Field Density Test Results Don't Match Observed Soil Behavior 1

[theCorkster]

Hopefully someone can help shed some light on a dilema I'm encountered on a fill construction problem.

We're involved as the geotechnical engineer on a project that involves construction of some minor fills (2 to 4 feet)for light weight, single-story structures. During construction the on-site laboratory for the owner was calculating failing relative compaction (RC) results. So with some side-by-side testing with the on-site laboratory we determined that soil moisture content corrections from oven-dried moistures (nuclear gauge indicating 2 to 3 percent higher than oven-drived); this is something we've seen with volcanic origin soils in the area. Using these corrections 90 percent relative compaction was achieved.

Further fill construction with a sandy clay with gravel/ clayey sand with gravel material excavated from about 3 to 5 feet in a detention basin has generated relative compaction ranging from about 82 to 89 percent RC. During this time our geologist has been on-site performing more side by side tests and monitoring the contractor's operations - soil 2 to 5 percent above optimum, loose lifts of 8 inches, and compaction with a Caterpillar 815 sheepsfoor compactor. Test pads cut 3 to 5 inches into the compacted fill cut through weathered gravel clasts and leaves a sheen on the surface where the clay content is greater.

With additional compaction, including loaded 635 scrapers and loaded water trucks, there is no visible deflection of the fill, nor is there an increase in the inplace density! Increasing the soil moisture content to 7 to 8 percent above optimum, followed by the same compaction process, yields similar numbers to those within 2 to 3 percentage points of optimum. Again, no deflection under fully loaded scrapers and water trucks.

Laboratory curves are being run by both labs and corrected for pluse 3/4 gravel when appropriate; regardless, the field compaction with a lot of effort falls in the 85 to 90 percent range. Both laboratories are generating curves and optimum moisture contents within inter-laboratory precisions (ASTM). And sand cone density tests in a small test pad compare nicely with the nuclear gauge densities.

I'm considering shifting to a method spec for compacting this material.

Any similar experience out there that might illuminate what I'm not seeing so I can get some sleep?

Thanks

 

[Mickney]

What are you filling over? Is it a soft, firm, stiff or very dense stratum?

Reason being, is that in the lab (at least our lab), we set our proctor molds on a 1 cubic foot block of concrete. This means we are compacting against a solid mass. This, however, may not be the case in the field, particularly if you are filling over a softer soil that does not provide the same resistance as the concrete block.

Just a thought as to why field values may not be lining up with lab values.

 

[WHEngineer]

This may seem like a silly question, but you would not believe how many times I've seen this problem. How long are you letting each proctor point set to equalize moisture content? With gravelly clay soils, I've seen it take several days to obtain a uniform moisture content. We actually just finished testifying in a case where another geotech firm was running quick proctors in the field on gravely clays - their optimum moistures were coming up 6 to 7 percent wet of optimum. They kept insisting that the contractor wet up the material even though the dry densities were good. Once wet to the geotechs optimum moisture, the soils would start deflecting. The contractor spent an extra $220k because they were wetting up soils that were actually running 95% compaction (ASTM D698-C.)

 

[theCorkster]

I've been away for my daughter's wedding and have finally been able to think about work again. Planning and executing a wedding is an excellent exercize - I'm glad all I had to do was be tasked and shooed on my way (over and over and over...)!


Oldestguy and dgillette;

Again, thanks. You guys get way outside the box!

I'm familiar with the USBR procedure. The procedure would have to be modified since it is based on D698 energy. However, this would address the gravel issue. I'll have to check with geolab in Lakewood to see what the cost might be.

Mickney;

Good thought. After stripping, we over-excavated from 1 to 2 feet to address loose surficial soil, rodent holes and root grubbing. We then proof rolled with subgrade with a loaded 653 scraper and when deflection was noted, we potholed to see what was causing it. In some locations we had wet clay derived from weathering of siltstone in the underlying formation and in some locations found porous soils. These areas were over-excavated (to as much as 5 feet beneath existing grade) and reconstructed with fill.

We've been able to achieve compaction with all site soils with the exception of a clayey sand with gravel material excavated beneath a depth of about 2 feet up to about 5 feet. In fact, because the site grade has been raised, the "problem child" has been placed on top of subgrade and/or fill that has exceeded 90 percent compaction. We've not had the compaction issue with other materials.

WHEngineer;

Compaction samples are processed in the lab at field moisture content over the 3/4, 3/8 and No. 4 screens. Moisture contents are run on the plus No. 4 and minus No. 4 materials for reference. Based on our experience, we either air dry back or add moisture to the recombined sample of plus and minus No. 4. Samples with clay cure for at least 16 hours; however, we've not tried curing for more than 16 hours primarily due to construction schedule and the contractor's needs.

In reflection I'd like to try the longer cure time to see if it does result in a change. With other volcanic soils with clay and silt derived from ash, I know that some moisture is merely absorbed to the surface of particles, and therefore these soils require significantly higher moisture contents (27 to 32 percent) to achieve compaction. However, with these soils in our area, these soils 1) do not require moisture conditioning for more than 16 hours and 2) generally reflect the lower dry density (90 to 90 pcf) and fall below the zero air voids.

More to follow as this continues to unfold...

 

[Aggie81]

To solve the problem with site compaction not corresponding with your lab curve, I would recommend going to

http://www.esol.net

web site and learn about site specific compaction energy. The web site offers a solution to having the correct site curve for a given sites lift thickness, soil property, and compactor combination. This software tool provides a solution to today's earthen fill construction industry in meeting compaction specifications.

 

[dgillette]

This looks a lot like advertising ESOL, Mr. Goldberg.

 

[Aggie81]

dgillette,

Thank you for your comment, I did not intend for my post to be an advertisement. The technology behind the software tool I mentioned appeared to have an application to the stated problem. I am new to this site and just happened to stumble across it while preforming another search. I was not aware of all the requirements for posting to the site, thank you again for bringing it to my attention.