Compaction testing & monitoring

What does compaction testing actually prove?

It shows how well material in the ground has been compacted, by comparing the dry density achieved on site with a reference density established in the laboratory for that same material. The comparison is normally expressed as a percentage of maximum dry density, sometimes called relative compaction. On its own a field density figure means very little; it only becomes an acceptance result once there is a reference to measure it against.

How is the laboratory reference density established?

A sample of the material is compacted in a mould at several different moisture contents, and dry density is plotted against moisture content to give a curve. The peak of that curve is the maximum dry density, and the moisture content at which it occurs is the optimum moisture content. For soils this is normally carried out to a compaction method in BS 1377-2, which absorbed the withdrawn Part 4; for unbound mixtures the Proctor method in BS EN 13286-2 is commonly specified.

Which laboratory compaction test is right for our material?

It depends on the material, the compactive effort your specification assumes and the particle size. BS 1377-2 offers light compaction with a 2.5 kg rammer, heavy compaction with a 4.5 kg rammer, and a vibrating hammer method that suits granular material. Each has limits on particle size and on how much of the sample may be retained on the 20 mm sieve, so coarse material may need a larger mould or a different approach.

What percentage of maximum dry density is usually specified, and where does that figure come from?

The figure comes from your project specification rather than from a universal standard, and it is commonly around 95%. On highways and infrastructure work using end-product compaction, the required percentage for each material class is given in the earthworks material acceptability table of Series 600 of the Specification for Highway Works. On other projects the designer sets it. Always work to the figure in your own documents rather than a remembered number.

Why do you take a moisture content with every density test?

Because the field test measures bulk density, and the acceptance comparison is made on dry density. The moisture content is what converts one to the other, so without it there is no figure to compare with the laboratory reference. The moisture content is useful in its own right as well: knowing how far the material sits from optimum usually explains why a layer has or has not reached the density required.

How does the moisture content of the material affect the compaction you can achieve?

Every material has an optimum moisture content at which a given compactive effort produces the highest dry density, and moving away from it in either direction reduces the density you can reach. Too dry and the particles resist rearranging; too wet and water occupies space the solids would otherwise fill, so extra rolling simply moves material around. This is why wet cohesive fill often cannot be brought up to specification by rolling alone.

When is an air voids requirement used instead of a percentage of maximum dry density?

Air voids criteria are normally used for cohesive fill, where the moisture content in the ground is what limits the density achievable. Series 600 applies a 5% air voids curve to certain selected cohesive material rather than a straight percentage of maximum dry density. Calculating air voids needs the particle density of the material as well as the field density and moisture content, so that determination has to be in the testing schedule.

Why has our compaction result come back low?

The usual causes are a layer placed too thick, too few passes, plant that is wrong for the material, or a moisture content too far from optimum. Less obvious causes are worth ruling out before reworking anything: the result may have been compared against a reference for a different material, oversize stone can distort a field density, and a soft layer beneath can prevent the layer above compacting at all. Send us the figures and we will help narrow it down.

Is a compaction test the same as a plate bearing or LWD test?

No. Compaction testing measures how densely material has been packed relative to a laboratory reference; plate bearing and LWD testing measure how stiff the layer is under an applied load. A layer can be properly compacted and still be too soft for its purpose if the material itself is weak, and a stiff surface can hide a poorly compacted layer beneath it. Specifications often call for both, for different parts of the works.

Do we need a new reference test if the material changes part way through?

Yes. A maximum dry density applies only to the material it was determined on, so a change in source, grading or geology needs its own reference before field results can be judged against it. In practice that means a new compaction test when a new borrow area opens, when an imported material changes supplier, or when the grading of what is being placed visibly moves. Mixed material is the most common reason for results that look inconsistent.

Which parts of compaction testing sit within your UKAS accredited scope?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. The laboratory compaction test and the field density method are separate entries, and site and laboratory work can carry different scope, so they are considered individually rather than as one service. Ask us to confirm whether the methods your specification names are covered, and each report will state the status of the result.

How is compaction testing and monitoring priced?

Price is driven by the number of laboratory reference tests, the number of field positions, how many visits the programme needs and how far the site is from the regional team. Standby time, out-of-hours working and any reinstatement count as well. A single reference covering a large uniform fill is a very different job from several materials with a test position every few metres, so send the specification and expected volumes for a relevant quotation.