Quality & Inspections · Mandatory · Construction
Compaction and Density Test Record
A record of compaction and density testing for fill materials, sub-bases, and road layers.
Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. BS Standards remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.
| Legal basis | BS Standards + Specification for Highways + Contract Spec |
| Why compact | Inadequately compacted fill creates differential settlement — cracks slabs, fails roads |
| Test methods | Nuclear density gauge (NDG) rapid non-destructive; Sand replacement (BS 1377 Part 9) accurate but slow; Dynamic Cone Penetrometer (DCP) measures strength/stiffness |
| Specification | Typically ≥95% MDD (Modified Proctor) |
| Frequency | Typically 1 test per 250m³ of compacted fill |
1. Compaction Testing — Verifying What Can't Be Seen
Compaction is one of the most invisible quality issues in construction. Fill that looks compact at the surface may be inadequately compacted at depth, and the consequences — differential settlement, cracked slabs, failed pavements — may not appear for months or years after construction. Compaction testing is the only reliable way to confirm that the required density has been achieved.
The compaction test record links each test result to a specific location on the site, the material being compacted, the test method used, and the laboratory reference values. Without a test location plan showing where each test was carried out, results cannot be related to specific areas of fill and are of limited value for demonstrating compliance.
Location is critical
Always link compaction test results to a plan showing test locations — results without location references are almost useless. Each test must be identifiable on a site plan so that areas of non-compliance can be located and remediated.
2. Compaction / Density Test Record — Content
The test record must capture every data point needed to demonstrate that the compacted fill meets the specified density requirement. Each row represents a single test at a specific location, and the columns track the material, method, result, and any remedial actions taken.
| Column | Description |
|---|---|
| Project / element | The project name and the specific area or element (e.g. road subbase, foundation backfill, platform fill) |
| Date and time | Date and time the test was carried out |
| Test location | Precise location referenced to a site plan or grid coordinate (e.g. chainage 150m, 2m from kerb line) |
| Material | Description of the fill material being tested (e.g. Type 1, 6F2, imported granular fill, cohesive fill) |
| Test method | Method used — Nuclear Density Gauge (NDG), Sand Replacement, Dynamic Cone Penetrometer (DCP) |
| Equipment | Equipment identification and calibration status (NDG serial number, calibration certificate date) |
| In-situ dry density | Measured dry density of the compacted fill in kg/m³ or Mg/m³ |
| In-situ moisture | Measured moisture content of the fill as a percentage |
| MDD (lab result) | Maximum Dry Density from the laboratory Proctor test for this material in kg/m³ or Mg/m³ |
| % MDD achieved | In-situ dry density expressed as a percentage of the laboratory MDD |
| Result | Pass or fail against the specified percentage MDD requirement (e.g. ≥95% MDD) |
| Remedial action | Details of any remedial action required for failed tests (re-compact, additional passes, moisture adjustment) |
| Re-test result | Result of re-test after remedial action, confirming compliance has been achieved |
| Tested by | Name and qualification of the person who carried out the test |
3. Common Mistakes
Testing only at the surface of a thick layer
NDG tests typically measure density in the top 150-200mm. If fill has been placed in thick layers (300mm+), the surface may be well compacted but the lower portion may be inadequately compacted. Layers must be placed and compacted within the specified maximum thickness, and testing must be representative of the full layer depth.
Not calibrating the NDG for the specific material
Nuclear density gauges must be calibrated for the specific material being tested. A gauge calibrated for granular fill will give inaccurate readings on cohesive fill. Calibration should be verified against sand replacement tests at the start of each new material type.
Not conducting Proctor (MDD) testing before compaction testing
The percentage MDD result requires a laboratory MDD value for the specific material being compacted. If the Proctor test has not been carried out before compaction testing begins, there is no reference value against which to assess compliance. The MDD must be determined for each distinct material type used on the project.
No test location plan
Compaction test results must be plotted on a site plan showing the location of each test. Without a location plan, it is impossible to identify which areas of the site have been tested, which have not, and which areas failed and were remediated. The plan is essential for demonstrating adequate coverage.
Not retesting after failed areas have been recompacted
When a compaction test fails, the area must be remediated (typically by additional compaction passes or moisture adjustment) and then retested. The retest result must be recorded in the test record to demonstrate that the area now achieves the specified density. An unremediated failure is a non-conformance.
4. Frequently Asked Questions
What does 95% MDD mean?▾
It means the in-situ dry density of the compacted fill must be at least 95% of the Maximum Dry Density determined in the laboratory Proctor test for that material. For example, if the laboratory MDD is 2,000 kg/m³, the in-situ density must be at least 1,900 kg/m³ to pass.
NDG vs sand replacement — which is better?▾
NDG is faster and allows many more tests per day, making it more practical for routine site testing. However, it requires operator licensing (due to the radioactive source) and must be calibrated for each material type. Sand replacement is more accurate for some materials, particularly cohesive soils, but is slow (one test per hour). Many specifications accept NDG for routine testing with sand replacement used for calibration and verification.
What is the DCP test?▾
The Dynamic Cone Penetrometer test measures the resistance of the ground to a driven cone. The rate of penetration (mm per blow) correlates with California Bearing Ratio (CBR), which is a measure of the bearing capacity and stiffness of the compacted fill. It is used to assess the structural adequacy of compacted layers, particularly for road subbase and capping.
What frequency is required for road subbase testing?▾
Typically 1 test per 250-500m² of compacted subbase, depending on the specification and the criticality of the application. The Specification for Highway Works and the contract specification will define the required frequency. Testing should be distributed to cover the full area and should include areas adjacent to edges and service trenches where compaction may be difficult.
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