Liquid Limit Vs Plastic Limit 7

[pelelo]

Engineers,

As all you may know PI = LL - PL.

Is is common the fact that someone reported a plastic limit (PL) being greater than Liquid limit (LL)?. I know that automatically, since PL > LL, PI is negative and any software could call it as Non Plastic, but I am not sure if this is ok in this case.

This test was performed on a common clay soil, therefore no special soil type was used.

Please advise.

 

[dik]

I've never heard of that, and I don't think it's possible. Likely an error. The first time I ever did Atterberg limits was as a summer student working on an airfield north of Winnipeg. I had no idea our clays were that plastic. My first reaction was to check the results and the second was to re-do the test. I was still skeptical and called the prof, and it was confirmed.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[emmgjld]

I have encountered some soils with a PL 1 or 2 points higher than the LL. Usually the silty clays & occ. very low plastic silts.

 

[EireChch]

I call BS.

 

[emmgjld]

The low border line area of the plasticity chart is not definitive!
Over the years, the authors have avoided discussing the CL-ML soils, probably for good reasons.

 

[dik]

or an error, typo or otherwise. Check with the guy that did the report.
-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[CarlB]

I haven't found a good explanation of the soil conditions that might lead to a negative PI, but seems to have been observed on occasion (such as by emmgjld above).
For example:

https://www.elementaryengineeringli...ering/soil-mechanics/plasticity-index-of-soil

"Also in some cases plastic limit of soil is found greater than the liquid limit or in another cases both PL and LL cannot be determined.

In all these conditions soil is considered to have plasticity index zero and soil is reported as Non-Plastic (NP).

Organic soils have high Liquid Limit and high Plastic Limit so they have very low value PI."

 

[dik]

CarlB... I don't think it can happen unless you have an unstable soil matrix. Maybe some weird type of deflocculated clay or something of that ilk. I've never heard of this in 50 years...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[GeoEnvGuy]

The lab should report non plastic. If the lab reported a negative value call the lab manager and ask them if this is a mistake or are they telling you the sample has negative plasticity index.

 

[BigH]

Perhaps the plastic limit thread was a bit off?

 

[iandig]

I think that there is either a fundamental error in the testing or the reporting. The plastic limit is the moisture at which the soil begins to behave in a plastic manner, below this moisture the soil is dry and friable. The liquid limit is the point where the soil begins to behave in a viscous, liquid manner. ergo, the soil cannot be both friable and viscous.
I have seen issues in the testing of both the plastic limit and the liquid limit, generally related to silts. A 'true' silt would be thixotropic in nature, and can behave in a strange way when determining the liquid limit by cone penetration, in particular if only a single point test is done in place of the more robust 4 point method.
I would ask for the liquid limit to be done by 4 point method and get a sedimentation test carried out, to assess the truE proportion of silt to clay particles in the soil sample.