Problem with Soil 1

[gatorn10ac]

Hello Everyone,

I have a problem with soil from the lab to the field. The lab ran Astm D698 which is generally specified for this area. (East Tennessee) The lab has run 3 different sample on the same material and they are getting a PCF of 83.0lbsb@35+ moisture. In the field I am getting 105-108% compaction and the soil moisture is 25.5 % and the soil is sticking to the drum of the roller. What next?????

 

[Focht3]

It sounds like you have a mismatch between the compaction standard (ASTM D 698) and the actual compactive effort. Run ASTM D 1557 and see if that curve agrees with the field results.

What are you building? What is the soil type? Atterberg limits? Gradation?



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[SirAl]

The Proctor that is being applied does not appear representative based upon the results and the behavior of the soils that you described. Is there a soils report available that documented the soils and conditions prior to construction? A description of the soils would be helpful. Atterberg limit results would be valuable information. Is the soils in the field being chemically modified with lime/Portland cement, etc. ? Also please provide a description of the compaction equipment that you are using. A few more details are required before constructive comments can be provided.

 

[gatorn10ac]

Thanks for the reply. The soil is being used for a building pad, small. The Compaction equipment being used is a Bomag "sheeps foot " roller.

I have not ask yet about the atterberg limits . I thought of running a 1557 on it and see how that relates. I also thought about running one point proctors in the field.

The soil being used is from an off site borrow source. There is no added chmicals to the soil.

I thought the same thing about the proctor not being representative of the soil being used, that is why the sample was re-sampled three times. The same results on each sample.

Could it be that the standard proctor does not aply to this sample ? It's a sandy clay with a trace of chirt.

Thanks again
Ken Carlton

 

[SirAl]

Unless you're dealing with a bentonitic condition, 35% would put a sandy clay near or even above the liquid limit value, at least in my neck of the woods.

I don't see much value in one-points unless the material is highly variable. Even at 25% moisture, I would suspect that your soils are well above optimum. As the moisture approaches optimum and compaction is getting close, the compaction equipment should start to walk on top with only a small amount of penetration of the the drum pads. I am assuming that you require 98% to 100% SPMDD.

 

[VAD]

At 83 pcf and 35% moisture I would want the lab to examine the material in question. What % chert is in the sample. Isthere organics in the soil.

 

[GeoPaveTraffic]

How are the field tests being performed? Nuclear, drive tube?? Are the field moistures from the Nuc?

I can't explain the high apparent water content, unless there is organic contamination of the soil.

The low density on the standard proctor could be caused by a very fat (high limit) clay. The relatively light hammer used in the standard test may not have enough energy to break up the pieces of clay.

Please keep us informed.

 

[Focht3]

All good comments - except I would do single point Proctors on the borrow material. Too much data is better than not enough...

Do you have a gradation on the material? Did you apply any rock correction factor to your lab results?

At 83 pcf and 35%, you don't have a CL material - moisture is too high. What other test data do you have on the borrow material?



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[gatorn10ac]

Hey everyone,

Thanks for the advise. What I did was had the lab run a modified proctor on the material. The modified fit alot better than the standard did. We placed the remaing fill and no problems.

Again, Thanks for the input.

Ken Carlton
g8rn10ac

 

[DBNodurf]

Wow, I hate to have missed all the fun! Once again, Focht3 hits the nail squarely on the head. The standard Proctor is NEVER representative of the field compaction effort unless someone is using the ole horse-drawn sheepsfoot roller the test was designed to emulate. For modern day compaction (modern meaning since World War I), the modified version is the most applicable.

Personally, I'd like to see the standard variety disappear, but its inertia in the design community (you know, the folks with the blinders on?) is similar in magnitude to that of earth's...

Anyway, just remember that no soil has a characteristic "optimum" moisture content; the optimum moisture content varies with the compactive effort... as you increase the compactive effort, the moisture-density relationship shifts "higher and drier;" i.e., the maximum dry density FOR THAT EFFORT increases, and the corresponding OMC decreases.

Be forewarned: somewhere there some graduates of East Carolina University's Construction Mgmt program that actually understand this concept; I'd hate to see them show up any geotechnical engineers out there!

D. Bruce Nothdurft, MSCE, PE, PG

 

[Focht3]

Thanks for the kind words, [blue]DBNodurf[/blue]. And I agree with everything you said - [red]except when you opined[/red] - ...I'd like to see the standard variety disappear... We use D1557 for the most part here in central Texas, but D698 is prevalent along the Texas Gulf Coast, and I specify D698 for expansive clays that will remain beneath lightly loaded foundations. Why? Because the higher dry densities associated with D1557 poses a significant risk of future expansive soil movements. Ditto for the lower compaction moistures with D1557 -

Each method has its place - that's why both continue to be used, sometimes on the same project...



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[SirAl]

Focht3
An important and often over looked point w.r.t. the swelling clay issue.

 

[BigH]

To all of you wanting to do away with Std Proctor - pax on you - your lab techs would be cursing a tad!! - giving up the Std Proctor for the "hernia" test?!

I've had a tendency to use Modified Proctor under foundations and when I want the contractor to take the compaction seriously. When you are doing site grading without later loading, e.g., landscaping, standard proctor is fine. What is wrong with 105% Standard compaction? Is it because it is greater than 100%? - and this doesn't "look" right? Okay, specify your modified compaction and get a result of 99%MDD. Happier??!

Yes, Focht3 is correct about the swelling clay problem - I understand in CA, they even specify 85% MDD (Modified) for swelling clays. Try to get this past an agency that only believes "the bigger the better."

Now back to the original question. 35% does seem high for a sandy clay, unless the clay is very fat.

I have run into a situation recently with compaction of fly ash (with 15% natural sand added (fine sand, some silt)) recently - we actually have a "two" hump curve. We have the first hump in the order of 22% moisture and a second hump in the order of 33%. The first has a MDD of about 1.4g/cc added and the second hump has an MDD of about 1.2 g/cc. Is there any way, gatorn10ac, that you could have such as well. Did you try any moisture levels down at 15 - 20% range in the lab testing?? Just a thought.

 

[DBNodurf]

Good point, John (Focht)!! That's why 2+ heads are better than one! I appreciate your contributions to this forum.