About Soil Compaction Calculator
The soil compaction calculator performs field compaction control by comparing the in-place dry unit weight of a compacted fill with the maximum dry unit weight measured in a laboratory Proctor test. It reports the relative compaction as a percentage and tells you immediately whether the fill meets the project target, such as 95 percent of the modified Proctor maximum.
Enter the field bulk (moist) unit weight, the field moisture content, the Proctor maximum dry density and optimum moisture content, the specific gravity of the soil solids, and your target relative compaction. The tool computes the field dry unit weight, relative compaction, degree of saturation, air voids, void ratio, porosity, and the zero-air-voids density at the field moisture.
How It Works
- Enter the field bulk unit weight (kN/m³) and the field moisture content w (%).
- The dry unit weight is found from gamma_d = gamma_bulk / (1 + w/100).
- Relative compaction is the field dry density divided by the Proctor maximum dry density, expressed as a percentage, and compared with your target.
- Using the specific gravity Gs and the unit weight of water, the tool also derives the void ratio, porosity, degree of saturation, air voids, and the zero-air-voids (fully saturated) density at the field moisture.
Worked Example
A field density test on a compacted fill gives a bulk unit weight of 20 kN/m³ at a moisture content of 10 percent. The dry unit weight is gamma_d = 20 / (1 + 0.10) = 18.18 kN/m³. The modified Proctor maximum dry density is 19 kN/m³, so the relative compaction is RC = 18.18 / 19 × 100 = 95.7 percent, which passes a 95 percent target. With Gs = 2.70 and gamma_w = 9.81 kN/m³ the void ratio is e = 2.70 × 9.81 / 18.18 − 1 = 0.457, the degree of saturation is S = 0.10 × 2.70 / 0.457 = 59.1 percent, and the air voids are about 12.8 percent.
Formulas
- Field dry unit weight
gamma_d = gamma_bulk / (1 + w/100)- Relative compaction
RC = (gamma_d_field / gamma_d_max) * 100- Zero-air-voids unit weight
gamma_zav = (Gs * gamma_w) / (1 + (w/100) * Gs)- Void ratio, saturation and air voids
e = Gs*gamma_w/gamma_d - 1 | S = (w/100)*Gs/e | air voids = n*(1 - S), n = e/(1+e)
Standards & References
- ASTM D698 Standard Proctor Compaction Test
- ASTM D1557 Modified Proctor Compaction Test
- ASTM D6938 Nuclear Density Gauge / Field Density Testing
Frequently Asked Questions
What is relative compaction and what target is typical?
Relative compaction is the ratio of the field dry unit weight to the laboratory Proctor maximum dry unit weight, expressed as a percentage. Earthwork specifications commonly require 90 to 95 percent of the modified Proctor maximum for structural fill, and 98 to 100 percent for critical applications such as pavement subgrade beneath roads.
Why convert bulk density to dry density before comparing?
The Proctor maximum dry density is defined on a dry-mass basis, so the field bulk (moist) unit weight must be converted to a dry unit weight using gamma_d = gamma_bulk / (1 + w/100) before computing relative compaction. Comparing a moist field density directly to a dry Proctor maximum would overstate the compaction achieved.
What is the zero-air-voids curve?
The zero-air-voids (ZAV) curve is the dry unit weight that would be reached if all air were removed at a given moisture content, that is full saturation. No compaction point can plot to the right of or above the ZAV line, so it provides an upper bound and a useful check on field and Proctor data.
Does compacting wet of optimum or dry of optimum matter?
Yes. Soil compacted dry of the optimum moisture content tends to be stiffer but more susceptible to swelling and collapse on wetting, while soil compacted wet of optimum is more ductile and less permeable but lower in strength. The calculator reports how far the field moisture is from the optimum so you can judge the field condition.