PE Civil · soil-state application
Moist density, dry density and relative compaction: an SI worked example
Convert field moist density to dry density before comparing it with the laboratory maximum. Work the calculation and check a changed water content.
Reviewed for the current ReasonSpan™ exam profile · Updated October 11, 2026By the ReasonSpan™ editorial team · Our content standards
Keep the field and laboratory quantities on the same basis
A field test reports moist density of 2,128 kg/m³ and gravimetric water content of 12%. The applicable laboratory maximum dry density is 2,000 kg/m³, and the project requires at least 95% relative compaction. Assume the field and laboratory tests represent the same material and applicable compaction procedure. Does this location meet the stated density criterion?
Water content is water mass divided by dry soil mass. Moist density includes the water, so divide by 1 + w, using 0.12 rather than 12. Dry density is 2,128/1.12 = 1,900 kg/m³. Relative compaction is 1,900/2,000 = 95.0%. The stated minimum is met exactly; this is not evidence about every location, moisture-window compliance or testing uncertainty.
Diagnose the attractive mistakes
| Result or method | Why it fails |
|---|---|
| 106.4% from 2,128/2,000 | Compares moist field density with dry laboratory density. |
| 1,872.6 kg/m³ from multiplying by 0.88 | Removing 12% of total moist mass uses the wrong water-content denominator. |
| 95% proves complete acceptance | The exercise checks one density criterion at one location; material, moisture, test method and sampling requirements remain. |
Change the water content
Keep moist density at 2,128 kg/m³ but change gravimetric water content to 16%. Before reading the result, decide whether dry density increases or decreases.
It decreases: 2,128/1.16 = 1,834.48 kg/m³, or 91.72% of the laboratory maximum. More of the same moist mass is water, leaving less dry soil mass. Do not apply a generic moisture adjustment instead of using the measured water content.
These are original instructional givens, separate from timed mocks. State assumptions before calculating and check the source edition required for your exam or project.
Sources and further reading
Exam requirements can change. Confirm current details with the organization that administers your exam and use the cited sources for the preparation principles summarized here.