The specification for a granular working platform layer asks for a CBR percentage. The material has stone in it, and nobody's sure a plunger will read it. So should it be a CBR or a plate bearing test, and can one stand in for the other?
They're different tests: a direct plunger California Bearing Ratio (CBR) test reports a percentage from a prepared surface, while a plate bearing test measures settlement under a known load. Stone over 20 mm usually rules out the plunger, so a derived CBR from a plate test can be used instead. Agree the calculation and its suitability with the project engineer.
Key takeaways
- A direct in-situ CBR drives a plunger into the prepared surface at about one millimetre per minute (BS 1377-9) and reports a percentage, and it's best suited to clays, silts, sands and fine gravels.
- A plate bearing test, usually to BS 1377-9, records load against settlement through a rigid circular plate and stresses roughly twice its diameter in depth.
- Coarse fill is where the plunger struggles. Once stone passes about 20 mm, you'd generally switch to a plate test and agree with the project engineer how any CBR will be derived from it.
- A derived CBR is a different measurement, so label it as derived and show the calculation.
In this guide
- Which test answers a CBR question on your ground?
- What does each test measure, and how deep does it read?
- When is a derived CBR from a plate test accepted?
- Frequently asked questions
- What to have ready before you book
Which test answers a CBR question on your ground?
That depends on the material and what the design needs. "CBR" can mean an in-situ, a laboratory or a derived result, and our CBR testing guide explains why one can't stand in for another.
Ask what the figure is for. Does the specification want the strength of the ground on the day, or settlement under a load? A CBR can be estimated from a Lightweight Deflectometer (LWD) surface modulus, but that's a correlation, not a measurement. If LWD or a Dynamic Cone Penetrometer (DCP) test is also an option, our guide to choosing between plate bearing, CBR, LWD and DCP compares all four.
What does each test measure, and how deep does it read?
The plunger reads the prepared surface, while the plate reads deeper.
In-situ CBR. A plunger is driven into the surface at a controlled rate while force is recorded against penetration. The load at set penetrations is compared with a standard reference material, and the ratio is reported as a percentage. It indicates the subgrade strength that pavement and platform thicknesses are designed from. See in-situ CBR testing.
Plate bearing test. A known load goes onto a rigid circular plate and the settlement is recorded. From that you can calculate a bearing pressure, a stiffness modulus or a derived CBR, depending on the design. Plates of 300, 450, 600 and 760 mm are commonly used, and as a rule of thumb a plate stresses ground to roughly twice its diameter in depth. See plate bearing testing.
CBR and plate bearing test compared
| Question | In-situ CBR (direct plunger) | Plate bearing test | Derived CBR from a plate test |
|---|---|---|---|
| What it measures | Plunger force against penetration, compared with a reference material | Settlement under a known load through a rigid circular plate | A CBR calculated from plate data using an agreed method |
| What it reports | A percentage | Load and settlement relationship, bearing pressure or stiffness modulus | A figure labelled as derived |
| Standard | BS 1377-9 | BS 1377-9 (usual plate loading method) | The calculation agreed with the project engineer |
| Depth and scale | The prepared surface under the plunger | Roughly twice the plate diameter (a 600 mm plate reads over a metre) | As for the plate test |
| Material limits | Best for clays, silts, sands and fine gravels. Generally unsuitable above about 20 mm particles | Suits coarse material and thick granular layers | Used where stone over 20 mm usually rules out the plunger |
| What the report shows | The CBR percentage | The load and settlement data, and the modulus or pressure requested | The calculation behind the figure, labelled as derived |
When is a derived CBR from a plate test accepted?
When the material's too coarse for a plunger and the project engineer has agreed the calculation and its suitability.
Start with the stone. Layers holding pieces bigger than about 20 mm are generally unsuitable, because one of them can end up under the plunger and the reading then reflects that stone rather than the layer around it.
Then the paperwork. The result is labelled as derived, and the report states the calculation so the reader can see how the figure was reached. A derived CBR can be calculated from plate bearing data where appropriate, but it isn't the same measurement as a direct plunger CBR. If a specification just says CBR, ask which one it means before anyone's on site.
When you don't need a derived CBR
If the specification asks for a modulus or a bearing pressure, the plate test gives you those figures.
If the material's clay, silt, sand or fine gravel and a CBR percentage is what's asked for, a direct plunger CBR is normally the one to run.
Use a plate test for settlement under a sustained known load, or when the design has to be proved at the pressure the plant or structure will apply. Not sure which test to choose? Give us a call.
Frequently asked questions
Why can't I run an in-situ CBR on gravelly fill?
Stone controls the result. Fill holding pieces larger than about 20 mm is generally unsuitable, because a single stone under the plunger can dictate the reading, so the figure no longer describes the fill as a whole. Run a plate test instead, and agree with your project engineer how a derived CBR will be calculated.
How deep does a 600 mm plate bearing test read?
Over a metre. As a rule of thumb a plate loads the ground to roughly twice its own diameter, so a 600 mm plate is testing more than a metre of material beneath it. A larger plate reaches deeper.
Who agrees how a derived CBR is calculated?
Your project engineer. The calculation and its suitability are agreed with them. It's distinct from a direct plunger CBR test, and the report should say it's derived.
Is a laboratory CBR the same as an in-situ CBR?
No. The laboratory test is run on a sample compacted in a mould to a density and moisture content you choose, so it shows how the material behaves in that set condition. The in-situ test reads the formation as it is on the day, with its real compaction and moisture.
What to have ready before you book
Send the specification, the number of test positions, your dates and how the site is accessed.
Tell us which result the specification wants: a CBR percentage, or a modulus or bearing pressure. That shows whether you need a plunger CBR, a plate test or a derived calculation.
Send your site postcode so we can confirm travel, access and availability. Tell us what you need.
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