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Consistency of clays

Consistency of clays

Consistency of clays


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

in my experience the clays I have regularly encounter do lose "strength" if disturbed, if only that is while sampling. Thus any short cut, such as the penetrometer or blow count, is viewed as a general indicator, not necessarily something to be relied on for design recommendations. It is used frequently to help determine which sample to then gets more testing that is likely to be more reliable.

RE: Consistency of clays

If you can get good quality undisturbed samples, i think many would agree that a traixial test is the most accurate to get strength data.

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

consistency is a descriptor. The grandfathers of geotechnical engineering related it to the clay's ability to be molded, indented, scratched with a fingernail, etc. That was then subsequently related to SPT N-values. Then correlations to unconfined compressive strength. I'm certain, we could also relate it to UU testing, etc.

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.


ípapß gordo ainÆt no madre flaca!

RE: Consistency of clays

Thanks engineers,

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.


RE: Consistency of clays

For relatively little extra time required consider upgrading a little. Assuming that disturbance effects are not still significant, a quick unconfined compression test on what would appear to be the least disturbed part of the SPT sample, run a Qu test taking maybe 20 seconds. Try for a sample height at about twice the diameter. A Remac valve spring tester will do the job and gives max pounds developed. I no longer am that active, but I had a chart on hand for differing diameter samples and had a significant range of input of pounds in one column with Qu next column, to simply be filled in on the log. Sure it may not be ASTM, but is a heck of a lot better at getting a usable number than a penetrometer. This one is a little dusty from sitting in my basement.

Here is from eBay.

This one is closer to the old model I have.

RE: Consistency of clays

Modern standards for soil description (e.g. ISO 14688) no longer conflate consistency with strength. Consistency is determined from Atterberg limits by means of a consistency index, whereas strength is determined from various tests. This means that, for example, soil can be described as having 'stiff' consistency but have a 'low' shear strength (su = 20 kPa to 40 kPa). For most soils the two are related, but there is not a clear relation for all soils, which is why 'consistency' and 'strength' are now considered separately. What you measure from SPT, PP, UU, etc. is strength, not consistency.

RE: Consistency of clays

Nice pics OG...I am just trying to imagine how many other interesting field tools may you have in that basement...

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

LRJ - interesting post, i dont think that that is the same in the UK. I considered consistency/strength to go hand in hand since it is based on undrained shear strength. I didnt think it was possible to get a stiff clay that had low strength? You may get a stiff sensitive clay that has a high peak and low residual undrained shear strength but the consistency term is still based on its peak strength.....thoughts?

RE: Consistency of clays

LRJ, same as EireChch, what you are saying is new to me...I would try to find that ISO standard...interesting.

RE: Consistency of clays

The British standard for soil descriptions is BS EN ISO 14688 (the British/European version of the global ISO standard). BS5930 (2010) refers to BS EN ISO 14688 when providing example descriptions (see the table at the bottom of page 114) which detail consistency and strength separately. It's been that way for many years now, but the industry is extremely slow to embrace new changes; this actually reinforces my belief that the geotechnical industry (worldwide) is extremely slow to adapt and still very much relies on how it has always been done.

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

LRJ, in that case, did you have N-values or other field strength tests? If yes, how they correlated with your DSS? Just curious. Anyways, that's why lab testing is always important...although sometimes clients do not want to pay for it.

RE: Consistency of clays

There were a variety of strength tests (though not SPTs) and they all confirmed the strength classification. Nonetheless, the confirmation of a strength classification for one particular soil unit on one particular site is not essential to highlight that strength and consistency are different properties.

RE: Consistency of clays

LRJ - thanks for that info. Very helpful. While i have been a geotech for approximately 5 years now (still a novice), I have only been in the working in the UK for approx 1 year. So maybe the industry is a bit slow to take it up as you said or maybe i just havent seen enough boreholes!

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

Consistency can be determined by:
(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

Apparently the term, "Consistency" has dual meanings. One (the one I relate to) is a reflection of strength, the other relates to water content and the Atterberg limits - i.e., not an expression of in-situ conditions.

For the latter, I'm familiar with the, "Liquidity Index."


ípapß gordo ainÆt no madre flaca!

RE: Consistency of clays

Consistency might have historically been conflated with strength, but it never should have been. This is why consistency and strength are now rightly recognised as distinct concepts.

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

EireChch - and this is where the British have deviated from "standard" North American practice (Terzaghi, for example) where we have undrained shear strengths ranging as very soft (up to 12.5 kPa), soft (12.5 to 25 kPa), firm (25 to 50 kPa) - sometimes called medium stiff, stiff (50 to 100 kPa), very stiff (100 to 200 kPa) and hard (greater than 200 kPa). I am sure you are aware but others in British practice may not be so they may err if reviewing borehole log descriptors from say Canadian or US practice. Until recently (nearly 40 years on) I hadn't realized that the BS standards (and now EN) uses a different system than found in Canada and the USA.

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