Compaction testing is a critical quality control process in earthworks and construction that verifies soil and fill materials have been compacted to the required density and strength.
In Australia, compaction testing follows AS 3798-2007 — Guidelines on Earthworks for Commercial and Residential Developments.
What Is Compaction Testing?
Compaction testing measures the density of compacted soil or fill and compares it to the maximum dry density achievable for that material (determined through standard or modified Proctor testing). The result is expressed as a percentage of maximum dry density — typically requiring 95% to 98% depending on the application.
Level 1 vs Level 2 Compaction
Level 1 Compaction (Supervised)
Level 1 compaction provides the highest level of quality assurance. It involves:
- Full-time supervision of the filling and compaction process
- Continuous testing of imported or site-won material
- Verification that every fill layer meets density requirements before the next layer is placed
- Detailed documentation of all materials, placement methods, and test results
- Highest assurance of compliance with AS 3798
When Level 1 is required:
- Structural fill beneath buildings and footings
- Road embankments and pavement subgrades
- Retaining wall backfill for critical structures
- Engineered fill for commercial and industrial developments
- Fill on steep slopes or unstable ground
Level 2 Compaction (Periodic)
Level 2 compaction involves periodic inspections and testing only. The fill layer is typically compacted before the technician arrives and is then tested to confirm it meets the required standard.
- Periodic attendance for spot testing
- Lower cost but lower assurance
- Relies on the contractor's quality control processes
- Suitable for less critical applications
When Level 2 is acceptable:
- General site filling and landscaping
- Non-structural fill
- Low-risk residential sites with proven soil conditions
- Temporary access roads and working platforms
The Compaction Testing Process
1. Material Classification
Before compaction begins, the fill material is tested to determine:
- Particle size distribution (AS 1289.3.6.1)
- Atterberg limits (plasticity index)
- Maximum dry density and optimum moisture content (Standard or Modified Proctor, AS 1289.5.1.1 or AS 1289.5.2.1)
2. Field Density Testing
Field density is measured using one of these methods:
| Method | Standard | Application |
|---|---|---|
| Nuclear Density Gauge (NDG) | AS 1289.5.8.1 | Fast, non-destructive, most common method |
| Sand Replacement | AS 1289.5.3.1 | Direct measurement, suitable for all soil types |
| Rubber Balloon | AS 1289.5.4.1 | Cohesive soils only |
| Core Cutter | AS 1289.5.1.1 | Fine-grained cohesive soils |
3. Moisture Content Determination
The in-situ moisture content is measured and compared to the optimum moisture content. Material that is too wet or too dry will not compact properly.
4. Density Ratio Calculation
$$ \text{Density Ratio} = \frac{\text{Field Dry Density}}{\text{Maximum Dry Density}} \times 100\% $$5. Reporting
A compaction test report includes:
- Test method used
- Location and depth of each test
- Field density and moisture content
- Maximum dry density and optimum moisture content
- Density ratio (percentage compaction)
- Pass/fail determination against specification requirements
Australian Standards for Compaction
| Standard | Title |
|---|---|
| AS 3798-2007 | Guidelines on Earthworks for Commercial and Residential Developments |
| AS 1289.5.1.1 | Determination of the dry density/moisture content relation of a soil using standard compactive effort (Standard Proctor) |
| AS 1289.5.2.1 | Determination of the dry density/moisture content relation of a soil using modified compactive effort (Modified Proctor) |
| AS 1289.5.3.1 | Determination of the field dry density of a soil — Sand replacement method |
| AS 1289.5.4.1 | Determination of the field dry density of a soil — Rubber balloon method |
| AS 1289.5.8.1 | Determination of field density and field moisture content of a soil using a nuclear surface moisture-density gauge |
| AS 1289.5.7.1 | Determination of the field moisture content of a soil |
Typical Compaction Requirements
| Application | Minimum Density Ratio | Test Standard |
|---|---|---|
| Structural fill (buildings) | 95% Modified Proctor | AS 3798 |
| Road subgrade | 95% Modified Proctor | Austroads |
| Road base / sub-base | 98% Modified Proctor | Austroads |
| Pavement subgrade | 95–98% Standard Proctor | Varies by spec |
| Residential slab fill | 95% Modified Proctor | AS 2870 |
| Pipe trench backfill | 90–95% Standard Proctor | Varies by spec |
| Landfill / waste containment | 95% Modified Proctor | EPA guidelines |
Common Problems and Solutions
| Problem | Cause | Solution |
|---|---|---|
| Low density | Moisture content too high or low | Adjust water content, re-compact |
| Soft / spongy layer | Over-compaction or saturated soil | Allow to dry, scarify and re-compact |
| Segregation | Poor material grading | Mix material, re-process |
| Varying results | Non-uniform fill material | Zone the fill, test each type separately |
| Failing compaction | Insufficient roller passes | Increase passes or adjust lift thickness |