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Liquidity index for material containing +#40Helpful Member! 

dgillette (Geotechnical) (OP)
2 Aug 06 18:18
As you probably know, Atterberg limit tests are run on the portion of the soil passing the #40 sieve.  If a soil contains a small amount of larger material, one could, I presume, calculate the liquidity index with an adjusted water content that ignores the +#40 material:

w-adj = wt of water / wt of -#40 * 100

In effect, this associates all the water with the -#40 fraction, assuming that the larger particles "float" within the -#40 matrix and do not aDsorb or aBsorb much water.  (The equation can be tweaked easily to allow the larger particles to adsorb or absorb some small amount of water.)

How much +#40 material can be present before the liquidity index ceases to be meaningful for things like remolded shear strength?
Helpful Member!  GeoPaveTraffic (Geotechnical)
3 Aug 06 9:25
I would say that as long as the majority of the +#40 particles are not in point to point contact that the liquidity index would be usefull for things like shear strength.  In those cases your description of the particles floating in a matrix of -#40 particles is the way I have described it in the past.  

In honesty, I have not been concerned about lager particles until the particle size is quite a bit larger, say 0.5 to 1 inch.  If you are concerned about the effect of the non-tested particles, run a shear strength test on undisturbed samples.  

Hope I helped.
BigH (Geotechnical)
4 Aug 06 12:31
I see the liquidity index to be really only "good" for fine grained soils anyway - so, as per USCS, your soil must contain >50% passing #200 sieve size.  The #40 sieve takes into account fine and medium sand - so I would say that if you have any significant (I know - quantify) material >#40, the liquidity index wouldn't be an appropriate index property in any event. Example:  You have a roadway base course - the Atterberg limits can be determined on the -#40 sieve - but the liquidity index is really meaningless for a roadway base anyway - the only reason to know the Atterbergs is to determine if the fines are potentially expansive/contractive which might lead to pavement breakup - also to determine binder properties.
   Good question though!

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