Dry Density vs. Wet Density

[summer05]

I need some help --- What is the difference between dry density and wet density in soil compaction? I have seen "dry density" used in Proctor Test results, and I also see people use "Dry Density" or "Wet Density" in nuclear Gage Testing, What are the differences and when and what should you specify for pipe trench and general earthwork? Thanks.

 

[MRM]

You should specify a minimum relative compaction value for pipe trench and general earthwork. Note that the dry density of the soil will be embedded in that relative compaction value (dry density as compacted divided by maximum dry density obtained during testing).

Dry density is used because it is esentially a "normalized" density value. That is, the moisture content and saturation variables are removed from the dry density value. How you arrive at that dry density (and percent compaction) does depend on moisture content however, which is where some specification for moisture limits should be considered based on your Proctor testing. Review the Proctor test for additional clarification on that.

When you see wet density used or reported in conjunction with nuclear gauge testing, it is shown probably for your information and is really sort of an intermediate step in arriving at the dry density value and ultimately, the percent compaction (or sometimes relative density) value that you've specified.

 

[BigH]

MRM does a nice job in explaining as an engineer. But, summer05, you seem to want the basics. The relationship between dry density and wet density is:

dry density = wet density divide by (1 + moisture content as a fraction).

e.g., Moisture content is 10%. Wet density is 110 pcf, then the dry density is 110/1.1 = 100 pcf.

 

[vollEngineer]

uh, right, so use percent compaction of DRY density in your spec.
As a CMT engineer in addition to geotech, I respectfully request you specify Modified Proctor (see ASTM D-1557) or Standard Proctor (ASTM D-698) and avoid the phone calls later