Should CPT shear values be supported by testing?
Should CPT shear values be supported by testing?
(OP)
I would like some opinions on whether the standard of practice is met if one relies exclusively on cone penetrometer (CPT) correlations to derive soil shear strength properties for design of critical facilities such as tall retaining walls, bridges, and large culverts, or whether such correlations should necessarily be supported by some site-specific measurements of shear strength, such as direct shear or triaxial tests. I also was wondering if the perceived reliability, and therefore the acceptaqbility in design, of stand-along CPT data depends on whether the materials are natural or man-placed fill, cohesive or non-cohesive.





RE: Should CPT shear values be supported by testing?
If I know
- the soils which have been penetrated,
- their specific cpt-profiles (cone resistance,sleeve friction as well as their characteristic curves) and
- their properties by my own experience (or reliable experience of others geotechnical engineers)
and (!)
if I got some basic test results (grading curves, water content, LOI),
I estimate strength and deformation parameters taking the cpt-data into account without further cost intensive testing. Without this it's hazardous!
RE: Should CPT shear values be supported by testing?
RE: Should CPT shear values be supported by testing?
Hope this helps.
f-d
¡papá gordo ain’t no madre flaca!
RE: Should CPT shear values be supported by testing?
I am amazed at what decisions are made with average to poor quality SPT values, by engineers working in 'out of town' areas. I am also amazed at the poor quality of field testing, sampling and laboratory testing which is often used, whether 'in town' or 'out of town' for individual engineers. I understand the canard of 'lots of local experience'. I also am currently redoing several jobs, as the 2nd Geotech, since the original work was insufficient.
RE: Should CPT shear values be supported by testing?
all in all, i think engineers should be careful in different geologies (whether 5 miles or 500 miles apart). likewise, the project specific "issues" will probably be the biggest factor. here in the Piedmont, you could go 500 feet and find a completely different geology...so with the way litigation goes these days, i suggest that all professionals be careful before they try to go out on a limb to help a client.
"no good deeds go unpunished"
RE: Should CPT shear values be supported by testing?
That's a typo, right? Should say "without the input..."?
RE: Should CPT shear values be supported by testing?
RE: Should CPT shear values be supported by testing?
RE: Should CPT shear values be supported by testing?
If you get a budget for intensive testing, take it and improve your experience. Most time the promoters don't want to spend money for real qualified soil testing, hence desingnig such facilities (e. g. deep excavations in soft to very soft soils) based on cpt's and index tests is our daily business. Don't try to base any geotechnical judgement about shear strength on single shear tests (it doesn't matter wether triax or direct sehr tests) cause you can easily be fooled by it's results due to the unavoidable variations of soil mechanics test results. Single shear tests are only suitable if you have got enough experience correlate their (otherwise maybe missguiding) results with.
RE: Should CPT shear values be supported by testing?
Many times my friends and I have discussed and debated the question you are asking. On smaller projects where we have lots of local experience we have just used the CPT data (combined with auger samples to get a soil PI) to model the Su values using a estimated Nkt value. Usually this invloves modeling the CPT data until a believable soil OCR value is reached. We then back calculate the Nkt factor to give us our Su values. This method has served us well when we are in area that we fairly certain the soil OCR values are near to 1.
On larger sites with more heavily loaded structures we always conduct some Nilcon vane or field vane tests. The Su values from the vane testing can then be used to fine tune your Nkt factor so that you determine the Su values more accurately. Using the site specific Nkt factor you can then get a good estimate of the soil OCR values. In the event that the client/developer wants a building with some form of unique or heavy loads we will get some piston tube samples.
All the soil modeling and back calculation of Nkt factors can be done very quickly with a CPT data analysis package or spread sheet program.
In the past what approach have you used?
I would be interested to hear your comments.
Coneboy
RE: Should CPT shear values be supported by testing?
It's not just about non-ideal soils (SGC mixtures), though. It's about using published correlations for any type of soil without thoroughly understanding the applicability of those correlations.
What I am really concerned about is the implication that mechanical properties are derived in a deterministic way from CPT data, when the CPT correlations used for them are actually empirical. For example, what is the justification for basing the in-situ shear strength of an SGC mixture on published CPT correlations developed on sands, if the data supporting those correlations do not include representative SGC mixtures?
On the other hand, if one references large-scale in situ testing of representative soils, or conducts one's own shear testing program to correlate the properties in a site-specific way, then I believe the CPT data can be useful to widen the scope of the testing and the results could be called deterministic.