Validity of Atterberg Limit charts that has been used in Geotechnical Engineering

[Reddevils]

Dear all,

I have been working for a small geotechnical firm in the US. I am responsible for writing geo-reports and sending it to the client. While I have bee writing report, I found something weird about taking PI of the soil that I never done in my life.

My boss wants to do only Liquid limits test and used the liquid limit obtained from laboratory to the table of default list of liquid limit, and plastic limit. He finally wants to use the Plasticity index obtained on this way for the further calculation.

I had checked with several research papers and thesis where they had done both Liquid limits and plastic limit tests and compared the value obtained from actual test to the value obtained from that table. These values are off by significant margin (sometimes >6). I don't know whether the way taking value of Plastic limits from table based on liquid limits is right or wrong?

Does any of your company use same chart as attached?

Thanks in advance.

 

[hokie66]

No, looks lazy as well as inaccurate. However, the chart could indicate the relationship observed within a specific geological area. Did your boss explain how the chart was developed?

 

[Reddevils]

Yes, I have discussed with him about it. He said, it is based on the several hundreds of experiments in research conducted in different part of the US. But, I still have not satisfied with his answers. I feel like we are cheating the clients by assuming plastic limits based on liquid limits.

 

[Ron]

Yes...his approach is lazy and not defensible unless you develop your own plastic limit correlation based on actual tests in local soils. There are two separate tests for a reason.

The plastic limit is more difficult to get right and is less repeatable than the liquid limit; however, it still needs to be run.

 

[AussieGeoEng]

Check out the paper below, where a method is proposed for determining wP based on the fall cone.

Feng, T. W. (2000). Fall-cone penetration and water content relationship of clays. Geotechnique, 50, 181-187.

I don't think this has really been picked up in practice, but in a discussion by Prof Mesri & Peck of a different method, they suggest that the Feng method may be reasonable.

O'kelly, B. C., Mesri, G. & Peck, R. B. (2011). A new method of measuring plastic limit of fine materials. Geotechnique, 61, 88-92.

 

[jgailla]

If I found out my testing company was doing this, I'd probably find another company to do the work.

 

[fattdad]

bad boss alert!

f-d

¡papá gordo ain’t no madre flaca!

 

[Erdbau]

Soils from the same geologic origin and mineralogy will generally plot in a straight line on a plasticity chart (LL vs PI). Often the line is roughly parallel to the A-Line (dividing line between silt and clay). You can probably make your own correlation for your local soils, but using data from other parts of the country probably isn't going to be the most accurate way to do things.

Roll out a couple of plastic limits on your own time, and see how well your boss' correlation really is.

You are cheating your clients if you tell them you are performing the limits tests in accordance with ASTM standards. If not, how much does the PI really govern the recommendations you are making, and how much of the recommendations are based on local experience?