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.
If a soil is clssified as no plastic, it means that it is not plastic at any water content. How can a liquid limit be defined without a plastic state.
A non plastic soil may behave like a liquid when liquified. However, that is due to the rise in pore pressure or excessive shear force applied to it. It has nothing to do with its water content.
I think the liquid limit only exists for a soil with three states of solid, plasticity and liquid at different water contents.
This probably has as much to do with philosophical issues as engineering, if we give something a name, does that mean it does what it says it does.
I believe that if you are trying to interpret a value, you should be aware of how it is derived at and the limitations of the way it was measured. You should never rely on a value in isolation. When considering a liquid limit for a non-plstic soil, you need to try and appreciate and understand the effect of it being non-plastic on the measurement of the liquid limit.
Sure there's a liquid limit for (non-plastic) sand. Go to the beach and build a sandcastle and you can drip the (liquid) sand out of your hand. From another point of view, the liquid limit means that any non-zero stress will lead (theoretically) to an infinite strain. Non-plastic sand should have a water content beyond which it will offer no resistance to stress. Just like Ron said. I'm not sure you could get any useful information out of a liquid limit done on non-plastic soil.
Rjeffery - so AASHTO T90 implies. Now if the plastic limit represents that point at which a soil turns from a solid state to a plastic state and the liquid limit is where the soil changes from a plastic state to a liquid state (remember - remoulded sample), it seems that a soil can be a solid at a point where is should be liquid. T90 says that if this happens, the soil is non-plastic. I would surmise that it is a problem with an exactness of definitions - the soil doesn't have a plastic state.
The debate goes on - philisophically, . . . Any real "hard" facts out there guys?
Hummmm. I will refer you to silty material with a heavy concentration of mica. It generally has a plastic limit higher than its liquid limit. but that was not your question. ask the people in Kobe, Japan. the sand that they build on is Non-plastic but with enough water and some vibration...
Rjeffery - granted, the Kobe problem is one of liquefaction - not of exceeding a liquid limit. The liquefaction potential is relational to the fines content (e.g., <5%, 15%, >35%) and on the relative density at which the material is found - but is this a "liquid limit" as defined under our testing procedures? I'd like to hear more about the mica concentration, etc. Any references? Couldn't this be an effect of the definition of the various states? A similar problem occurs with particle size analysis where you try to match a sieve portion (say from #100 to #200 sieve size) to the results of a hydrometer test on the material. Hmmmmm . . .
anicdotally, (i know i spelt it wrong[e?]) here in Northern VA, silty material has a minerology that sometimes contains asbestos and and its chemically similar cousins, like mica. It seems that the shape of the mica/asbestos has a great deal to do with the plasticity, and its reaction to vibration. when attempting to perform the LL i have has samples that tear in the cup when grooved with the flat ASTM tool (not the AASHTO, though) but will close in one blow and have a sheen of moisture in the surface. The closure is due to a lack of adheasion to the cup. but when rolled as prescribed in either methods the PL is reached well before the LL can be established (If at all)...
Rjeffery - Two standards that I know of:
British Standard BS 1377-2-4.3
Indian Standard IS 2720 (Part 5) - 1985
I got a four page bibliography from UMASS on the Fall Cone. Lots of papers from Geotechnique back in 1980s. I'm sorry I don't have the URL off-hand - you might be able to search. One paper:
Kumoto and Houlsby (2001) Theory and Practice of the Fall Cone Test, Geotechnique V51, No 8 pp 701-712
Some silts have a liquid limit, but you can't roll the threads even near the dia. to get a plastic limit (following ASTM 4318). Goes straight from liquid to plastic.
Some auger cutting samples I got once had a liquid limit, but the plastic limit we couldn't roll. Turned out that the drillers hit rock and didn't note it, and that was the ground calcareous rock. (Legitimate soil?)
Got some stuff from CMEC that almost defied soil mechnaics once.
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