Why Have My Plate CBR Results Failed?
A failed plate CBR or ground-strength result can stop the next pavement layer, delay concrete placement or create uncertainty about the performance of a working platform.
The immediate response is often to add more roller passes and repeat the test. Although this may sometimes improve the result, it can treat the symptom rather than address the underlying reason for failure.
Low plate CBR results can be influenced by compaction, moisture condition, material composition, layer thickness, underlying ground conditions and the way the test was undertaken. Before remedial work begins, it is therefore important to confirm what was tested, whether the correct procedure was followed and what the result represents.
“A low plate or CBR result is rarely fixed by another roller pass until you understand whether the weakness is in the test layer, its moisture condition or the material below it.”
What Does a Plate CBR Result Measure?
“Plate CBR” is widely used as a site term, but a plate loading test and an in-situ California Bearing Ratio test are not the same test. BS 1377-9:2025 lists them as separate testing methods.
A plate loading test measures the load–settlement response beneath a plate and can be used to determine parameters such as deformation or subgrade reaction. An in-situ CBR test measures resistance to penetration using the CBR procedure.
Some reports convert a plate loading result into a nominal or equivalent CBR using an empirical correlation. This can be useful where the project design and specification accept the correlation, but it is not automatically interchangeable with a directly measured in-situ CBR result.
CTS reports the test undertaken in accordance with its documented Plate Loading Test Procedure, Procedure 15. This ensures that customers understand whether the reported value is a measured in-situ result or an equivalent CBR derived from plate loading.
Before investigating an apparent failure, confirm:
- The testing method used
- The required target parameter
- Whether the result is measured or derived
- The plate size
- The test level
- The acceptance limit
- The applicable project specification
CTS Procedure 15 also sets out requirements for test-area preparation, plate selection, equipment setup, loading increments and reporting. Following a documented procedure helps ensure that the result accurately represents the conditions at the test location.
Common Causes of Failed Plate CBR Results
Several factors can cause a plate loading or CBR result to fall below its specified target. The result should be reviewed alongside the site conditions, material properties and testing records before a cause is determined.
1. The Layer Is Under-Compacted
If the material particles have not been brought into a dense and stable arrangement, the initial application of load can cause further movement rather than a stiff response.
Common causes of under-compaction include:
- Lifts that are too thick for the selected compaction plant
- Too few roller passes
- An unsuitable roller type
- Excessive travel speed
- Poor overlap between roller passes
- Weak compaction at edges or around obstructions
Coarse material can also bridge at the surface while voided material remains underneath. The surface may appear tight and resist a boot heel, but the plate loading test influences a greater volume of material. Visual appearance alone is therefore not proof of uniform compaction.
Before concluding that a layer is under-compacted, CTS verifies that the test was undertaken in accordance with its documented procedure. This includes checking the plate seating, loading sequence and equipment setup to confirm that the result is representative of the tested area.
2. The Material Is Too Wet
Water occupies voids and, particularly in cohesive or fine-rich materials, can generate pore pressure during compaction and loading.
The roller may pump or weave, while the surface can appear sealed even though the layer below remains soft. Continuing to add roller passes can make the condition worse.
Factors that can move the material outside the moisture range in which it can be compacted effectively include:
- Recent rainfall
- Poor site drainage
- An exposed formation
- Waterlogged areas
- Material taken from a wet stockpile
The moisture condition should be considered alongside the relevant laboratory compaction relationship rather than assessed using a generic moisture percentage.
3. The Material Is Too Dry
Dry material is not necessarily strong or well-compacted material.
Without sufficient moisture to assist particle rearrangement, some soils and unbound mixtures can resist compaction and fail to achieve the required dry density.
Dry cohesive material may break into hard clods rather than form a uniform layer. It can also produce a deceptively firm surface that loses performance after becoming wet.
The relevant comparison is between the in-situ material condition and its laboratory compaction and moisture relationship.
