A formation on soft ground is due to be stabilised next week and the programme assumes lime, with the plant already pencilled in. Nobody has seen a chemical result for the soil yet, and sulfate testing before lime stabilisation closes that gap.
Get the sulfate result, and the related chemical results, to the designer before the binder is chosen. Test second and the designer is reacting to a decision already made. This guide covers which determinations to ask for and where chemical testing sits in the sequence, and it also says when sulfate screening isn't the question.
Key takeaways
- Sulfate and related chemical results should reach the designer before the binder is chosen, so lime or cement is picked on measured soil chemistry.
- Hixtra's chemical testing covers sulfate, pH, chloride and organic matter, so one schedule can ask for all four.
- Stabilised soils have their own reference, BS EN 13286-47, and it applies to the stabilised material, not the untreated soil.
- Sulfate screening answers a binder question, so for stiffness or bearing on a formation, ask for LWD or plate bearing testing instead.
In this guide
- Why does the sulfate test come before choosing lime or cement?
- What does chemical testing cover before stabilisation?
- Which tests sit where in a stabilisation sequence?
- When you don't need a sulfate test before lime stabilisation
- Frequently asked questions
- What to have ready before you book
Why does the sulfate test come before choosing lime or cement?
The designer specifies the binder. To do that on measured soil chemistry rather than an assumption carried over from the programme, they need the sulfate result and the related results first.
The wrong order is easy to spot, because a binder gets named in the tender allowance and plant and labour are lined up around it. That's backwards. The chemical result turns up afterwards, if at all, and by then the designer isn't choosing but checking whether a choice already made can be defended.
This doesn't need a long investigation: one early request will do, with results going to whoever owns the binder decision. Once the binder is set, stabilised soil testing takes over and looks at the treated material.
What does chemical testing cover before stabilisation?
Hixtra's chemical testing assesses soils, fill and aggregates for sulfate, pH, chloride and organic matter. BS 1377-3 is the reference for chemical testing of soils, and four determinations are short enough to request as a set. Four results, one schedule.
Why ask for the related results with the sulfate result?
One request means you won't need a second round of sampling. Sulfate is the headline figure, but the related determinations can sit on the same schedule, and the designer may want any of them. Ask for sulfate alone, learn later that pH was wanted too, and someone has to go back to the ground.
What each result means for the binder is the designer's call, made against the specification. Your job is to have everything to hand.
Which tests sit where in a stabilisation sequence?
Chemical testing comes first. Binder choice follows it, then stabilised soil testing and the earthworks checks come after, in the order a contractor would meet them on a job.
The order of tests around a stabilisation decision
| Stage | Test | Reference | What the result tells the reader |
|---|---|---|---|
| Before the binder is chosen | Chemical testing | BS 1377-3 | Sulfate, pH, chloride and organic matter results for the designer |
| After the binder is chosen | Stabilised soil testing | BS EN 13286-47 | Testing of the stabilised material |
| During earthworks | MCV testing (Moisture Condition Value) | None named | A moisture condition check on earthworks material |
| During and after placement | Earthworks testing | Series 600 of the Specification for Highway Works | The earthworks specification the checks refer to |
Only the first row feeds the binder decision. The rest follow from it. Earthworks testing is tied to Series 600 of the Specification for Highway Works, so checks on the placed material should match whatever the contract specification calls up.
When you don't need a sulfate test before lime stabilisation
Skip it when there's no binder planned. Skip it too when the question is stiffness or bearing.
In the first case, the earthworks design names no lime or cement, so there's no binder decision for a sulfate result to inform and no reason to commission it out of habit.
The second case is a formation that needs proving for stiffness. LWD testing measures the response of formation, subbase and engineered fill using a portable Lightweight Deflectometer (LWD). The result is a dynamic surface modulus in MPa. CD 225 of the Design Manual for Roads and Bridges sets pavement foundation classes by surface modulus, starting at 50 MPa.
One limit matters on soft ground. Soft cohesive soils are the weak point, because clays deform plastically and give little elastic recovery for the equipment to read. If you need settlement under a sustained known load, plate bearing testing is the better fit.
If you're unsure about the specification or which test to choose, give us a call and talk it through before booking anything.
Frequently asked questions
Do I need a sulfate test before lime stabilisation?
Yes, whenever lime or cement is a candidate binder, because the designer needs the sulfate result and the related chemical results before choosing.
What else should be tested alongside sulfate before choosing lime or cement?
pH, chloride and organic matter. Hixtra's chemical testing covers all four determinations, sulfate included, so a single schedule can ask for the lot.
Who needs the sulfate results before the binder is chosen?
The designer, first. Contractors and consultants who have priced or programmed lime should see the same results too, because the programme leans on that choice. A result that arrives after the binder is fixed can only confirm or contradict a decision already made.
Is sulfate testing the same as testing the stabilised soil?
No. Sulfate testing is chemical testing of the soil ahead of the binder decision. BS EN 13286-47 is the reference for stabilised soils, and it applies to the stabilised material. They're at different points in the sequence, so one can't stand in for the other.
What to have ready before you book
Send the site postcode, specification, test positions, access details and preferred dates, with a project contact and any deadline. Testing is coordinated centrally and delivered across England and Wales, so use your regional contact number to discuss availability for your site.
Request a quote or tell us what you need, and say which binder the programme currently assumes.
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