How to calculate soil compaction percentage
Percent compaction compares the density achieved in the field with the maximum dry density from a laboratory Proctor test. Divide the field wet density by one plus the moisture content as a decimal to get field dry density, then divide that by the laboratory maximum. A wet density of 125 pcf at 12% moisture gives a dry density of 111.6 pcf, which is 93.0% of a 120 pcf maximum.
Which Proctor test matters as much as the number itself. ASTM D698 (Standard Proctor) uses lower compactive effort and suits embankments and general fill, while ASTM D1557 (Modified Proctor) uses higher effort and is typical for pavement subgrade, base course and structural fill. The same soil returns a higher maximum dry density under D1557, so 95% of Modified is a considerably denser requirement than 95% of Standard.
Moisture control drives the result. Wet of optimum the roller pumps the lift instead of densifying it; dry of optimum the particles will not reorient. Most specifications hold field moisture within about two percentage points of optimum. Field density is measured by sand cone (ASTM D1556) or nuclear gauge (ASTM D6938), and lift thickness and roller passes need to be controlled alongside the test results. Treat the assessment here as a reference and let the geotechnical engineer of record make the final acceptance call.
Frequently Asked Questions
Divide the field dry density by the maximum dry density from the laboratory Proctor test and multiply by 100. Field dry density is the wet density divided by one plus the moisture content expressed as a decimal.
ASTM D698 (Standard Proctor) applies lower compactive effort and is common for embankments and general fill, while ASTM D1557 (Modified Proctor) uses higher effort and is typical for pavement subgrade, base and structural fill. The same soil yields a higher maximum dry density under D1557, so always state which test the percentage refers to.
Wet of optimum the roller pumps and the lift will not densify, so let it dry before rolling again. Dry of optimum the particles will not reorient, so add water and remix. Most specifications keep field moisture within about two percentage points of optimum.