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Compaction Testing

Compaction Testing

Compaction Testing

In the past week I had a city official, plot the results of a compaction test on the Proctor and reject the test because it fell "outside" a "reasonable range" from the Curve.

here is the quote (names removed)

"As far as the plotting of the test results against the proctor curve used, that has been a common practice as a check, of sorts, to assure that the correct proctor is indeed being utilized. When the test results for the density test in question were plotted against the selected proctor for this material, they fell well outside the proctor curve, rather than on or within the curve envelope as one would expect. That being said, I don't know if it is an inappropriate material proctor that was utilized, or the high moisture content in the test that has provided questionable results."

"To get us through this particular test for acceptance, I'm sure XXXXX would need at least a test report that displays the dry density and moisture results that fall within the reasonable expected range as plotted against the proctor curve. If you can provide such report, engineering would be ok recommending approval of the compaction test"

the standard calls for 95%, no reference to Moisture, and while moisture was elevated (22-27%) in silty claye soils it is not out of reason. By the way compaction exceeded the requirement.

I have been doing this for over 30 years and have never heard of the practice.

Has any one else heard of this, and if you have can you tell me how to establish the "reasonable range"


RE: Compaction Testing

In my mind, based on the information provided, the "city official" obviously does not understand the nuances of compaction; is he a geotechnical engineer by training or ???? . It is an interesting topic that for something that one considers so "elementary" - many of the foundation handbooks (for instance Winterkorn and Fang) have larger chapters on compaction than on some other geotechnical topics.

You failed to mention if it was a standard proctor curve - or an modified curve. Does the "city official" understand the difference? Further, heavy compaction equipment will lower yet more the optimum moisture content and increased dry density due to the larger compaction energies involved.

Saying that, which I am sure you are aware, there are a number of issues that need to be addressed - namely, where does the test results fall in relation to the zero air voids curve. That is a much larger issue - because if they fall to the right of the ZAV line, then it is a theoretical impossibility. Secondly, while the moisture contents are high - they might give the right compaction but be detrimental to the purpose of the fill - you must have been using a fairly highly plastic clay.

So, in short, no, I have not had any run-ins of what you have described; I have had problems with some labs on the ZAV issue.

RE: Compaction Testing

Sometimes in screwy cases like this, assuming you have their confidence as "knowing more", here is a possible way I have gotten around using percent compaction as the accepting criteria. What is the purpose of the compacting work anyhow? Is it to provide a suitable subgrade for pavements? A good degree or density so that settlement of trenches does not occur? etc?. Then, for clays, I have found that other testing can be used. For instance, if some degree of "bearing capacity" is needed, as for a subgrade, the unconfined compressive strength needed is check for. A simple test with the pocket penetrometer for tight clays is the way to go. It is common that the moisture content that is acceptable for this check is well beyond the peak of the compaction curve, making it not necessary to do a lot of drying of the soil.

Another check for subgrades is run a loaded truck over it and measure the rut depth. One inch rut is a severe test, but may have to be adopted. 2" is easier and usually adequate.

For trench backfill, check the natural ground density and accept the backfill if it equals that density. Many a time I have found natural ground around 80 percent of Modified proctor, yet suitable.

RE: Compaction Testing

Data please. preferably a redacted curve with the point on it, and classification results if available. Any info on the test itself is also useful.

putting the point on the curve is good oversight practice by the GE that the tester reports to (and self-check) but there is a bag of ways to mess up testing. I usually put the point down to see if (like BigH said) if it is right of the ZAV line. Our SOP was for techs to do a 1-pt field proctor at the start of each day, observed change of materials or cut location, and failing test. 1-pts help the tech understand the soils better and help relieve some of the 'wrong proctor' peanut gallery out there.

Only based on the quote from the official said "high moisture content" and "outside the curve".... it does sound like the test would be outside the ZAV line which would mean testing error or wrong material. There are plenty of good acceptable explanations for dry passing soil outside the curve, wet/outside/passing not so much... some testing possiblities for that scenario: If the soils have gravel larger than 3/8", there is a possibility that the in-place test sieved out the stone and penalized the test with higher Moisture Content of the fines, a host of errors with nuke gauge calibration of the moisture content, technician using uncalibrated equipment (wrong scale, unlevel scale, scale exposed to wind...), burning clays, burning asphalt if there are reclaimed materials, math/note-taking.....

Also, it sounds like we are talking about 1 individual compaction test. Makes me wonder how many passing tests were overlooked to determine that a corrective retest is necessary. This isn't nuclear work, if the test was wet of ZAV, the Geotech Egr should be given the opportunity to say that test must be bad and disregard. Now, if there only is 1 test... i'll be taking sides with the AHJ on this one.

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