Subgrade Optimum Moisture Content
Subgrade Optimum Moisture Content
(OP)
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!
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!





RE: Subgrade Optimum Moisture Content
RE: Subgrade Optimum Moisture Content
f-d
¡papá gordo ain't no madre flaca!
RE: Subgrade Optimum Moisture Content
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
RE: Subgrade Optimum Moisture Content
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?
RE: Subgrade Optimum Moisture Content