In-Situ Density Testing
Before a compacted layer gets signed off and covered, you need proof it meets the density your specification requires. In-situ density testing measures the achieved compaction of soil, fill, or subbase directly on site, so you can confirm the layer is ready to move on from before work continues. Site Testing Services (National) Ltd carries out density testing across England and Wales, using the method that suits your site.
What is in-situ density testing?
In-situ density testing measures the dry density of a compacted layer as it stands on site, and expresses it as a percentage of the maximum dry density (MDD) established for that material in the laboratory. This percentage compaction figure is what your specification is actually checking against, since inadequate compaction leads to settlement, rutting, and water finding its way into layers it should not reach.
This is different from the laboratory dry density/moisture content relationship test, which establishes the target MDD and optimum moisture content (OMC) for a material before construction begins. Our Laboratory Testing service covers that step. In-situ density testing is the field check that confirms the compacted layer has actually reached that target.
Sand Replacement Method vs Nuclear Density Gauge
Two methods cover almost all in-situ density testing on UK sites, and each has a genuine place depending on your project.
Sand Replacement Method
The sand replacement method is the reference method described in BS 1377-9:1990. We excavate a small hole in the compacted layer, weigh the material removed, then fill the hole with sand of a known bulk density to calculate its volume. The excavated soil is retained for laboratory moisture content testing, which allows dry density to be calculated accurately.
It is a manual, slightly slower process than a gauge reading, but it needs no specialist licensing and is widely accepted as the benchmark method against which other techniques are checked.
Nuclear Density Gauge
A nuclear density gauge gives an immediate, non-destructive reading by measuring the attenuation of gamma radiation through the compacted material. Results are available in moments, without excavating the layer, which makes the gauge method faster on site.
Because the gauge contains a radioactive source, its use is governed by the Ionising Radiations Regulations 2017 (IRR17). A Radiation Protection Adviser must be involved, the source needs to be registered, and specific procedures govern how the gauge is stored, transported, and used on site, all of which is worth weighing against the equipment cost if you are deciding between running density testing in-house or bringing in a specialist contractor.
Neither method is universally “better.” Sand replacement remains the reference method and needs no licensing, while the nuclear gauge trades a licensing and handling overhead for speed and a non-destructive result. We can advise which suits your site, timescale, and specification.
| Method | Standard | Speed | Site requirements |
| Sand Replacement | BS 1377-9:1990 | Slower, manual excavation, lab moisture content follow-up | No specialist licensing |
| Nuclear Density Gauge | IRR17-regulated equipment | Fast, immediate on-site reading | RPA arrangements, source registration, transport/storage procedures |
How Density Testing Relates to Your Compaction Spec
Your specification will typically require the field-measured dry density to reach a set percentage of the maximum dry density established in the lab for that material. The precise figure varies by project, material, and specifying body, so it is worth confirming the exact threshold in your contract documents before testing begins. Our team can talk through what your specification requires and recommend the testing approach and frequency to match it.
Density Testing Procedure
Site Preparation
We identify a representative, undisturbed area of the compacted layer for testing, avoiding disturbed edges or areas affected by recent plant movement.
Testing
For sand replacement, we excavate the test hole, retain the material, and backfill with calibrated sand to determine volume. For the nuclear gauge method, we position the gauge on the prepared surface and take a reading in backscatter mode, or in direct transmission mode where a probe is inserted into the material, depending on which suits the layer being tested.
Data & Reporting
Results are calculated as dry density and expressed as a percentage of the target MDD, so you can see immediately whether the layer meets your specification. Reports are typically available the same day.
Applications
- Compaction sign-off for road and pavement subbase and capping layers
- Verification of filled ground before construction proceeds
- Car park and hardstanding construction quality control
- Reinstatement checks on utility trenches
- General earthworks quality control across a site
Why Choose STS for Density Testing?
We offer both sand replacement and nuclear gauge density testing, so you get the method that fits your site rather than whichever one happens to be available. Our UKAS-accredited laboratory supports the moisture content and MDD/OMC testing that field results are measured against, and our technicians can advise on which method and testing frequency suits your specification. Where your project also needs CBR or plate bearing data, we can coordinate all three tests on the same site visit.
Frequently Asked Questions
What is in-situ density testing? It is a field test that measures the achieved dry density of a compacted soil, fill, or subbase layer on site, to confirm it meets the compaction level required by your specification.
Sand replacement or nuclear gauge, which do I need? Sand replacement needs no specialist licensing and is the reference method under BS 1377-9. A nuclear gauge gives a faster, non-destructive reading but requires the operator to hold Radiation Protection Adviser arrangements and source registration under IRR17. Either can be right depending on your site and timescale.
How does field density relate to the lab MDD test? The laboratory dry density/moisture content relationship test establishes the maximum dry density and optimum moisture content for your material. Field density testing measures what has actually been achieved on site and expresses it as a percentage of that target.
How quickly are results available? Nuclear gauge readings are available immediately on site. Sand replacement results are usually available the same day, once the retained sample’s moisture content has been determined.
Is nuclear gauge testing safe and regulated? Yes. It is carried out under the Ionising Radiations Regulations 2017, which govern source registration, handling, transport, and the involvement of a Radiation Protection Adviser.
What standards do you test to? Our sand replacement testing follows BS 1377-9:1990, and our laboratory MDD/OMC testing that field results are compared against is UKAS-accredited to BS 1377 and ISO standards.
Get in touch
Call Site Testing Services (National) Ltd on 0330 6339 988 to discuss your compaction testing requirements and get a free quote, anywhere in England and Wales.
