I. Core Principles of Compaction Degree Testing
Subgrade Compaction Degree = (Measured Dry Density on Site / Maximum Standard Dry Density in Indoor Testing) × 100%
Only when the compaction degree meets the design specifications (e.g., ≥95% for highway subgrades) can it be considered "effectively compacted."
This indicator directly reflects the density of the soil and is one of the mandatory control indicators for engineering quality acceptance.
II. Commonly Used Compaction Degree Testing Methods and Applicable Scenarios
1. Sand Cone Method
Principle: The dry density is calculated by replacing the volume of the test pit with standard sand and combining this with the mass of the excavated soil sample.
Advantages: Accurate results, widely used for on-site testing of various soil types.
Applicable Conditions: Suitable for fine-grained soils, sandy soils, and gravelly soils, especially suitable for routine on-site testing.
Note: The appropriate size of the sand cone (e.g., Φ100mm or Φ150mm) must be selected according to the maximum aggregate size.
2. Ring Sampling Method
Principle: A ring sample is cut to obtain an undisturbed soil sample, its volume and mass are measured, and wet and dry densities are calculated.
Advantages: Simple operation, low equipment cost.
Limitations: Only applicable to fine-grained soils without aggregates, and is a destructive test.
3. Nuclear Density Analyzer Method
Principle: Utilizes gamma rays or neutron beams to penetrate the soil, rapidly measuring wet density and moisture content, and automatically converting to dry density.
Advantages: Non-destructive, fast testing speed (<60 seconds), suitable for large-area rapid sampling.
Requirements: Must be calibrated and compared with the sand cone method before use to ensure data accuracy.
4. Other Auxiliary Methods
Dynamic Cone Penetrator Method: Determines density by penetration depth, suitable for preliminary assessment.
Electromagnetic Wave Method: Non-contact detection, can be used for continuous monitoring, but requires subsequent calibration.
III. Key Steps for Determining "Compaction Achievement"
1. Determine the Design Compaction Standard
Expressways and Class I Highways: Subgrade compaction degree ≥ 95% (0-80cm)
Class II Highways: Compaction degree ≥ 93%
Adjustments may be made for particularly arid or excessively wet areas, but must be confirmed by the design team.
2. Layer-by-Layer Testing and Section-by-Section Acceptance
After each layer of filling (usually ≤ 30cm thick), compaction degree sampling should be conducted to ensure "layer-by-layer compliance."
3. Comprehensive Judgment Using Multiple Methods
The sand cone method or ring cutter method should be the primary control method, with a nuclear density meter used for rapid verification.
Supplemented by on-site observation: such as roller wheel track height difference < 3mm, no obvious surface undulations or spring-like phenomena.
4. Data Recording and Traceability
All testing points should have coordinate records to form a testing report, which serves as the basis for project acceptance.
