Subgrade Optimum Moisture Content

[cveg05]

I'm overseeing the construction of a road which has required a 2' undercut of the in situ soil. The 2' undercut is being replaced with engineered fill which is a gravelly clay and is being placed in 8" lifts.
The proctor that was performed on this engineered fill indicated that optimum moisture content is ~24%. My crews have been watering the material as they roll it to keep close to this OMC. When we had a nuclear density guage test performed the compaction was adequate at just over 99%, however the moisture content was only 11%. My spec states that the moisture content must be much closer to the OMC.
My question is, if you get the proper compaction in place, how important is it that the OMC is reached?

Thanks!

 

[Ron]

Not important. I suspect also that your Proctor is not correct. This is evidenced by your results of 99% at 11% moisture, when optimum is 24%(?). I am also assuming this is a standard Proctor, not a modified Proctor. Typically with clayey materials, compaction has to be done at slightly over optimum, while the material is drying back toward optimum to get the most efficiency.

 

[fattdad]

I'd agree. You're not referencing the correct Proctor value. Take a bulk sample from the test location, return it to the lab and run a Proctor point. See if the point falls on the lab curve that you are referencing. This has to be done "dry" of optimum, so make sure it's not too wet (i.e., a ball formed in your hand is crumbly).

f-d

¡papá gordo ain’t no madre flaca!

 

[AussieGeoEng]

I would tend to disagree - moisture content is important. If the test was completed on different samples, then that is another story.

A relative compaction of 99% can be achieved dry of optimum - it would however require significantly more effort. I.e similar to the shift in the compaction curves from standard to modified.

There are documented case studies where "collapse" or "hydrocompression" or similar name has occured where earthfill was placed with little control (either relative compaction or moisture range). E.g. BRANDON, T. L., DUNCAN, J. M. & GARDNER, T. N. 1990. Hydrocompression Settlement of Deep Fills. Journal of Geotechnical Engineering, 116, 1536-1548.

A high relative compaction for a (reactive) clayey soil dry of optimum may induce more problems than it solves. With a higher compaction a higher density is achieved which results in higher clay fractions in a unit volume and will result in swell.

In saying all the above though, an assessment for your project would need to be made with more details of the testing results as suggested by f-d.

An interesting presentation can be found at:

http://www.youtube.com/watch?v=3K6TJdoQ4yQ

 

[TigerGuy]

cveg,

you describe the material as gravelly clay. How much gravel, and what is the top size? It sounds to me like the lab sieved out the gravel and ran the proctor on the clay portion only. Then at the project site, the rock is driving your moisture down and density up, giving a false passing test.

I deal with contractors that believe the solution to high moisture and low density is to blend rock into the fill, so the test results pass.

If the lab is running the proctor correctly, how are you verifying a consistent amount of rock throughout the fill?

 

[IronTriangle]

Is geo grid being used at all on this particular job?