Not a tonne rating
A 5 percent CBR does not create a 5 tonne limit. There is no direct percentage-to-vehicle conversion.
CBR EXPLAINED · GROUND PROTECTION AND TRACKWAY
CBR measures ground response. It does not approve a vehicle.
California Bearing Ratio is a penetration test used to characterise subgrade and unbound material under defined conditions. It is useful evidence for access planning, but it is not a direct bearing pressure, a tonne limit or a substitute for the complete project assessment.
THE SHORT ANSWER
An accurate project CBR requires appropriate testing of the relevant ground. A higher measured CBR means the tested material resisted the standard plunger more strongly under the recorded test conditions. It does not mean the ground can carry the same percentage of a vehicle weight. Actual suitability still depends on moisture, density, soil structure, weak layers, load footprint, traffic, turning, duration, preparation and the mat system.
01 · DEFINITION
What California Bearing Ratio actually measures.
CBR is a dimensionless index derived from a penetration test. A standard plunger is driven into the test material and the measured force at specified penetrations is compared with a standard reference material. The result is expressed as a percentage.
The number therefore describes penetration resistance for that material, in that condition and under that test procedure. It is widely used in pavement and foundation work as an indicator of subgrade support, but it is not the same as allowable bearing pressure in kilopascals and it does not directly state what vehicle can use a route.
Not a site-wide constant
One result may not represent soft spots, trenches, made ground, services, edges or changing drainage elsewhere on the route.
Not permanent
Moisture, disturbance, remoulding and construction activity can change the material response after testing.
02 · EVIDENCE QUALITY01 · STRONGEST 02 03 04 05 · SCREENING
Use the strongest ground evidence available.
CBR evidence is not binary. The right question is what the value represents, how it was obtained and whether it describes the route at the time it will be used. A measured result is still specific to its location, depth, material condition, moisture state and test method.
Project ground investigation
Relevant investigation and laboratory results with location, depth, material condition and test basis.
In-situ CBR
A direct field test at a defined location and condition under BS 1377-9:2025.
DCP estimate
A penetration profile converted through an appropriate correlation. Useful for identifying weak layers, but still an estimate.
Site observations
Surface, drainage, rutting, standing water, recent weather, photographs and verified site knowledge.
Mapped parent material
Useful regional screening context at 1 km resolution. It does not measure CBR at the work area.
03 · WHY RESULTS CHANGE
CBR belongs to a material and its condition.
Research underpinning UK pavement practice shows that moisture and soil structure materially affect subgrade CBR. Disturbed or remoulded soil can respond differently from the same soil in an undisturbed state. A result should therefore travel with its test condition, location and date.
Moisture and drainage
Wetting can reduce the penetration resistance of many subgrades, particularly cohesive soils. Standing water and poor drainage are operational warnings, not just cosmetic issues.
Density and preparation
Compaction, capping, granular build-up and separation layers can change the supporting system. The designed and constructed build-up matters more than a generic promise that stone improves CBR.
Soil structure and disturbance
Excavation, trafficking and remoulding can change response near formation. Made ground and reinstated trenches require particular caution because variability may govern.
Depth and weak layers
A sound surface can conceal a weaker layer below. Tests and observations must be interpreted at relevant depths and across the actual route.
Season is a risk flag, not a conversion formula. Wet winter conditions often increase the chance of weaker near-surface support, but there is no defensible universal rule that converts an August CBR into a February CBR.
04 · MAPPED DATA
What TrackSpec can infer, and what it cannot.
TrackSpec uses the postcode to locate the project and consults the British Geological Survey Soil Parent Material Model at 1 km resolution. That can add screening context about the dominant mapped parent material and likely soil characteristics.
BGS explicitly states that the 1 km dataset must not be used at finer resolution and must not replace detailed site investigation. It covers natural geological deposits, not man-made construction or materials.
TrackMatz does not describe the postcode output as a site CBR rating. TrackSpec uses mapped context alongside the project inputs to guide an indicative product class. A measured CBR requires appropriate testing of the actual ground.
05 · FROM CBR TO A PROJECT DECISION
CBR is one input. The load path is the decision.
A mat spreads load and protects the surface, but it does not change the underlying soil into a stronger material. Whether a system is suitable depends on how load reaches the ground and how often that loading is repeated.
| Decision input | Why it matters | What TrackMatz needs |
|---|---|---|
| Ground evidence | Defines the quality and uncertainty of the support assumption. | Test information where available, surface description, drainage, photographs and recent weather. |
| Load footprint | Gross weight alone hides axle, wheel, track or outrigger concentration. | Vehicle or plant type, gross weight, axle arrangement and contact footprint. |
| Traffic and movement | Repeated passes, turning, braking and edge loading can govern performance. | Pass frequency, route direction, turning areas and operating controls. |
| Duration and programme | Longer exposure increases the importance of drainage, inspection and changing conditions. | Install date, removal date and the weakest likely programme conditions. |
| Preparation and layout | Formation, separation, joints, straight runs, interfaces and unsupported areas affect load transfer. | Route dimensions, access, offload, laydown, preparation and required layout. |
06 · PRODUCT GUIDANCE
Published figures describe products. They do not create CBR bands.
