A piling platform is built. The specification asks for "CBR". The fill has stone in it. A rig's due on the programme, and the clause needs a number someone can sign off.
Choosing between plate bearing, CBR, LWD or DCP starts with the quantity the clause asks for, not the name of the test. Plate bearing gives settlement under load, in-situ CBR gives a plunger ratio, LWD gives a surface modulus and DCP gives a penetration rate.
Key takeaways
- A plate bearing test gives a load and settlement relationship, from which a bearing pressure, a stiffness modulus or a derived CBR can be calculated.
- An LWD gives a dynamic surface modulus in megapascals (MPa). CD 225 of the Design Manual for Roads and Bridges sets pavement foundation classes by surface modulus, starting at 50 MPa.
- In-situ, laboratory and derived CBR are different methods. Agree any derived calculation with the project engineer.
- DCP gives a penetration rate in millimetres per blow. It's approximate, so an in-situ CBR or plate bearing test usually carries more weight where either is possible.
In this guide
- Start with what the clause asks for
- How the four tests compare
- When the clause says CBR but the ground says otherwise
- Frequently asked questions
- What to have ready before you book
Start with what the clause asks for
Plate bearing testing is usually carried out to the plate loading method in BS 1377-9, with the plate size, the load increments and the maximum applied pressure set by your specification. Plates of 300, 450, 600 and 760 mm diameter are commonly used. As a rule of thumb a plate stresses a depth of ground roughly twice its diameter, so a 600 mm plate is assessing over a metre of material, not just the surface.
In-situ CBR testing drives a cylindrical plunger into the prepared surface at about one millimetre per minute, normally to the method in BS 1377-9. California Bearing Ratio (CBR) compares the load at set penetrations with a standard reference material, and the ratio is reported as a percentage.
A Lightweight Deflectometer (LWD) drops a known weight onto a plate and records how far the surface deflects, and the result is a dynamic surface modulus in MPa. On highway schemes the requirement for LWD testing is set by the specification, with Series 800 of the Specification for Highway Works covering performance testing of pavement foundations.
How the four tests compare
Each test answers a different question. None stands in for another without agreement.
| Test | What you get | Reference | Best suited to | Main limit |
|---|---|---|---|---|
| Plate bearing | Load and settlement, giving bearing pressure, stiffness modulus or derived CBR | BS 1377-9 | Working platforms, crane and piling positions, thick granular layers, coarse material | A derived CBR is a different measurement from a plunger CBR |
| In-situ CBR | A percentage from plunger load against penetration | BS 1377-9 | Clays, silts, sands and fine gravels | Particles over about 20 mm are generally unsuitable |
| Laboratory CBR | A percentage on a remoulded specimen | BS 1377-2 | Material that can be prepared to a chosen density and moisture | Reports the specimen's defined state, not the ground on the day |
| LWD | Dynamic surface modulus in MPa | No single British Standard; manufacturer's procedure with a recognised method such as ASTM E2583 | Granular and recycled materials, capping, subbase, made ground | Soft cohesive soils |
| DCP | Penetration rate in mm per blow | No British Standard for soils; a recognised method such as ASTM D6951 with the equipment's procedure | Shallow layers and boundaries between materials | Approximate; CBR correlations unreliable in mixed material |
A laboratory result and a field result can both be called CBR. They describe different things. Check which one the clause means. DCP testing is light and quick, and needs no plant.
When the clause says CBR but the ground says otherwise
If the material won't suit the plunger, another test may fit better. Agree it with the project engineer first.
Stone content
Material containing particles larger than about 20 mm is generally unsuitable for a plunger CBR, because a single stone under the plunger controls the reading, so a plate test with an agreed derived calculation is then the sensible route.
Derived and inferred CBR
A derived CBR can be calculated from plate bearing data where appropriate. It's distinct from a direct plunger CBR. Label it as derived.
A CBR can be estimated from an LWD surface modulus, but that's a correlation, not a measurement. A CBR inferred from a DCP penetration rate is an estimate too. For the DCP, the published relationships work best in homogeneous fine grained soils and become unreliable in mixed material, made ground or anything with significant stone content.
When it's not the right test
An LWD is a good check on a placed layer, but it's a poor way of finding out what's happening deeper down.
A DCP gives an approximate assessment rather than a direct measurement, so where an in-situ CBR or plate bearing test is possible, those usually carry more weight.
Laboratory CBR testing reports a specimen remoulded to a chosen density and moisture, while an in-situ CBR measures the ground as it was on the day.
Frequently asked questions
Is an LWD result the same as a CBR result?
No. An LWD reports a dynamic surface modulus in megapascals, calculated from the force applied and the deflection it produced, and a CBR can be estimated from that modulus, but it's a correlation and has to be appropriate to the material. Check first that your specification will accept a correlated value.
Can a plate bearing test give me a CBR value?
A derived CBR can be calculated from plate bearing data where appropriate, but it is a different measurement to a direct plunger CBR and should be labelled as derived in the report.
What does a DCP result in millimetres per blow tell me?
The smaller the figure, the stiffer the material, and a clear change in the rate marks a boundary between materials of different resistance.
Which test suits a crane hardstanding or piling platform?
Plate bearing usually fits. The plate applies a sustained load close to the stress the real plant will impose, which is why it suits crane and piling positions, and where the test checks a working platform, the target pressures come from the platform design.
What to have ready before you book
Send the site postcode, the specification, the test positions, access details and preferred dates. Add a project contact and any deadline.
On-site attendance covers England and Wales.
UKAS accreditation applies to the particular methods listed in Hixtra’s current schedule, rather than every service. Ask us to confirm the accreditation status of the testing you require.
Send your site postcode so we can confirm travel, access and availability. Request a quote.
.svg)