CBR Test

The California Bearing Ratio (CBR) Test is an in-situ test that simulates wheel loads and measures the ground’s resistance to penetration compared to a standard crushed rock material. Developed by the California State Highway Department, it is essential to determine the mechanical strength of the subgrade and/or sub-base in road and pavement design and construction.

If you require testing for working platforms, crane outrigger pads, piling mats, etc., please visit our Plate Bearing Test page.

Sub Surface provides CBR Tests nationwide, as described in BS1377: Part 9: 1990: Soils for Civil Engineering Purposes. We offer a professional service with fast mobilisation and quick turnaround results to meet project demands.

Types of In-situ CBR Tests

CBR Tests are performed on the subgrade and/or sub-base of a road. There are two main types of in-situ tests that can be carried out, depending on the particle size of the ground being tested:

CBR Test – A standard CBR Test can be carried out if the particle size is less than 20mm.
Modulus of Subgrade Reaction Test – A Modulus of Subgrade Reaction Test is carried out when the particle size is greater than 20mm.

MModulus of Subgrade Reaction Test (CBR Test) in Bolton, Greater Manchester — Modulus of Subgrade Reaction Test (CBR Test) in Bolton, Greater Manchester

Performing a CBR Test

A standard CBR Test is performed using a cylindrical plunger and loading ring attached to the rear of a fully loaded four-wheel-drive vehicle, which is jacked up to provide a stable deadweight. The test is typically carried out on the subgrade at the proposed formation level.

Steps involved in the CBR Test:

Test Location: Representative areas are selected for testing, every 20 to 30 metres along the road or pavement.
Test Preparation: If the road has not yet been excavated down to the proposed formation level, a hole approximately 300mm in diameter is hand-dug to the formation depth.
Equipment Setup: A cylindrical plunger is placed on the ground to be tested and attached to a loading ring, which is connected to the rear of a fully loaded four-wheel drive vehicle. A dial gauge is attached to the equipment to measure ground penetration.
Load Application: Pressure is applied to the plunger, causing it to penetrate the ground at a rate of 1.0mm per minute. The applied pressure is recorded at every 0.5mm penetration until the plunger has penetrated at least 5.0mm.
CBR Calculation: The pressure recorded to penetrate the ground 2.5mm and 5.0mm is used to calculate the CBR Value.

Performing a Modulus of Subgrade Reaction (MSR) Test

An MSR test is performed using the same apparatus as a Plate Bearing Test, typically using a 600mm diameter plate. The test can be carried out on the subgrade at the proposed formation level or on the subbase once installed.

Steps involved in the MSR Test:

Site Preparation: The test area should be cleared and levelled. If the road has not yet been excavated down to the proposed formation level, a hole to is excavated to formation level for the excavator to track over or track into, if necessary (see photo below), depending on the test depth and the size of the excavator.
Test Preparation: A thin bed of sand is placed on the ground at each test location to fill in any surface voids.
Plate Placement: A circular steel plate, typically 600mm in diameter, is placed on the ground to be tested.
Load Application: A load is applied to the plate in increments using a hydraulic cylinder and jack positioned beneath a tracked excavator.
Settlement Measurement: The settlement of the ground is recorded at each load increment until the settlement has stabilised.
Test Completion: The load is increased at regular intervals until the settlement of the plate reaches 1.25mm.
CBR Calculation: The Modulus of Subgrade reaction (k-value) of the ground is calculated using the load required for a 1.25mm settlement, which is then converted into a CBR value.

Modulus of Subgrade Reaction Test (CBR Test) in Warrington, Cheshire

How Many CBR Tests or MSR Tests Do I need?

CBR and MSR Tests should be undertaken every 20 to 30 metres to determine the CBR value of the subgrade or sub-base as a percentage. The CBR value is then be used to design roads or pavements in accordance with Highways England – Design Manual for Roads and Bridges (HE-DMRB-P CD 225 Revision 1 Design for New Pavement Foundations).

At Sub Surface, we can perform up to ten CBR Tests and eight MSR Tests per day, depending to site conditions and test depths. We offer fast mobilisation and a professional, reliable, and competitively priced service nationwide.

The Importance of CBR Testing

CBR Tests play a crucial role in construction and civil engineering by ensuring ground stability and safety for roads and pavements. These tests help assess the settlement characteristics of the ground, providing essential data for informed decision-making.

Key Benefits of CBR Tests:

Enhances Ground Stability & Safety – Reduces the risk of premature road and pavement failures.
Optimises Design and Construction – Provides accurate data to determine the required subbase thickness for roads and pavements, improving efficiency and reducing unnecessary material costs and waste.
Mitigates Project Delays and Costly Repairs – Identifies potential ground issues early, preventing unexpected failures that could lead to costly delays.
Compliance with Industry Standards – Meeting legal and engineering requirements, enabling roads to be adopted and ensuring reliable, accurate and reproducible results by adhering to British Standards and regulations such as:
- BS1377: Part 4: 1990 – Methods for test for soils for civil engineering purposes (compaction-related tests),
- BS1377: Part 9: 1990 – Methods for test for soils for civil engineering purposes (in-situ tests),
- Eurocode 7 (BS EN 1997-2) – Geotechnical design (ground investigation and testing).

CBR Test in Liverpool, Merseyside

Understanding CBR Test Results

CBR values are expressed as a percentage, with higher values indicating more competent strata. Clays typically achieve a result of around 2%, well-compacted sands up to around 10% and very high-quality subbase between 80% and 100%.

Factors Affecting CBR Test Results

Several factors influence CBR values, including:

Soil Type: Clayey soils generally have a lower CBR value, while granular soils tend to have a higher CBR value. In some cases, you could consider replacing the weaker soils or using stabilisation techniques such as geogrids or lime/cement stabilisation to increase CBR values.
Moisture Content: Higher moisture levels reduce soil strength, so installing better drainage may improve the CBR values.
Compaction: Higher compaction leads to better strength and increased CBR values.

Request a Quote

We are happy to assist with any questions regarding CBR testing or if you would like a quote. Please do not hesitate to contact your Regional Sub Surface Office.

Other In-situ Tests and Services: Plate Bearing Test, Percolation and Soakaway Tests, Ground Investigation

CBR Test FAQs

Do I need a CBR Test, MSR Test or a Plate Bearing Test?

A CBR Test and an MSR Test are used in the design of roads, pavements, car parks, etc. The CBR test is used for materials with a particle size less than 20mm, while the MSR Test is used for materials with a particle size of greater than 20mm. A Plate Bearing Test is used to determine the ultimate bearing capacity and immediate settlement of the ground for working platforms, crane outrigger pads, and piling mats. If you need a plate bearing test, please visit our Plate Bearing Test page.

How long does a CBR Test take?

A CBR Test takes approximately 30 minutes and an MSR Test usually takes a minimum of 45 minutes. However, the actual time depends on the ground conditions encountered on-site and the testing depth. We can typically carry out up to ten CBR Tests and up to eight MSR Tests per day.

How quickly will I receive the results of a CBR Test or an MSR Test?

Depending on the number of tests conducted, the results can usually be made available the next working day.

What equipment is needed for a CBR Test?

A cylindrical plunger, loading ring, dial gauge to measure penetration, vehicle jacks, and a fully loaded four-wheel drive vehicle are required to perform a CBR Test.

What equipment is needed for an MSR Test?

A circular steel plate (usually 600mm in diameter), a hydraulic cylinder and jack, dial gauges and a tracked excavator are needed to perform an MSR Test.

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