plastic limit of soils 1

[johnengr]

Recently I had some soil tested for Atterberg limits for shrink swell determination per the IBC. The testing lab report noted that the plastic limit was zero as they could not get the sample to thread down to the 1/8" requirement. This in turn forced the Plasticity Index to equal the liquid limit which in this sample was 66. Per USDA classification procedures the soil was a clay loam. I have read in army manuals that the 1/8" d is not a strict govering factor if the soil has been throughly wetted. I would appreciate any guidence concerning the classification of a soil that will not neck down to 1/8" d, would this soil then be classified as a silt? If this is the case then should the Atterberg limits not be used for shrink swell but rather the Expansive index.

Thanks

 

[woofar]

Hi, well first up the atterberg limits should not be used to determine the reactivity of a soil. The atterbergs can give you an idea of something that may be expansive, however the actual determination should be left to the shrink/swell test.

Obtaining a plastic limit from a low plasticity material requires quite a deal of experience on the part of the technician. Your material does sound like it is quite silty however the only way to classify it as a silt is by the use of hydrometer analysis. As for the 1/8" d , if the thread crumbles before it reaches 1/8" then it is considered at it's plastic limit when it starts to crumble, provided that the crumbling is due to moisture loss and not due to presssure from your hand or from laminations coming from the remoulding process. The 1/8" limit is just an end game figure so that if you get down to 1/8" without any crumbling then the material should be remoulded into a new thread and then rolled back down again. Many technicians believe that they have to roll it down to 1/8" first and then get it to crumble, it would pay those techs. to sit down and reread their test methods. The plastic limit can never equal the liquid limit.

With low plasticity silty type materials you can use the Linear Shrinkage(LS) to give a very good estimate of the PI by the equation PI=2*LS. This relationship only reliably holds true for the fines associated with crushed rock road-base, there is a similar relationship with many natural materials however it is best to do extensive comparitive testing on all of your local materials before you start using this relationship. A range of 1.5*LS to 2.5LS should cover most materials.

P.S. In your post you seem to be using Plasticity Index(PI) and Plastic Limit(PL) interchangeably which is not correct, PI=LL-PL

HTH
Michael

 

[Ron]

I agree with woofar....get a hydrometer test done so you know the silt quantity. If it is non-plastic as you have indicated, it likely has a very high silt content and relatively high shrinkage potential, but less so for expansion (depending on the material, shrinkage and expansion potential are not equal).

 

[johnengr]

Thanks woofar and ron for you information. The 2006 IRC requires a determination of the expansion index in accordance with ASTM D 4829 or the Atterberg limit info testing. What is your opinion on this test for expansive soil identification and degree of severity?
Thanks-John

 

[fattdad]

You have a sample that classifies as elastic silt (LL=66, P=some low value). It seems hard to believe that the sample is non-plastic, however.

What are you designing? Where is the project located? What is the local occurance of shrink-swell soils?

I'm not sure that the hydrometer test would properly address the problem. You could find that you have 90 percent silt-sized particals, but the remining 10 percent could be smectite. Weight percentages cannot properly address your problem.

Regarding the "shrink-swell test", it does provide an extreme case condition, but often wildly overestimates the actual subgrade performance - this goes back to local occurance.

Here in Central Virginia, we have plenty of shrink-swell soils (triassic basin) and elastic silts are often the worst.

f-d

¡papá gordo ain’t no madre flaca!

 

[STVU]

Liquid limit of 66% and no plasticity? Is this a unique formation? Usually such a high liquid limit will yield substantial moisture content,plasticity and a PL value.

Anyway, unless you have a unique geological formation, I would classify the material as plastic silt.

You mentioned shrink/swell determination in your post-are you looking for earthwork fluff factors or are you looking for expansive characteristics?

If you want earthwork factors, then we need to get densities of the borrow soils and as compacted soils. If it is expansive soil properties you need, then we need to do expansion index test.

 

[GeoPaveTraffic]

I agree with fattdad.

Check the local experience with this material. You may also want to have the plastic limit rechecked.

Another consideration is the natural moisture content of the site soils. Shrink/swell problems only occur if the moisture content changes. If the soil is wet and there is little chance of it drying, then not much of a problem. Same is ture if the the soil is currently dry and not much chance of the water content increasing.

 

[fattdad]

On the matter of lab work. For those in the forum that do not have their own soil laboratory and perform their own compliance testing (i.e., AMRL proficency testing), I really recommend that every so often you take a sample, divide it by three and send it out to three labs for routine testing (i.e., PI, percent sand). You may be dealing with a laboratory that isn't refined enough to determine whether there is a plastic limit or not (i.e., inexperienced thread-roller).

On the matter of moisture change, this is the main reason I don't like the "potential volume change" test. It takes a dry sample, soaks it and measures swell. These extreme conditions may never be realized at the foundation bearing depth.

On the matter of mitigation if you truly have shrink-swell soils, remove yard irrigation next to the foundation, remove trees and lower the foundations a couple feet (in Central Virginia we typically go from 2 to 4 ft). There's some depth where surface desication just doesn't extend.

Hope some of this helps.

f-d

¡papá gordo ain’t no madre flaca!