4. The Material Is Unsuitable or Variable
Particle-size distribution, fines content, plasticity, particle shape and aggregate strength can all affect the material’s bearing response.
Excess plastic fines can make performance highly sensitive to moisture. A gap-graded or segregated layer may lack the smaller particles required to create a dense structure. Conversely, an excess of fine material can prevent effective stone-to-stone interlock.
Weak, weathered or degradable particles may also break down during compaction and alter the material grading after delivery.
Recycled materials require particular control because their source and proportions can vary. Localised pockets of soil, masonry, asphalt planings, organic matter or other contaminants can produce significantly different test results within the same area, even where the stockpile was supplied under one material description.
5. The Weakness Is Below the Tested Layer
A plate loading test does not assess only the top few millimetres of the surface. The stress influence extends into the construction below the plate.
A thin, well-compacted sub-base can therefore produce a poor result when it is positioned above:
- A soft formation
- A loose lower lift
- A service trench or crossing
- A waterlogged pocket
- An area with insufficient construction thickness
Variable layer thickness can make these conditions particularly difficult to identify from the surface.
For this reason, a failed result should not automatically be attributed to the most recently placed layer or the team that installed it. The result may be identifying a system-level issue that has been covered by the latest layer.
Before attributing the failure to the underlying ground, the test methodology should also be reviewed. CTS Procedure 15 places particular emphasis on correct test-area preparation, equipment setup and plate seating to ensure that the result represents the site conditions.
Where an underlying issue is suspected, trial pits, level records, probing or depth-profile testing may provide more useful evidence than repeatedly testing the same surface.
6. The Site or Test Conditions Do Not Match the Intended Assessment
The site and test conditions can affect whether the result accurately represents the area being assessed.
Factors that may distort a plate loading result include:
- A sloping or disturbed test surface
- Poor plate seating
- An unsuitable reaction arrangement
- Testing close to an edge or excavation
- Testing directly above an unrepresentative inclusion
- Incompatible loading and reaction arrangements
A technically valid result can also appear to fail if it is compared with a target intended for a different construction layer, test method or design condition.
The test method, location, level and acceptance criteria should therefore be confirmed before conclusions are drawn.
How Should a Failed Plate CBR Result Be Investigated?
The most effective route to achieving a reliable result is usually to diagnose the cause before beginning remedial work.
A structured investigation should include the following steps.
1. Verify the Testing Requirement
Check the relevant:
- Drawing
- Pavement design
- Earthworks specification
- Testing method
- Target value
- Test level
- Acceptance criteria
This confirms that the result is being compared against the correct project requirement.
2. Confirm That the Correct Testing Procedure Was Followed
Confirm that the plate loading test was completed in accordance with CTS Procedure 15.
This review should include:
- The calibration status of the equipment
- The plate size and selection
- Preparation of the test area
- Plate seating
- Equipment setup
- The loading sequence
- The loading increments
- Reporting requirements
This step helps establish whether the reported result is representative of the conditions at the testing location.
3. Map the Pattern of Results
Plot passing and failing locations against:
- Haul routes
- Roller runs
- Layer edges
- Service trenches
- Drainage features
- Changes in material source
- Variations in layer thickness
A clear pattern can help establish whether the issue is isolated or affects a wider area.
4. Review Site Conditions and Technician Records
Inspect the area before disturbing it and review the observations recorded by the testing technician.
Relevant observations may include:
- Rutting
- Pumping
- Cracking
- Loose surface texture
- Segregation
- Moisture conditions
- Ground instability
- Site levels
- Recent or adverse weather conditions
Proof rolling can help expose variability, although it does not determine the CBR value itself.
5. Measure Moisture and Density
Compare the in-situ moisture and density with the appropriate laboratory compaction relationship and the specified density or end-product requirement.
This can help determine whether the material is capable of achieving the required performance under its current moisture condition.
