The specification says CBR, the formation is being trimmed, and someone on site asks for a field CBR test. They mean the plunger test carried out on the ground itself, which we list as in-situ CBR testing. That page covers how the test is done. This guide is about the decisions around it.
Key takeaways
- A field CBR is the in-situ plunger CBR. The method page explains how it's carried out.
- Decide first what the designer will do with the value, because in-situ, laboratory and derived CBR answer different questions.
- Book the visit for when the formation is in the state it'll be built on, then protect it.
- A low number isn't a failure. It usually changes what goes on top.
- Ground with stone over about 20 mm generally needs a plate bearing test instead.
Start with the question the number has to answer
CBR isn't one test. It's a family of methods, and in-situ, laboratory and derived CBR are separate measurements rather than options you can swap. So before you book a field CBR, ask the designer what the value is for.
If the design wants the strength of the formation as it stands, at its natural moisture and as compacted, a field test gives that directly. If it has to allow for the wettest likely condition, that's normally handled by laboratory CBR testing on a soaked specimen, not by hoping for a dry week on site. And if the material is too coarse for a plunger, the figure may have to be a derived CBR, calculated from plate bearing load and settlement data and labelled as such.
Our CBR testing guide sets the three out side by side.
Time the test around the formation, not the diary
CBR is strongly influenced by moisture content. A clay formation that was trimmed dry can drop well below its earlier CBR after rain, or once plant has run over the open formation.
So a few habits pay off. Test soon after trimming. Keep traffic off the formation before the visit. Cover or blind it once the positions are done.
If the formation was trimmed some days ago and it has rained since, say so when you book. Each value describes the ground at the moment it was tested, so aim for the condition the road or platform will actually be built on.
Reading a field CBR result
The percentage compares how hard the ground resists the plunger with the resistance a standard crushed stone gives under the same load. A higher figure generally means a stronger subgrade and less construction needed above it. It's about the subgrade, not the layers you'll build on it.
So a low reading isn't something to argue with. It tells the designer about the ground they've got, and the FAQ below covers what normally happens next.
Two positions on the same clay can disagree, sometimes by more than you'd expect. Ask about a moisture content sample at each position. With it, each value can be read against the material's natural and optimum moisture contents, which usually explains the gap.
On highway work, think about where the value sits for CD 225. The relationship it uses to turn CBR into a subgrade surface modulus is intended for values of roughly 2 to 12 per cent.
And don't expect the field figure to match a laboratory one. A soaked laboratory value is usually the lower of the two, while a well prepared dry formation can read higher.
How a field CBR fits with laboratory and derived CBR
| Option | What it tells you | Use it when |
|---|---|---|
| Field (in-situ) CBR | The formation as it is, compaction and moisture included | Fine soils, a vehicle can reach each position, and the design wants the condition on site |
| Soaked laboratory CBR | What a remoulded specimen gives in a defined, wetted state | The design has to cover the wettest likely condition |
| Derived CBR from a plate test | A figure calculated from load and settlement, labelled as derived | Stone over about 20 mm rules out the plunger |
| LWD or DCP correlation | An estimate, not a direct measurement | A quick check where the material and specification allow it |
When a field CBR is not the right test
Coarse ground is generally the main reason. With stone over about 20 mm, a single stone can govern the reading, so a plate bearing test, which loads a much larger area, is normally the better route. The plunger can't be used on capping either.
Access comes next. If the test vehicle can't get to a position, LWD or DCP testing is the usual fallback, with any CBR from them treated as an estimate.
For a fuller comparison, see our guide on choosing between plate bearing, CBR, LWD and DCP.
Frequently asked questions
Is a field CBR test the same as in-situ CBR testing?
Yes. Field CBR is another name for the CBR test carried out on site, using the traditional plunger method rather than a specimen prepared in the laboratory. An in-situ CBR measures the ground exactly as it was on the day, compaction, moisture and all, while a laboratory CBR reports what the material can do in a defined state.
Does a low field CBR mean the formation has failed?
No. A low value usually means the design needs more capping or subbase, or some ground improvement, and the designer decides which.
Can laboratory CBR samples be taken on the same visit?
Yes. While we're on site, the technician can take a bulk disturbed sample from formation level and bring it back to our laboratory. Ask us how much material is needed before anyone digs.
Why do two field CBR positions on the same site give different numbers?
Moisture is the usual cause. A moisture content from each test position lets the values be read against the material's natural and optimum moisture contents, so ask for one when you book.
What to have ready
On the day of the visit, the technician generally needs three things: the formation trimmed flat at the level being tested, a vehicle route to every position that's safe to use, and room to work around each one.
When you enquire, tell us what the designer has asked for and we'll match the testing to it. Add the postcode, how many positions you need, your dates and how the site is accessed. Send a photograph of the exposed material if you are unsure. Where you can, group your CBR positions, any plate tests and any sampling into one visit. Tell us what you need.
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