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Atterberg Limits

Atterberg Limits

Atterberg Limits


We all know that both the liquid and plastic limits are percentages, and the subtraction between LL and PL is the result of what we know as plasticity index (PI).

My question is, the correct way of expressing this plasticity index (PI), do you think it is by adding the percentage?.

For example:

LL =30%, PL = 12%, PI = 18%.

OR do you think without the percentage (PI = 18) is ok?.

I asked you this because I noticed many geotech books use different terminologies. There are some which only include them using the LL and PL, but not for the PI. There are other few which do include percentages as units for LL, LP and PI. And there are others which do not use the percentage symbol at all.

Please advise.

RE: Atterberg Limits

I have always presented the values as just an integer....no "percent" reference.

RE: Atterberg Limits

For those that use these numbers, such as meeting a spec., it doesn't matter since that person knows what the number stands for.

RE: Atterberg Limits


Thanks a lot for your replies.

RE: Atterberg Limits

I don't use percentages in reporting Atterberg limits.

That said, it is of some value to read the actual ASTM Standard to learn more about the reporting requirements.


ípapß gordo ainÆt no madre flaca!

RE: Atterberg Limits

I guess I'm the odd man out on this one. I always use percentages. That said, as long as you are consistent, I don't see that it matters either way.

Mike Lambert

RE: Atterberg Limits

I write the percentage, but I also think that it doesnt matter as long as you are consistent.

RE: Atterberg Limits

The original units are % so including the % symbol would be the technically correct thing to do. Geotechnical engineers are unlikely to misinterpret the data, however, though I guess there are some extreme circumstances where adding the % sign is a useful clarification (e.g. in cases where PI is very large, add the % sign may avoid any potential for confusion).

RE: Atterberg Limits


ASTM D4318-05, in the report section, it says "19.1.4 Liquid limit, plastic limit, and plasticity index to the nearest whole number, omitting the percent designation.".

RE: Atterberg Limits

from Holtz and Kovacs, Page 39

RE: Atterberg Limits

D 4318 - 00:

For Liquid Limit: "The liquid limit, LL, is the average of the two trial
liquid-limit values, to the nearest whole number (without the
percent designation)." (Clause 14.1.1)

For Plastic Limit: "Compute the average of the two water contents (trial
plastic limits) and round to the nearest whole number. This
value is the plastic limit, PL." (Clause 17.1)

Plasticity Index: does not say that PI is reported as whole number, but must be as "Both LL and PL are whole numbers. If either the
liquid limit or plastic limit could not be determined, or if the
plastic limit is equal to or greater than the liquid limit, report
the soil as nonplastic, NP." (Clause 18.1.1)

And for the record - I only use whole numbers (integers) - never the percent sign - and when in a soils report (say for percentage of sand sized particles), we always used "percent" and never "%".

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