Relative Compaction is a measure used in geotechnical and civil engineering (soil mechanics) to express how well a soil has been compacted in the field compared to a reference maximum dry density obtained in the laboratory.
Formula
\[ RC = \frac{\gamma_{d,\text{field}}}{\gamma_{d,\text{max}}} \times 100\% \]Or equivalently using dry density (\(\rho_d\)):
\[ RC = \frac{\rho_{d,\text{field}}}{\rho_{d,\text{max(lab)}}} \times 100\% \]Where
| Symbol | Description | Typical Unit |
|---|---|---|
| \(RC\) | Relative compaction | % |
| \(\gamma_{d,\text{field}}\) (or \(\rho_{d,\text{field}}\)) | Dry unit weight (or dry density) achieved in the field | kN/m³ (or kg/m³) |
| \(\gamma_{d,\text{max}}\) (or \(\rho_{d,\text{max}}\)) | Maximum dry unit weight (or dry density) from lab test (e.g., Standard/Modified Proctor) | kN/m³ (or kg/m³) |
Key Notes
- The maximum dry density is obtained from a laboratory compaction test such as the Standard Proctor or Modified Proctor test.
- Field dry density is commonly measured using methods like the sand cone, nuclear density gauge, or rubber balloon method.
- Typical compaction specifications require RC ≥ 90–95% for embankments and RC ≥ 95–100% for critical structural fills and pavement subgrades.
Relationship to Relative Density
For granular (cohesionless) soils, Relative Compaction can be correlated with Relative Density (\(D_r\)) by an empirical relationship (Lee & Singh, 1971):
\[ RC = 80 + 0.2 \, D_r \]where \(D_r\) is expressed as a percentage.