Liquid Limit

[BigH]

I guess I can double post this! Perhaps you all will be a bit deeper with any answer - help.

Does a soil that is classified as non-plastic have a liquid limit? Simple question, but . . .

 

[Ron]

BigH...
I answered in the other forum in which you posted and then noticed right after hitting "submit" that you had asked the question. My apologies for the simplistic answer...considering your experience and capability, I think you are trying to elicit some interesting discussion for which you already know most of the answers if not all!!!

Here's my post in the other forum...
"It has a liquid limit but no plastic limit, therefore no Plasticity index. The liquid limit alone can have some usefulness, but is generally paired with the plastic limit for defining characteristics of clayey materials, not "non-plastic" materials. For instance, there are correlations between liquid limit and consolidation, between liquid limit and liquefaction potential, etc."

Ron

 

[iandig]

My understanding of the definition of the liquid limit is the moisture content at which the material acts like a liquid and not a solid. I think the difficulty in having confidence in the value is that materials which are non-plastic, often don't behave in a predictable manner when you test them for the liquid limit. I am speaking here specifically about when you use the cone penetrometer, as a material becomes less plastic, they become more susceptible to what I have always called dilatency (although I stand to be corrected for using the wrong term, what I mean by dilatency is that when you shake a silt, the water rises to the surface, we always called this dilatency in the army, and I can't get out of the habit now - sorry).
Anyway going back to non-plastic soils (fines passing the 425µm sieve)you can measure a 'liquid limit' but in my opinion the results will be very difficult to accurately reproduce. This is because any vibration/knocking/tapping of the sample effects its structure, the excess water required to 'liquify' the solids is rapidly released and the solids 'settle'. Water comes to the surface, and the solids are effectively compacted, thus reducing the penetration of the cone thus reducing the liquid limit for the smae 'overall' moisutre content.
Simple answer? -> yes you can measure the moisture content at which you achieve 20mm penetration with the cone penetrometer which has been taken to correspond to the liquid limit, but the test method may not be sufficiently repeatable for soils which are non-plastic. I guess it all comes down to what you are trying to prove, or more importantly who is trying to use the information.
We used to have to determine the plasticity index of rock armour, the material had no fines (less than 0.0001% of the overall mass), but if you tried hard enough you could get enough to do a test. But was the fact that these were plastic of any relevence when you consider the sample as a whole?

 

[BigH]

iandig - thanks - on your last point, it is well taken from another problem on our site where a material had only 20% passing the 0.425mm and had a plastic limit above the specified value. Not too much to really affect the behaviour but enough to flunk the sample. There is something called the plasticity modulus (%PI x %passing 425mm sieve) - then you use this number to judge the effects (300 to 1200 is okay for gravel roads).
Keep the comments coming - just want to see what the overall take is.
Chou - and enjoy the Chinese New Year.
Howard aka BigH

 

[GeoPaveTraffic]

I agree with the tone so far. A liquid limit could be ran and would be valid as a liquid limit. However, I would be concerned about the repeatability of the test and the range from multiple tests on the same material may be quite large. iandig, makes a good point about who will be using the data and for what purpose.

When I worked as a consulting engineer on a large levee project in the mid-west US, I had our lab run many liquid limit tests on non-plastic silts or sands with a lot of clay. To me it provided more information on how the material might perform under varrious loading conditions.

Of course the lab manager stamped NON ASTM TEST on all of them, but I could live with that.

 

[BigH]

Thanks for the responses so far - where are you others? This could be an interesting topic to kick about!

 

[jdonville]

All,

My company uses gINT to prepare logs and record our lab information. The software will permit entry of LL test results even if the soil is non-plastic (that is, the PL test cannot be performed -> PL = 0).

From the gINT help file:

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ASTM specification D 4318 (Liquid Limit, Plastic Limit, and Plasticity Index of Soils), section 20.1.4 [...] states in part: "If the Liquid Limit or Plastic Limit tests could not be performed, or if the Plastic Limit is equal to or greater than the Liquid Limits, report the soil as nonplastic, NP."

In southeastern Alaska there are soils where the Liquid Limit test can be performed and yields values in the range of 20 but the Plastic Limit test cannot be run. Your first reaction might be to input a [Plastic Limit] value of "0" and expect the ASTM functions to classify such a soil as a clay since the PI would be 20. Section 20.1.4 says otherwise, that is, since the Plastic Limit test could not be run the soil is non-plastic and therefore a silt. The ASTM Classification function in gINT accommodates this condition.

-----

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[BigH]

Thanks for your input. I guess it is the concept. Only a plastic soil should have a "limit". I cannot see that a liquid limit/non-plastic soil has any meaning. Without the plasticity it will, even if fine grained, act as a cohesionless soil. One point, why does one let gINT classify a soil? . . . or are we just ...
The original query was brought to the fore by a specification that I saw where it said that the soil must have a Plastic Index of less than 6 and a liquid limit of less than 45. So, if the soil has a liquid limit of 47, say, but is non-plastic, can I use it in my base course? (Granted, this is for the purposes of classifying the finer material in a crushed stone base course - in Ontario, the crushed stone must be non-plastic; in most other jurisdictions, they seem to permit PI of up to 6.)