Consistency of clays
Consistency of clays
(OP)
Experts,
I think this have been a topic many engineers have argued about.
The consistency of clays, how is more reliable to determine it?, based on the SPT-N blowcounts or based on the unconfined compressive strength (UCS)?.
I understand it is easier to do it based on the blowcounts than sending the sample to a lab to get the consistency. What do you think is the right way?.
In my practice, I tend to determine the consistency based on the UCS provided by the pocket penetrometer. I understand that the pocket penetrometer is not the most accurate device.
I just don't use consistency based on the blow counts.
Please share your thoughts.
I think this have been a topic many engineers have argued about.
The consistency of clays, how is more reliable to determine it?, based on the SPT-N blowcounts or based on the unconfined compressive strength (UCS)?.
I understand it is easier to do it based on the blowcounts than sending the sample to a lab to get the consistency. What do you think is the right way?.
In my practice, I tend to determine the consistency based on the UCS provided by the pocket penetrometer. I understand that the pocket penetrometer is not the most accurate device.
I just don't use consistency based on the blow counts.
Please share your thoughts.





RE: Consistency of clays
RE: Consistency of clays
If you dont have the luxury of that, which it sounds like you may not, then my next best would be a field vane.
After that your looking at SPT-N to get a correlation to shear strength. There was a thread recently on that topic.
Why dont you tell us what you have available to you i.e. borehole types, lab facilities, CPTs, DMTs???
RE: Consistency of clays
That said, it's an index descriptor used for correlations, at least in my mind.
In certain terrain, I'd want to have my pocket penetrometer (marine clays, etc.). In other terrain, I wouldn't be as concerned (i.e., alluvial clays and silts, for example where sensitivity is not expected).
With a sense of these index values, you'd then have an obligation to collect a real undisturbed sample (or perform in-situ testing) to actually learn the behavior of the deposit. I don't do design of fine-grained soil based on index values.
So, for the benefit of mapping soil layers, N-value and/or pocket penetrometer is all I'd use. For the benefit of actually learning the undrained shear strength, consolidation characteristics, friction angle, etc., I'd do actual sampling and testing for the design purpose.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Consistency of clays
EireChch, I don't have anything available, just the SPT-N blowcounts and the pocket penetrometer.
Naturally, using both methods, not all the time I will get the same "consistency".
I am working with other engineers and geologists, and they base the clay consistency on the blow counts. To my knowledge as fattdad said, consistency based on blowcounts is a old method, and UCS seems to be more recent.
I have been using pocket penetrometer for few years, but whenever I have arguments with someone, I want to have good reason as to why I don't consider the blowcounts as consistency but use the PP instead.
Thanks
RE: Consistency of clays
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RE: Consistency of clays
RE: Consistency of clays
pelelo, I think that consistency descriptions from N-values or PP readings are just that: descriptions, and as explained for several people above, for design calculations this is not relevant.
Anyways, I expect that the values of unconfined compressive strengths for determining the consistency of clays which you are using, are from correlations from SPT N-values proposed by Terzaghi and Peck(qu = 12.5 N (in kPa)). So, the PP criteria was based actually on N-values.
Bottom line, I think that there is not a strong justification for what to use for the consistency description... you may just need to say that your preference is to use PP readings and indicate that your consistency descriptions shown in your reports are from PP readings.
RE: Consistency of clays
RE: Consistency of clays
RE: Consistency of clays
I have personally encountered some strange clay which feels very stiff to touch (therefore having a 'stiff' consistency) but which achieved 'medium' undrained shear strength in the DSS test (and this was for a number of tests on good quality samples). This anomaly was likely due to secondary constituents within the clay; if I recall correctly that clay was intermixed with chalk particles.
RE: Consistency of clays
RE: Consistency of clays
RE: Consistency of clays
I included the Table 5 from BS EN ISO 14688-2 below for those interested. Consistency is based only on the indentation of working it by hand, nail or a pen knife with strength based on undrained shear strength. I copied the example description from the Table from the bottom of page 114 of BS5930 - Stiff closely sheared medium strength orange mottled brown slightly sandy slightly gravelly CLAY. Since this clay is 'medium strength' Cu is 40-75kPa.
Thanks again.
PS - are you based in the UK, if so, where abouts?
RE: Consistency of clays
(1) A manual test according to ISO 14688-1:2004 - Section 5.13; or
(2) Atterberg limit tests, to obtain the consistency index (IC), as per ISO 14688-2:2004 - Section 5.4.
Approach (1) is usually suitable for field identification and approach (2) following laboratory testing. The references I've provided have been updated/repeated in other standards; I referenced those sections as it was easier for me to find than in the new standards.
And yes, I'm currently based in south east UK, but have worked elsewhere too. The standard is (supposedly) an international standard.
RE: Consistency of clays
For the latter, I'm familiar with the, "Liquidity Index."
f-d
ípapß gordo ainÆt no madre flaca!
RE: Consistency of clays
Also, consistency index is different to liquidity index. See here: https://en.wikipedia.org/wiki/Atterberg_limits#Liq...
ISO 14688 does state that liquidity index may be used as an alternative classification to consistency index, but it doesn't provide any bounds for a specific description when using liquidity index.
RE: Consistency of clays