TrackMatz publishes current supplier guidance so clients can compare the four classes. We do not turn those figures into a fixed low, medium or high CBR tonne matrix.
| Class | Published product position | Site qualification |
|---|---|---|
| TM10 Current specification | Pedestrian-first. Supplier guidance up to 10 tonnes suggested and tested up to 20 tonnes depending on ground conditions. | Occasional cars require ground and traffic assessment. Commercial vehicles, trucks and plant normally require a higher class. |
| TM50 Current specification | Supplier guidance up to 20 tonnes on softer ground and more than 50 tonnes on firm ground. | CBR, footprint and traffic assessed. Cranes, outriggers, steel tracks and heavy repeated traffic require a higher class or specific review. |
| TM120 Current specification | Supplier guidance for loads up to 60 tonnes and vehicles up to 100 tonnes, subject to configuration and ground conditions. | Complete project review required. Cranes, outriggers and steel tracks remain behind written or engineered review. |
| TM200 Current specification | Supplier load-bearing guidance in excess of 200 tonnes, dependent on ground conditions, plus 415 tonnes/m² or 605 psi compressive capacity. | Engineered project review for heavy lift and concentrated loads. The system must not bridge unsupported ground. |
A much lighter vehicle can still be unsuitable on very weak, saturated, disturbed or unsupported ground. Product guidance is not automatic site permission.
07 · WHEN MORE EVIDENCE IS NEEDED
Escalate the evidence with the risk.
- Heavy or concentrated loads, including cranes and outriggers
- Steel-tracked plant or movements outside published product conditions
- Made ground, void risk, reinstated trenches or unsupported areas
- Weak, saturated, pumping or visibly deforming ground
- High traffic, repeated turning or a long programme through changing seasons
- Critical access where loss of route would stop the project or create safety risk
- Conflicting or poor-quality ground information
- Any project requiring a temporary works design or formal engineering approval
TrackMatz can review project information and arrange a project-specific survey. Formal ground investigation or engineering remains a competent specialist responsibility where the risk profile requires it.
08 · CLEAR ANSWERS
CBR questions clients actually ask.
What does a CBR percentage mean?
It is the ratio between the force needed to achieve specified penetration into the test material and the force needed for a standard reference material, expressed as a percentage. It is an index of penetration resistance under the test conditions, not a vehicle weight percentage.
Can I get an accurate CBR from a postcode?
No. A postcode can locate regional mapped context, but an accurate project CBR requires appropriate testing of the relevant ground. The BGS 1 km parent-material model is a screening dataset and must not replace site investigation.
Does a mat increase the soil CBR?
No. A mat can distribute load, protect the surface and provide an operating platform, but it does not change the underlying soil test result. A designed and constructed foundation build-up may improve the supporting system.
What CBR is required for heavy plant?
There is no universal threshold. Suitability depends on ground evidence, footprint, traffic, turning, duration, preparation, layout and the selected system. Heavy or concentrated loads may require formal engineering review.
Does TrackSpec calculate the site CBR?
No. TrackSpec combines user inputs with mapped parent-material screening context and current product rules to give an indicative class and mat quantity. It does not measure site CBR or replace investigation.
Which British Standard covers in-situ CBR testing?
BS 1377-9:2025 covers in-situ soil tests, including the in-situ California Bearing Ratio test. Laboratory compaction-related soil testing is covered separately by BS 1377-4:1990.
09 · PRIMARY SOURCES
References that can be checked directly.
BSI, BS 1377-9:2025
Current British Standard for in-situ soil testing. Its scope includes the in-situ CBR test and dynamic cone penetration testing. Check the BSI record.
Current British Standard for in-situ soil testing. Its scope includes the in-situ CBR test and dynamic cone penetration testing. Check the BSI record.
BSI, BS 1377-4:1990
British Standard for compaction-related laboratory tests on soils for civil engineering purposes. Check the BSI record.
British Standard for compaction-related laboratory tests on soils for civil engineering purposes. Check the BSI record.
British Geological Survey, Soil Parent Material Model
The open 1 km model provides six generalised parent-material parameters. The BGS user guide states that it must not be used at finer resolution or replace detailed site investigation. Dataset record and technical limitations.
The open 1 km model provides six generalised parent-material parameters. The BGS user guide states that it must not be used at finer resolution or replace detailed site investigation. Dataset record and technical limitations.
Transport Research Laboratory, LR 1132
Foundational UK pavement research explaining the limited accuracy of CBR and the influence of moisture and soil structure on subgrade response. Read the report.
Foundational UK pavement research explaining the limited accuracy of CBR and the influence of moisture and soil structure on subgrade response. Read the report.
Transport Research Laboratory, DCP guidance
Explains how dynamic cone penetration profiles can estimate strength and identify anomalously weak layers using an appropriate UK correlation. Read the TRL guidance.
Explains how dynamic cone penetration profiles can estimate strength and identify anomalously weak layers using an appropriate UK correlation. Read the TRL guidance.
Scientific references checked 16 July 2026. This guide supports access planning and procurement discussion. It is not a temporary works design, site investigation or engineering approval.
10 · PROJECT OUTPUT
What your project team receives.
TrackMatz turns the available project information into a practical procurement starting point. The level of certainty depends on the quality of the evidence supplied.
Project information review
Ground conditions, vehicles, load footprints, traffic, duration, layout and programme considered together.
Clear product position
An indicative TrackSpec starting point where appropriate, followed by TrackMatz review before proposal.
Written proposal
The proposed product class and quantity, based on the project information available at that stage.
Current specifications
Relevant product specification sheets available for technical and procurement review.
Visible assumptions
Important limitations, exclusions and missing information identified rather than hidden behind a headline load figure.
Proportionate escalation
A clear request for further evidence, testing, investigation or specialist engineering where the risk requires it.
The objective is a defensible procurement decision, not false precision. TrackMatz does not present mapped postcode context as a measured site CBR or treat supplier figures as automatic permission to proceed.
NEXT STEP
Bring us the ground, load and programme.
For a broad access requirement, compare track mats for hire. For a surface-led brief, see ground protection mats hire. For a defined vehicle route or haul road, see temporary roadway hire. TrackMatz reviews the project details before a proposal is issued.
Same-Day Proposal · 24-Hour Mobilisation · National UK Coverage✉ enquiries@trackmatz.co.uk📞 01785 290390
