Compaction level reduces with moisture content?

[EireChch]

Hi all,

I am working on a project where there is a debate as to how the material was QA/QC'd to 90% compaction (standard proctor), however after settlement of some infrastructural, the compaction level is ~80%. The material is well graded sand with trace fine gravel.

One theory being floated around is that the material has a very bell shaped OMC/MDD curve. Someone has proposed that the material was compacted to 90% however as there were rainfall events / drying events, the moisture content has changed. As the moisture content changes, the density also changes. Refer to below sketch, we have an OMC/MDD line similar to line 1 which is sensitive to changes in MC.

I dont buy it to be honest. I argued that the purpose to the OMC/MDD curve is on moisture content during construction, ideally you are at OMC +/-2% (or whatever your spec says), if you are too wet or too dry you will likely find it difficult to achieve 90%MDD.

Infrastructure is supported on granular material and doesn't routinely settle due to changes in MC, does it?! Yes if there is changes to water level as a reduction in groundwater level can increase in effictive stress in soil causing additional settlement, but not if the moisture content changes from 14% to 8% after placement?

 

[EireChch]

Thanks Ron / BighH.

Your point re the upper 50mm being loose / not well compacted is a good one. We see this all the time while doing DCP testing. No confinement in the upper 100mm at least so comes back as failing your target blow count. I just apply the rule that the average blow from 150-250 is similar to what the upper 100mm would be when confined. Then recommend a last proof roll before reinforcement is placed and hope there is minimal disturbance after that.

Ron - Its funny because this comment came in a meeting from another consultant with 20+ years experience. I queried it and said that once placed there should be not be any change in dry density but they were adamant, so i didnt push much further in the meeting.

I have since provided a simple example with calculations and the response was " you are discussing theory, I am discussing what happens in the real world"....I guess some people wont listen