Optimum Water Content for Sand With Minimal Fines

[CivilTechSK]

Hello,

I'm currently at a mining project involving re-construction of dikes for their tailings area. The material being used is a sandy soil with approx. 10% fines. Compaction testing is required for these areas, and I'm questioning the results I'm getting for my proctor.

For my optimum water content, I keep getting approx. 14.0% optimum. I have done multiple proctors on the same material, and keep getting consistent results with respect to the optimum. My cross-shift, however, has performed a couple proctors on the same material and it resulted in approx. 6.5% optimum. The material does change slightly throughout the site, but it should not be that much different. Also, there have been additional proctors on material with more aggregate present, and they have resulted in a higher optimum than 6.5%. This might indicate that 6.5% could also be too low. I know sometimes with cohesionless soils, if they are too dry, the densities can rise and fall giving a false indication that optimum has been achieved. In my experience, I have never done a sand proctor without aggregate, and achieved an optimum around 6.0%.

We are both using the same ASTM methods, and my proctors don't start to "bleed" until approx. 15.0%, indicating it's just past optimum. I am not re-using any material from previous points either.

Does anybody have any suggestions as to why we keep resulting in such different optimums? I should also note, our maximum densities are pretty much identical in comparison.

 

[oldestguy]

Provide a few curves by each. My bet is the sand is very uniform in gradations, little range of sizes, similar to a bunch of marbles.

 

[CivilTechSK]

Attached are a couple curves from each of us.

 

[Ron]

Were these standard or modified Proctors?

14% for standard Proctor would not be particularly unusual. It's a bit high for modified.

Also....your specific gravities are all over the place! Typically sand specific gravities don't vary as much as you have shown. With the lower optimum moisture content the curves can't be compared to the ZAV curve.

Check your specific gravity assumptions.

 

[CivilTechSK]

Proctor was a standard effort.

Another reason I thought 6.7% was somewhat low for the optimum was because the specific gravity seems too low to be close to the ZAV curve. The S.G. would have to be approx. 2.1 for the wet points to be close to the ZAV curve, and that's too low in my opinion.

The material does have iron ore present in it, and it changes with location. That's why the S.G. range from 2.55 - 2.65, even though they are assumptions.

 

[Ron]

Assuming the iron ore is hematite, your specific gravity assumptions are still low at 2.55-2.65.
Most natural sands will be between 2.6 and 2.7
Hematite has a specific gravity of about 5, so it doesn't take much of it to run your specific gravity way up.

Get the SG right so you can check saturation and get the ZAV curves right. I suspect your curve algorithm is also using the SG which is throwing your curves off.

Plot your curves manually as a check

 

[fattdad]

The one-point is your friend!

Take three of the laboratory proctors and trace them all on one piece of paper. Go in the field with a concrete block, mold and hammer. Take the sand fill, get it below optimum and run a one-point. Walk that one-point onto your traced, "Family of curves" (after all it's a family of curves for your site) and see which curve the new one-point most relates to.

f-d

ípapß gordo ainÆt no madre flaca!