How to Calculate Percent Compaction

Calculation Objective

Percent compaction compares field dry density with the maximum dry density from the applicable laboratory Proctor curve. It is also called relative compaction.

Wet density must first be converted to dry density. Moisture content is expressed as a decimal in the equation.

Example Inputs

• Field wet density = 124.0 pcf.
• Field moisture content = 10.0 percent.
• Laboratory maximum dry density = 118.0 pcf.
• Laboratory optimum moisture content = 11.0 percent.
• Required relative compaction = 95.0 percent.
• Allowed moisture deviation = plus or minus 2 percentage points from OMC.

Step 1: Convert Moisture to Decimal

Ten percent moisture is $10/100=0.10$. Using 10 directly in the denominator would be a major error.

Step 2: Calculate Field Dry Density

This is the density of dry soil solids per total field volume after removing the calculated contribution of water mass.

Step 3: Calculate Relative Compaction

The result is above the entered minimum of 95.0 percent.

Step 4: Calculate the Minimum Required Dry Density

This provides a useful direct check: field dry density of 112.73 pcf is greater than the required 112.10 pcf.

Step 5: Check Moisture

The entered acceptance band is OMC plus or minus 2 percentage points:

The field moisture content of 10.0 percent is inside that band. Based on the entered values, both density and moisture criteria pass.

What the Result Does Not Prove

A passing calculation does not prove that the entire lift is acceptable. Confirm that the field point is representative, the correct Proctor curve was used, the soil source has not changed, the test method is valid for the material, and all required corrections were applied.

The result also does not address lift thickness, proof-roll response, pumping, segregation, bonding between lifts, or unsuitable material outside the test point.

Metric Example

The same calculation works with kN/m3 or kg/m3 when field wet density and laboratory MDD use the same unit. If wet density is 19.5 kN/m3 at 12 percent moisture, field dry density is 17.41 kN/m3. If MDD is 18.2 kN/m3, relative compaction is 95.7 percent.

Common Mistakes

• Dividing field wet density directly by laboratory MDD.
• Entering 10 percent moisture as 10 instead of 0.10.
• Mixing pcf, kg/m3, and kN/m3 in one calculation.
• Using OMC as the denominator instead of MDD.
• Comparing with a Standard curve when Modified Proctor governs, or the reverse.
• Rounding intermediate values so aggressively that a borderline result changes status.

Frequently Asked Questions

Can percent compaction exceed 100 percent?

Yes. Review material match, curve validity, corrections, field effort, and test variability when it does.

Is relative compaction the same as relative density?

No. Relative compaction compares field dry density with Proctor MDD. Relative density is a different concept used primarily for granular soils and compares in-place void ratio or density with laboratory maximum and minimum index states.

How much precision should be reported?

Follow the test standard and project requirements. Avoid presenting more precision than the input measurements support.