We have a project with about 50,000cy fill, with maximum fill at about 3'.  It has brought up a question.  We are calling out our testing to be performed in accordance with ASTM D-6938 (field nuclear gauge test).  What is the difference between this test method and a labratory test such as D1557?  When is a labratory test needed vs a field test.

I am asking this from a very elimentary standpoint as my experince is of a Hydrologic Engineer and sticking my nose into this project.

Thanks

Typically, when you are doing the field density tests for compaction control, you'll have a specification such as, "95 percent of the maximum dry density as determined in the laboratory using ASTM D-1557, Modified Proctor. The lab test is a basis to confirm that the compacted field density is achieving some percentage of a laboratory standard.

Problem is the laboratory standard can vary by grain size distribution.  So, not one lab test can be used for the entire job.  It's always correct to do more "Proctors" in the laboratory as quality assurance on the field program.

f-d

¡papá gordo ain't no madre flaca!

Continuing with f-d's explanation...

ASTM D6938 is a method to determine the in-place density and moisture content of the soil or a soil-aggregate mixture such as a pavement base material.

The in-place density is first determined as the wet density, then the moisture content at the same location is determined.  Let say for example that your wet density of the in-place material was 114.2 pounds per cubic foot.  Then the moisture content is determined and found to be 11.3 percent.  From these two values, the dry density is determined by the equation

 ((wet density/(1+moisture content))=dry density

So your dry density is 102.6 pcf

Meanwhile, a sample of the material in the field has been taken back to the laboratory to perform the D1557 test (commonly called a Modified Proctor).  This test is the moisture-density relationship of laboratory compacted soil.

In the laboratory, the technicians divide sample into 4 specimens and allow them to dry.  They then add moisture at different amounts so as to develop a relationship curve between moisture and dry density.  The moistened soil is then compacted in the lab using standardized procedures so that the same amount of compaction energy is put into each specimen.

After compaction the wet density and the moisture content are determined to find the dry density of each specimen.

The dry density is then plotted against the moist content of the specimen at the time of compaction and a curve is developed.  The peak of this curve is called the maximum dry density and the corresponding moisture content is called the optimum moisture content.  Let's say that such a test was done on your soil and it was found that the maximum dry density was 109.6 pcf and the optimum moisture content was 11.1 percent.

Your in-place density divided by the maximum dry density is the amount of compaction you have achieved in the field.  Usually that is specified to be somewhere between 95 and 100 percent, depending on the application.  For your case...

102.6/109.6 yields 94 percent compaction, or slightly failing the typical specification requirement.  Looking at the results, we see that the in-place moisture content is almost at optimum, so this tells us that there just needs to be more compactive effort put forth in the field...roll it some more, then retest it.

 

excellent, Thanks!

Great explanation Ron.

f-d hit on the point that can cause lots of job control problems.  The lazy way is to do the minimum amount of lab work, with the assumption that the field tech can relate his field test soil to an appropriate lab test.  That is where the field control can go very wrong.

To play it safe, if in doubt, run another lab test using the same soil that you tested in the field.  Easier said than done, since by the time the lab work is done, the area tested on the job has been covered up.

Don't make the mistake of thinking you can run sufficient numbers of lab tests ahead of the job to avoid having to run any while the work is going on.  That is a start, but seldom can be assumed as adequate lab testing.

More grief also can come from making "a gravel correction".

Also - how badly does the fill "break down" when heavily compacted . . ?  This can change the "real" maximum dry density from that determined in the lab.

Also, it is imperative that a sieve be run before the sieve to determine method of compaction.

Additionally, there is one other thing to consider when it comes to ASTM D1557 (this is more of a statement/question).  When the laboratory is classifying which method to use (there are three different methods based on ASTM D1557), it is important to note that this standard cannot be used when the amount retained on the 3/4" material is greater than 30%.  However, it states in the standard that there is no ASTM that is available to determine maximum dry density of this material.

Now the question:  What do you guys do when encountered with this?  I usually just tell the contractor to find a new material with not as much "large" material in it as, from my experience, something with this much "large" gravel would not really compact well anyways.

"...something with this much "large" gravel would not really compact well anyways" Not always true. Depends on the contractor and his equipment. Experience counts.

As I previously stated "..., from my experience,..." then your quote.

Alternatively, what would you do then to determine a max. density?  Use D1557 even though it's not the correct standard to use?

Probably many still do it the old fashioned way - a hard drive of a steel toed boot!

BigH,

That caused me to laugh.  I actually saw an older engineer do exactly that about 9 years ago when I first started off as a tester, bang the ground with the heel of his boot and say "yep, good enough to place the footing" and walk away.  Thanks for the chuckle.

Try AASHTO T-180 - this has been used with up to 50% plus 3/4 inch material.

If there is more than this we typically work up a method specification!

AASHTO T 180 has also limited the amount of +3/4" material to 30%.  If you read through the ASTM it references performing a method spec.  Also note that they indicate you should use engineering judgment if the material is gap graded (technically still only applies if less than 30% +3/4").

The heel of your boot and a 1/2-in steel T-probe are actually really good screening tools to figure out whether additional nuc gauge testing is needed, and where.

You can cover a lot of ground a lot faster.  Of course, techs and contractors usually want numbers that say "pass" or "fail" according to a certain criteria (i.e. the Proctor results), so no one has to make a judgement call in the field that might come back to bite them later.