Effective Compaction Depth

General request for opinions on the following:

I work for a local geotech firm that services areas in the upper to lower coastal plains of SC. We typically run into sites that require the upper XX amount of cleaner sands (SP to SP-SM/C) to be “densified in-place”. My general thought on this is that this is really only applicable for sites that have soils below or near optimum especially for soils dirtier than 12% fines.

Am I crazy to think that a contractor can essentially moisture condition (flood clean sands) areas that we need to consolidate and vibratory roll in soils with 26 ton smooth drum then back check the “densification” process with a DCP and proofrolling? For an example of 3 feet below grade, is it a pipe dream to think I will get compactive effort at this depth? Subgrade soils are predominantly a medium SP-SM below optimum supporting very light loads.


One of my first jobs that i did all the field tech work on used a processed fill that was a clean SP. The surface always jostled around during compaction effort even with water but the lower regions were always confined and compacting. After a few failed tests and scratching our heads, we bladed in and tested the lift below. Found a whole lot more compaction. Finished the whole job testing the 2nd to last lift all the way up. We had a smooth drum, a water truck, and a sheepsfoot. The lift would get spread, then the sheepsfoot ride-on would hit it, then the water truck would soak it, then the smooth drum would finish before next lift. I doubt the smooth drum was even really necessary, but it was there and the guy was working.

I wouldn’t make a practice of using a DCP to check for compaction on new projects. It is too easy to pin the blame if something bad happens and project specs rarely recognize the DCP test. The DCP is more of a tool when you’re doing forensic work when the test has no effect on the quality of the work, or the DCP can be used to supplement the compaction testing program.

[Conventional wisdom is smooth drum for sands and gravels and is certainly true for well graded materials. In the case of that SP the feet divots did help with allowing the uniform dispersal of moisture deep into the lift and having a drivable surface for the water truck to quickly deliver the water to it.]

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