A CBR test and a plate bearing test both tell you something about how the ground behaves, but they measure different properties and answer different design questions. A CBR (California Bearing Ratio) test is a strength index, comparing how far a standard piston pushes into the soil against a standard crushed-stone reference, and it is used mainly to size pavements, roads and hardstanding. A plate bearing test loads a rigid steel plate against a reaction on the actual ground and records the settlement, giving a direct measurement of bearing capacity, settlement behaviour and the modulus of subgrade reaction, which is what you need to sign off a foundation formation or a working platform for tracked plant. Put simply, CBR tells you how strong a material is for pavement design, and a plate bearing test tells you how the ground as built responds to a real applied load.
Getting the two confused leads to specifying the wrong test and paying for data that does not answer the question in front of you. This guide sets out what each test actually measures, how it is carried out, and how to decide which one your project needs.
What a CBR test measures
The California Bearing Ratio was developed by the California Division of Highways in the late 1920s and formally adopted in 1935, specifically to help engineers select economical pavement and base thicknesses that could still carry the anticipated traffic loads. It has since become a standard method in British, American and other codes worldwide.
The test itself is empirical. A standard-sized piston is pushed into a prepared soil sample at a controlled rate, and the resistance to penetration is compared against the resistance of a well-graded crushed stone reference material. The result is expressed as a percentage: a soft, weak subgrade might return a low single-figure CBR, while a strong granular sub-base returns a much higher value. The test does not characterise any property of the soil other than its resistance to penetration, but that index correlates well with pavement performance and is trusted for road and runway design.
In the UK, the laboratory CBR method sits within BS 1377 Part 4, the compaction-related tests in the British Standard methods of test for soils. Most CBR testing is done in the laboratory on compacted specimens, often soaked first to simulate the worst-case moisture the pavement will face. A CBR can also be measured in the field, and the dynamic cone penetrometer (DCP) is widely used to estimate in-situ CBR quickly across a site.
What a plate bearing test measures
A plate bearing test, or plate load test, is always an in-situ test. A rigid steel plate is bedded onto the surface being assessed and loaded in stages against a reaction, which is provided either by kentledge, meaning a stacked dead weight such as a loaded lorry or excavator, or by a machine acting as the reaction. The settlement under each load increment is recorded.
From that load-settlement data an engineer reads off the bearing capacity, the settlement at the design load and, where required, the modulus of subgrade reaction used in slab and pavement design. Because it loads the ground as actually built rather than a remoulded laboratory sample, it is one of the most direct ways to verify that a formation, a piling platform or a granular sub-base will carry the loads placed on it. In the UK it is carried out to BS 1377 Part 9, the standard covering in-situ tests on soils, and where it supports a working platform for tracked plant the results are checked against BR 470, the BRE good practice guide for ground-supported working platforms.
CBR test versus plate bearing test at a glance
| CBR test | Plate bearing test | |
|---|---|---|
| What it measures | A strength index from penetration resistance | Bearing capacity, settlement and modulus of subgrade reaction |
| UK standard | BS 1377 Part 4 (laboratory method) | BS 1377 Part 9 (in-situ) |
| Where it is done | Usually the laboratory; in-situ variants exist | Always on site, in-situ |
| Typical use | Pavement, road and hardstanding thickness design | Foundation formation and working platform or piling mat sign-off |
| Output | A CBR value as a percentage | Load-settlement data and derived design values |
| Reference guidance | Highway and pavement design methods | BR 470 for working platforms |
When to specify each
Reach for a CBR test when you are designing a pavement construction and need a strength value to feed into the layer-thickness calculation. Roads, car parks, external hardstanding and haul routes are all sized using CBR, and a DCP survey is a fast way to build up a picture of subgrade strength across a large area before design is finalised.
Reach for a plate bearing test when you need to prove, on the ground as constructed, that a formation or platform will carry a known load. Signing off a piling mat before a rig tracks onto it, confirming a foundation formation meets the assumed design bearing value, or validating a compacted sub-base after earthworks are all plate bearing jobs. Where a temporary works coordinator or checking engineer needs to accept a working platform against BR 470, the plate bearing test is the direct evidence they are looking for.
Many projects use both at different stages: CBR during design to size the construction, and a plate bearing test during construction to verify the finished ground performs as assumed. They are complementary rather than interchangeable, and the right choice comes down to whether you need a strength index for design or a load-response measurement for sign-off.
Common questions
Is a CBR test the same as a plate bearing test?
No. A CBR test is an empirical strength index that compares the ground’s penetration resistance against a standard crushed stone, and it is used mainly for pavement design. A plate bearing test is an in-situ load test that measures how the ground settles under a loaded plate, giving bearing capacity and settlement directly. They measure different things and are specified for different purposes.
Which test do I need for a working platform or piling mat?
A plate bearing test. It is carried out to BS 1377 Part 9 and gives a direct, in-situ measurement of how the completed platform performs under load, which can then be checked against the platform design and BR 470 before a piling rig or crane tracks onto it. A CBR value alone does not confirm the finished platform in the way a plate load test does.
Can a CBR test be done on site rather than in a laboratory?
Yes. While most CBR testing is done in the laboratory on prepared and often soaked specimens, an in-situ field CBR can be carried out, and the dynamic cone penetrometer is commonly used to estimate CBR values in place. Field and laboratory CBR values do not always correlate exactly, because soaking and moisture conditions differ, so the method should be chosen to suit what the design needs.
What does a CBR value actually tell you?
It gives a percentage strength index relative to a standard crushed-stone material. A low CBR indicates a weak subgrade that needs a thicker pavement construction or ground improvement, while a high CBR indicates a strong, well-compacted material. Pavement design methods use the value directly to select layer thicknesses for the expected traffic.
If your project needs the ground proven under a real applied load, whether that is a foundation formation or a working platform for tracked plant, our plate bearing testing is carried out to BS 1377 Part 9 with results presented for checking against BR 470. For more on the method itself, see our guide on plate bearing tests explained, and if your works also involve fixings into concrete or masonry, our guide on what an anchor pull test is covers on-site load testing of anchors.