Soil testing.

[Big Jimbo]

I know that when a soil test for plasticity is done, a sample is taken, dried, and put through a 425micron filter. The material that passes through is then tested for plasticity. Would anybody know what is involved in a Modified plasticity test ? Is it something to do with adding back in the material that does not pass through the filter ? I can't seem to find the relevant info online.

[alfaTom]

Do you mean the test to get the Modified plasticity index instead of the normal plasticity index?

If so then it's just the results of the plasticity test (Ip) multiplied by the percentage of particles that made it through the filter in the first place.

 

[Big Jimbo]

@alfaTom So if you had say 4kilo of material. Dried and filtered. Say 1/2 a kilo of material goes through the 425micron filter. Is the other 3.5kilo just discarded ?

[Big Jimbo]

Say that the 1/2 kilo was all clay, but the other material was say sand with gravel. That means that the 7/8 of the material is not taken into account ?

[markc]

Why 425 microns? Who came up with 1/60 inch as a measure 

Edited July 14, 2023 by markc

[Big Jimbo]

The 425 because that is what i believe is the size used to filter the material before the plasticity test is done.

[markc]

3 minutes ago, Big Jimbo said:

The 425 because that is what i believe is the size used to filter the material before the plasticity test is done.

Yes I know it’s the filter size, my point was why That particular size? Is it a natural phenomenon for say size of grains of sand?

[Big Jimbo]

I don't know. I just know that that is the size used by the labs.

[alfaTom]

3 hours ago, Big Jimbo said:

Say that the 1/2 kilo was all clay, but the other material was say sand with gravel. That means that the 7/8 of the material is not taken into account ?

I believe that's what the modified result is - a way to scale up to the overall sample size. The 0.425mm filter is used as part of a control method for testing as the results can be reproduced consistently and the modified plasticity index can then be calculated if we know the fraction that passed through the filter vs the total sample.
My understanding is that it's this modified index that gives you the overall figure as if the soil had not been disturbed for the purposes of the test.

This is the SI report for one of our boreholes. You'll note the Plasticity Index and Modified Plasticity Index remain the same until we get to 3.0m depth where the Soil Fraction >0.425mm changes drastically.
At this point the Modified Index reduces.

 

The test method for Plasticity Index was BS 1377 Part 2 1990 Test No 5.4, and the Modified Plasticity Index was BRE Digest 240: 1993.
Having a google finds the below theory if it's any use to you (otherwise you'd have to buy a copy of BRE 240)
 

 

Hope that helps

[Big Jimbo]

`Thank you @alfaTom. I understand it now. Cheers fella.