that? Don't tell me to ask the structural designer as he/she didn't care to discuss that in their report.

 

[cloa]

geobdg- you need to make a new thread.

http://www.eng-tips.com/threadminder.cfm?pid=1529

Use translation assistance for Engineers forum

Note the rules include No Student posting

 

[hokie66]

geobdg,
Good one. Right over the head...

 

[IJR]

I talked to the geotech.

He gave me c = 2 to 5 N (kPa), where N is the SPT

Thank you for your time

respects
IJR

 

[Mccoy]

I talked to the geotech.

He gave me c = 2 to 5 N (kPa), where N is the SPT

He kept pretty much on the cautious side

 

[IJR]

McCoy

What should it be then , some text says c=10 N (kPa) where N is the SPT. Is that what you mean?

respects
IJR

 

[BigH]

First off, we all (and don't deny that we don't or haven't) used correlations of SPT vs undrained shear strength. The OR says he can't find it in any book - come on now! Almost all textbooks have the tables that show N vs Su (or cu for some of you) - see attached.

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 . . .

 

[BigH]

Sorry - this is the other attachment - guess we can't post two attachments in a post . . .