What formula is used to determine percent maximum density?

• A. (Wet density / maximum density) × 100
• B. (Calculated dry density / maximum density from Proctor) × 100, rounded to the nearest whole percent
• C. (Maximum density / dry density) × 100
• D. (Calculated dry density × maximum density) / 100

Answer: (B)

Percent maximum density shows how close the in-place soil is to its Proctor maximum dry density. The key is to work with dry densities, not wet densities, because moisture can mask how compacted the soil truly is.

First, determine the dry density from field data. If you have the field wet density and the moisture content, dry density = wet density divided by (1 plus the moisture content). This gives the actual soil density without the influence of water.

Next, use the maximum dry density obtained from the Proctor test for that soil. This is the reference value representing the best achievable compaction under the test conditions.

Then compute percent maximum density by taking the ratio of the calculated field dry density to the Proctor maximum dry density, and multiply by 100. Finally, round to the nearest whole percent. For example, if the field dry density is 110 pcf and the Proctor maximum is 120 pcf, you get (110 / 120) × 100 = 91.7%, which rounds to 92%.

Using wet density would mix in moisture content and isn’t the standard reference for compaction effectiveness. Taking the inverse ratio or multiplying the densities and dividing by 100 would not yield a meaningful percentage of compaction.