6. Test the Material
Laboratory testing can help establish whether the material is suitable and capable of meeting the target.
Relevant testing may include:
- Particle-size distribution
- Water content
- Plasticity
- Laboratory compaction
- CBR-related testing
- Material classification
7. Investigate Below the Surface
Confirm the layer thickness and assess the condition of the formation or lower lifts.
Controlled trial pits, probing or another investigation agreed with the responsible engineer may be required where the evidence indicates a deeper problem.
8. Remediate a Defined Area and Retest
Once the likely cause has been identified, remediate a clearly defined area before repeating the test.
The test method, test level and acceptance criteria should remain consistent so that the new result is genuinely comparable with the original result.
Matching the Remedial Work to the Cause
The appropriate remedial action will depend on the cause of the failed plate CBR result.
Under-compacted material may require:
- Scarification
- Moisture conditioning
- Placement in thinner lifts
- A revised roller pattern
- Different compaction equipment
Wet cohesive material may require aeration and time, blending, stabilisation or replacement. Continuing to roll material while it is pumping is unlikely to improve its performance.
Dry material may require controlled water addition and thorough mixing rather than spraying water only onto the surface.
Unsuitable or segregated material may require removal, reprocessing or blending under an approved procedure.
Where the underlying layer is weak, the solution may involve excavation, improved drainage, stabilisation, increased construction thickness or a geosynthetic system. However, these are engineering design decisions.
A plate loading or CBR result identifies performance. It does not, by itself, select the appropriate engineering solution.
How CTS Can Support Plate Loading and CBR Testing
CTS undertakes plate loading testing, equivalent CBR determination, laboratory soils testing and supporting geotechnical investigations across the UK.
Our technical teams combine field observations with laboratory results to help identify whether a failed result relates to:
- Moisture condition
- Insufficient compaction
- Material suitability or variability
- Testing conditions
- Layer thickness
- Underlying ground conditions
Testing is completed in accordance with documented CTS procedures, including requirements for test-area preparation, equipment setup, plate selection, loading sequence and reporting.
By connecting the site result with moisture, density, grading, plasticity and underlying ground conditions, CTS can help ensure that remedial work is based on evidence rather than repeated roller passes and repeated tests.
Contact CTS to discuss plate loading testing, equivalent CBR determination or technical support for a failed ground-strength result.
Frequently Asked Questions About Plate CBR Results
Is a Plate Loading Test the Same as an In-Situ CBR Test?
No. BS 1377-9:2025 identifies plate loading and in-situ CBR testing as separate methods.
A plate loading test assesses the load–settlement response beneath a plate. An in-situ CBR test measures resistance to penetration using the CBR procedure.
What Is an Equivalent CBR Result?
An equivalent CBR is a value derived from a plate loading result using an empirical correlation.
It may be used where the project design and specification accept the correlation, but it is not automatically interchangeable with a directly measured in-situ CBR result.
Why Has My Plate CBR Test Failed?
Common causes include insufficient compaction, unsuitable moisture conditions, variable or inappropriate materials, insufficient layer thickness, weak underlying ground or site and test conditions that do not represent the intended assessment.
The testing method and records should also be reviewed before the cause is determined.
Will Additional Roller Passes Fix a Failed Result?
Not necessarily. Additional roller passes may help where the material is genuinely under-compacted and remains within a suitable moisture range.
However, further rolling may be ineffective or make conditions worse where the material is too wet, unsuitable or positioned above weak underlying ground.
Can Moisture Affect Plate CBR Results?
Yes. Material that is either too wet or too dry may fail to compact effectively or achieve the required bearing response.
The site moisture condition should be compared with the material’s laboratory compaction relationship.
What Should Be Checked Before Retesting?
Before retesting, confirm the testing requirement, test method, equipment setup, plate size, loading sequence, site conditions and acceptance criteria.
The underlying cause should be identified and a defined area remediated before a comparable test is undertaken.