Stabilised soil testing

What testing does a lime or cement stabilisation job need from start to finish?

It splits into three stages: testing the untreated soil, designing and proving the mix, then controlling and accepting the work on site. Before treatment you need classification, moisture content, plasticity, grading and a chemical suite covering sulfate, sulfide and organic matter. The mix design follows, and site control then covers binder spread rate, pulverisation, moisture content or MCV, in-situ density and finally strength or CBR on cured specimens.

How is a stabilisation mix design developed, and what laboratory work sits behind it?

The design is built from trial mixes of your own soil at several binder contents, prepared, compacted, cured and tested in the laboratory. A compaction test establishes the maximum dry density and optimum moisture content for each treated mix, specimens are then cured under controlled conditions and tested for unconfined compressive strength or CBR, with swelling checks added where sulfate is present. The design binder content is the one that meets the specified performance with a margin.

What is the initial consumption of lime test and why is it done first?

It establishes how much lime the soil absorbs before any lasting cementing reaction begins, determined by adding increasing lime doses and tracking the pH of the mixture. Clay minerals take up lime for modification first, so a design that ignores that demand will be under-dosed. The ICL result, usually determined to the method in BS 1924, is the baseline the trial mixes are built on rather than the final design content.

Why does sulfate and sulfide content matter in lime and cement stabilisation?

Because sulfate, lime or cement and water together can form expansive minerals that swell long after the layer has been built and accepted. The damage shows as heave, cracking and loss of level weeks or months later, by which point the pavement or structure above is in the way. Sulfides such as pyrite are the bigger trap, since they sit harmlessly in the ground until oxidised and then generate fresh sulfate.

What is tested for sulfate, and what happens if the levels come back high?

The usual suite is water-soluble sulfate, acid-soluble sulfate and total potential sulfate, the last of which accounts for sulfide minerals that may oxidise later. Where results are elevated, a swelling or soaking test is normally added to the mix design, such as the accelerated swelling test in BS EN 13286-49 or linear swelling measured with CBR to BS EN 13286-47. The outcome may be a different binder, a longer mellowing period or a decision not to stabilise that material.

What is the mellowing period, and does the material need testing either side of it?

Mellowing is the pause between the first mixing pass and final mixing, which lets quicklime slake fully and moisture redistribute so the clay breaks down. Series 600 of the Specification for Highway Works commonly sets this at not less than 24 hours and not more than 72 hours for lime. Moisture and pulverisation are normally checked after mellowing and before final compaction, because that is when the material has to meet the requirement.

What testing is carried out while the stabilisation plant is working?

Site control usually covers binder spread rate, mixing depth, pulverisation, moisture content or MCV and in-situ density as each layer is compacted. These have to be quick, because the mixing train, the roller and the testing all move together and a result that arrives an hour late is of no use to anyone. Heavy rain between passes changes the moisture condition and often means re-testing before compaction is allowed to continue.

How long after mixing does the layer have to be compacted?

Cement-treated material has a short window, commonly around two hours from the addition of the binder, after which compacting it breaks bonds that have already formed. Lime-treated soil is more forgiving once it has mellowed, and slag-based binders give longer again. The window is set by the binder and the specification, which is why the tests around compaction have to be planned against it rather than fitted in afterwards.

How is the finished stabilised layer proved: strength, CBR, or both?

It depends what the layer is. Capping is usually accepted on CBR after curing, while a bound subbase is normally accepted on unconfined compressive strength. Specimens are made from the mixed material, cured for the period the specification sets and tested at the stated age, commonly 7 or 28 days. A soaked CBR or an immersion strength check is added where durability under water has to be demonstrated.

Does a stabilisation job need a site laboratory, or will planned visits do?

Planned visits suit a shorter job with a fixed programme; a site laboratory earns its place when stabilisation is running daily and results are controlling the plant. On a continuous earthworks contract, doing the moisture, MCV, pulverisation and density work on site takes travel and sample transport off the critical path. Tell us the length of the programme and the daily output and we will advise which makes sense.

Which parts of a stabilisation testing programme are covered by your UKAS accreditation?

That has to be answered test by test rather than as a whole programme. 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. Send us the testing schedule for the stabilisation works and ask us to confirm whether the tests you need are covered, so the position is clear before work starts.

What do you need to price a full stabilisation testing programme?

Send the site postcode, the specification or testing schedule, the soil types involved, the binder being used and the area or volume to be treated. It also helps to know whether the mix design work is already done or needs including, the expected daily output and the start date. Quantities and the number of site visits move the cost far more than any single test does.