Swelling & soaking tests

What does a laboratory swelling test measure?

It measures how much a soil or a treated material expands once it has free access to water, reported either as a linear rise in a mould or as a change in volume. That matters because a material which swells after it has been placed and compacted can lift the capping, slab or pavement above it, often weeks after the works are finished. Swelling is assessed on prepared specimens conditioned and soaked to an agreed procedure.

Why would my specification ask for a soaked CBR rather than an unsoaked one?

Because soaking represents the worst-case moisture condition the layer could reach in service, and a subgrade that performs well when dry can lose much of its strength once saturated. A soaked specimen is compacted in the normal way and then immersed before penetration, so the design value reflects wet-weather or high water table conditions. Swell during the soaking period is normally recorded at the same time.

When are swelling and soaking tests required, and who usually asks for them?

They are most often required on stabilised soils, on sulfate-bearing ground and on expansive clays, and the request usually comes from the earthworks designer or the contractor's technical team before a stabilisation mix is approved. Highway and infrastructure specifications commonly call for swelling trials as part of that approval. Soaked testing is also asked for where a saturated subgrade condition has to be proved for pavement design.

Why does sulfate in the ground cause a problem with lime or cement stabilisation?

Because sulfate reacting with calcium from the binder and alumina from the clay can form expansive minerals such as ettringite, which draw in water and generate very high swelling pressures. The damage shows up weeks or months after treatment, as heave in a stabilised capping or subbase. Sulfides such as pyrite make matters worse, because pulverisation and the raised pH cause them to oxidise and release further sulfate.

Which sulfate tests should be done before a stabilisation design is settled?

Sulfate screening normally covers water-soluble sulfate on a 2:1 extract, acid-soluble sulfate and total potential sulfate, the last of which accounts for sulfides that could oxidise later. TRL Report 447 sets out the methods commonly used. Results above roughly 0.25 per cent as sulfate usually prompt swelling trials and consideration of a different binder, but the thresholds that apply to your works are the ones in your specification.

Which standards are swelling and soaking tests carried out to?

Linear swelling is commonly determined to BS EN 13286-47, the same method that covers CBR and immediate bearing index, with the specimen restrained in a CBR mould. The accelerated swelling test for soil treated with lime or a hydraulic binder is BS EN 13286-49, carried out on unconfined specimens. Soaked CBR follows the CBR method your specification names. Tell us which your testing schedule calls for.

How long is the soaking period, and what is the surcharge for?

A soaked CBR is normally immersed for four days, while swelling trials on stabilised material run much longer, commonly 28 days and extended to 56 where sulfides are present. The accelerated method uses seven days fully immersed at 40 degrees after a conditioning period, which brings the reaction on faster. Annular surcharge discs sit on the specimen to represent the weight of the construction that will overlie the layer.

What swell value counts as a pass?

That is set by your specification, but the limits used in highway stabilisation work are commonly an average of no more than 5 mm with no individual specimen above 10 mm in the restrained mould test, and no more than 5 per cent volumetric expansion in the accelerated test. Agree the acceptance criteria with us before testing begins, because they govern how the specimens are prepared and conditioned.

What happens if the mix swells too much?

An excessive swell result means the mix as designed is not safe to use on that material, so the design changes rather than the test being repeated. The usual routes are a different binder, often one containing ground granulated blastfurnace slag, a reduced lime content, or leaving that material untreated and dealing with it another way. Retesting a revised mix takes as long as the first round, so allow for it in the programme.

What sort of sample do the swelling tests need, and in what condition?

These tests use compacted specimens, so we need bulk material rather than intact tube samples, and enough of it to prepare the full set your specification requires. Send it in sealed containers at its natural moisture content and clearly referenced, because material that dries out in transit will not compact to the condition the design assumed. Confirm the quantity and containers with us before sampling.

Are swelling and soaking tests covered by your UKAS accreditation?

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, rather than to every service. Swelling and soaking covers several different procedures, soaking periods and conditioning regimes, so ask us to confirm the status of the exact method your specification names.

What should we send to get a quotation for swelling and soaking tests?

Send the material type, the binder and addition rates if it is being treated, the method and soaking period required, the number of specimens and your programme dates. The specification or testing schedule is the single most useful document. Cost is driven mainly by specimen numbers, mix preparation and the length of the conditioning period, since a long soak occupies laboratory space for weeks rather than days.