On-site density & compaction testing

How is density measured on site?

A known volume of the compacted layer is removed, the material taken out is weighed, and the volume of the hole is measured, which gives the bulk density in place. A moisture content is then determined so the result can be reported as a dry density and compared with the laboratory reference. Sand replacement and core cutter methods are described among the in-situ tests in BS 1377-9; non-nuclear equipment measures indirectly and is correlated back to one of them.

How does the sand replacement method work?

A hole is excavated in the compacted layer, the material removed is weighed and sampled for moisture content, and the volume of the hole is then measured by running calibrated sand into it from a pouring cylinder. Two sizes are used: a small pouring cylinder for finer material and a large pouring cylinder for coarser layers. It is the method most specifications treat as the reference, and it works on most granular and cohesive layers.

When is the core cutter method used instead?

The core cutter suits cohesive soils that are free of stone and firm enough to hold their shape. A steel tube of known volume is driven into the layer and recovered full, so the density follows directly from the weight of material in a known volume, which makes it quick and repeatable. It cannot be used in granular material, in stony ground, or where driving the tube would disturb the layer being tested.

Do you use a nuclear density gauge?

The field methods we normally use are sand replacement, the core cutter and suitable non-nuclear equipment. A nuclear gauge is a recognised and fast method, but it contains a radioactive source, which brings licensing, transport, storage and site permission obligations, and a good many clients and sites will not accept one. If your specification names a nuclear gauge specifically, tell us and we will discuss how the requirement can be met.

How accurate is non-nuclear electromagnetic equipment?

It can be reliable, but only once it has been correlated to a direct method on the actual material being tested. Electromagnetic gauges, usually referenced to ASTM D7830, infer density and water content from the material's electrical response, so readings depend on soil type, mineralogy and good contact with a level surface. Normal practice is to calibrate against sand replacement results on that material, then use the gauge for the bulk of the readings.

How long does each density test take, and how many can you do in a visit?

A sand replacement test usually takes around half an hour at each position, a core cutter rather less, and a correlated non-nuclear reading only a few minutes once the correlation is established. How many positions fit into a visit depends on the travel between them, the state of the layer and whether moisture contents are being dried on site or at the laboratory. We agree the expected number of positions when the visit is booked.

How are test positions chosen across a layer?

Positions should represent the whole layer, not the parts that are easiest to reach. The number comes from your specification or testing schedule, and the locations are normally spread across the area and deliberately include where compaction is hardest to achieve: edges, the ends of runs, around structures and drainage, and anywhere the roller has had to work in confined space. Random or stratified positioning is often specified to avoid selective testing.

What needs to be ready on site before a density testing visit?

The layer needs to be fully compacted, trimmed to level and clear of plant, with a safe working area around each position. Testing a layer that is still being worked, standing in water or already covered by the next lift wastes the visit. Let us know the access route to each position, because equipment and calibrated sand have to be carried in, and tell us if any position sits in a trafficked area.

Does the test leave a hole, and does it need making good?

Yes, a hole is left and it has to be reinstated. Sand replacement and core cutter tests each remove a small volume of the layer, typically around 150 mm deep and deeper again for coarse material, and the position is backfilled and compacted afterwards. On a finished or trafficked surface it is worth agreeing beforehand who reinstates and to what standard, since reinstating through a bound surface is a different job from filling a hole in soil.

Which materials are difficult to measure density in?

Coarse granular fill, rockfill and very stony ground are the hardest, because a single large stone in or beside the test hole distorts the result and the hole has to be big enough for the material to be represented. Very soft or wet cohesive material can collapse into the hole, and a thin layer over a hard base can be influenced by what lies beneath it. Where a layer is genuinely untestable by direct measurement, method compaction records are often the better route.

Are the field density methods on your UKAS schedule?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope, and on-site work and laboratory work can carry different entries, so the field density method and the moisture content determination are considered separately. Ask us to confirm whether the methods your specification names are covered before the visit is booked.

What should we send you before you price a density testing visit?

Send the site postcode, the specification or testing schedule, the material being tested, the number of positions and layers, and your preferred dates. Tell us whether a laboratory maximum dry density already exists for the material, because field results cannot be assessed without one. Access, ground conditions and any need to work outside normal hours or under traffic management also affect the visit, so mention those as well.