Depth Correction for Clays

[kyjocro]

I understand that the SPT "N" values are corrected for both hammer efficiency and overburden. According to my Foundation Engineering textbook (Das), the overburden correction is only performed for sands. I was curious if anyone can explain why overburden affects the N value correction for sands and not clays. I assume confining stresses in sand layers are dependent on vertical stresses and not so much in clays. Any insight would be appreciated.

 

[fattdad]

N-values in clays are not very reliable from the onset. Those correlations that use N-1 values are typically used in correlating relative density or performing seismic evaluations. I know there are others, but usually for the benefit of correlation to the behavior of sands.

I like jamming a pocket penetrometer in clays and taking sufficient tubes for UU or CU testing.

f-d

¡papá gordo ain’t no madre flaca!

 

[AussieGeoEng]

f-d,

Just out of interest, do you still do a lot of UU tests? Came across this discussion by Ladd last week and found it very interesting.

LADD, C. C. 2009. Discussion of "Stability of I-Walls in New Orleans during Hurricane Katrina" by J. M. Duncan, T. L. Brandon, S. G. Wright, and N. Vroman. Journal of Geotechnical and Geoenvironmental Engineering, 135, 1999-2002.

 

[fattdad]

Yes, we do UU testing or require our consultants to do them (I work for a state highway department). I haven't read the paper you cited. If you explain the relevance to your OP I'd be glad to hear what's on your mind. I'm actually planning on visiting Tom Brandon tomorrow and will likely be participating in a workshop on fully softened clay behavior later in the Summer with both Tom and Mike. They were both on my committee and I enjoy visting them when I can.

f-d

¡papá gordo ain’t no madre flaca!

 

[AussieGeoEng]

I was wondering what sort of reliance do you put on UU testing? i.e. what would be your ratio of UU to CU testing?

Obviously somebody does, from the response to the discussion: "Since Hurricane Katrina, about 3,500 consolidation tests, 6,000 CPTs, and 22,000 UU triaxial (Q) tests have been performed by the New Orleans District".

 

[dgillette]

kyjocro - As fattdad hints at, the blowcount in sand at a given RD varies with overburden stress. For liquefaction analysis, everything is adjusted to an equivalent overburden stress of 1 tsf ~ 1 atm ~ 1 kgf/cm2 ~ 100 kPa. For estimating RD, see NAVFAC DM-7 (for example) for correlations between N and RD, with separate curves for different overburden stresses.

AussieGeoEng - I consider UU tests to be for filling in the spaces between locations of CU tests. I'd hate to cite a specific ratio of UU to CU tests, because that would depend on geology, budget, consequences of over- or underestimating strength etc. Some may argue for very few UU tests, and more oedometer tests, using stress history for that point with parameters from SHANSEP analysis of CU tests (C.C. Ladd's Casagrande lecture - link shown below).

Quoting Ladd: "But more fundamentally, reliance on UUC tests to estimate su(ave) depends on a fortuitous cancellation of
three errors:
1. The fast rate of shearing (60%/hr) causes an increase in the measured su;
2. Shearing in triaxial compression also causes an increase in su since it ignores the effects of anisotropy, which lowers su with increasing ? angle;
3. Sample disturbance causes a decrease in su. These compensating errors cannot be controlled and only pure luck will yield a strength equal to su(ave), i.e., such that disturbance offsets the higher strength due to fast shearing in triaxial compression."

Recommended Practice for Soft Ground Site Characterization:
Arthur Casagrande Lecture
12th Panamerican Conference on Soil Mechanics and Geotechnical Engineering
Massachusetts Institute of Technology
Cambridge, MA USA
June 22 – 25, 2003

http://www.geoengineer.org/onlinelibrary/paper_details.php?paperid=2049

fattdad! - see you in August.

 

[fattdad]

I hope I can make it. I have to take my daughter to Bucknell for her freshman year in college. I really hope I can make it. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

fattdad - tell her not to root against the Big Red!