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Questionable Modified Proctor test

Questionable Modified Proctor test

Questionable Modified Proctor test

(OP)
I had a contractor using a backfill material that was tested using the modified proctor method yielding an optimum water content of 6.8% and max dry unit weight of 21.28kN/m3. They recently "retested" the backfill material getting a new corrected wc of 10.7% and max dry unit weight of 19.94 kN/m3. Thats a pretty drastic change in optimum wc and a drop in the max dry unit weight on the same material in my opinion. I am now getitng compaction test results with many coming above 100%, which itself not a huge deal, but several have been above 102% and even 103%. I am questioning these results now and while I am not concerned with the values around 100% I am worried that any test that resulted in a 95% to 97% percent compaction are not "true" values and may be lower than the specification requirement of 95% compaction. Thoughts?

RE: Questionable Modified Proctor test

What type of soil is it? There are soils with double highs, although I haven't seen any.

RE: Questionable Modified Proctor test

If you feel that way, you could re check with a Nuclear Densometer.

Thats of course if it is possible, maybe concrete has been poured on top or whatever. If that is the case you might be best letting sleeping dogs ly!

If additional fill has been placed, then excavating down to maybe 1.5m and preforming some Nukes might be an option.

Even if the "true" values were 90% they may have gained density over time due to additional fill on top providing a surcharge or dissipation of porewater pressure (if wetter than optimum).

If more than 1.5m of fill has been placed, again let sleeping dogs ly....Its only going to be marginally below the true 95% value (hopefully) and will that have much of an effect.

RE: Questionable Modified Proctor test

(OP)
Its structural fill gravel with sand and some silt present. Previous and current modified proctor tests showed standard bell curves no double dips. 90% passing 2" sieve, 82% passing 1.5", 66% passing 1", 44% passing No 4, 11.5% passing the 200 sieve. LL was 28 and PL was 21 with a PI of 7. They had been having field labor remove "plus size" aggregate during compaction as I already brought that up as an issue and now are getintg a screen on site to do it more thoroughly.

All the compaction test results were recorded wiht the nuclear denso.

RE: Questionable Modified Proctor test

With the discrepancy, I'd redo the testing; you should not have that variance.

Dik

RE: Questionable Modified Proctor test

What size lab compaction molds and what "gravel correction"? I'd do some double checking with the large size sand cone method. How was the nuc calibrated?

RE: Questionable Modified Proctor test

(OP)
The mold volume was 2131.1 cubic cm or 130 cubic inch. Done to ASTM D1557-12 and procedure C was used. Didnt see what if any gravel correction on the data sheet. Dont have the calibration of the Nuc readily available but when they were doing compaction with the prior testing densities were in the high 80s (required recompaction) up to 96 and 97% with only two tests getting over 100% none over 102%. While this is no subsititue for seeing the claibration method and results I dont think the nuc is off any.

RE: Questionable Modified Proctor test

Getting compaction results of over 100 percent is not unusual if using a well graded material, good compaction methods and the moisture content is at or very near optimum. That doesn't concern me unless the results are routinely in the 104-105% range.

Soils as you have described can vary in their Proctor value rather easily because of variations in the particle sizes at any location.

Depending on the area to be tested, multiple Proctor tests might be required.

RE: Questionable Modified Proctor test

My chime is that too often a variable back-fill material is classified based on 1 density test, which is then carried over to every density check conducted. Silt is relatively light, stone (gravel) can be relatively dense and the various ratios of these particles can throw your density testing off.

My first thought would be that the Mod. density of the material varies more than was anticipated, be it due to variable gravel content or otherwise. I would get a couple of extra tests run and use an average Mod. if possible.

All the best,
Mike

RE: Questionable Modified Proctor test

Are these 2 proctor results from the same lab? You may need to use an independent 3rd party lab to validate the results.

RE: Questionable Modified Proctor test

mod proctors are harder to check-point in the field. i don't like mods specified for structural fill.

RE: Questionable Modified Proctor test

dsg2...depends on the soil type. In the coastal plains soils that I deal with, check points usually work fine, particularly if the fines content is a bit higher than in clean fine sands. Our sands are poorly graded (SP) and can sometimes yield wacky Proctor results, but overall pretty good. We don't typically use the standard Proctor except in non-structural embankment soils.

Assuming you work mostly in glacial till and residual soils, you probably have sharply peaked Modified Proctor curves, high unit weights and little room for moisture ranges. Let me know if I'm off base....I'm doing some writing and still have time to change a few things if necessary!

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