CBR Testing Procedure

CBR Testing is done by measuring the pressure needed to penetrate soil using a plunger over a standard surface area. The pressure result is divided by the required pressure to perform an identical penetration of crushed rock material. The CBR value depends on the hardness of the surface. Harder surfaces deliver a higher CBR rate. Clay has a 2% value and various other sands have a 10% value.

The test is performed with a round standard cylinder at a rate of 1.25 mm/min required for the penetration of a standard material. Tests are performed on compacted or regular soils, in dry and wet conditions. The results are compared to determine the quality of the soil. Usually, the test is done for street structures or foundations to identify the thickness of materials required for construction. A higher CBR value requires thinner layers of materials required to be used, which can lower the construction costs.

CBR testing is possible only for materials, which have a 20mm particle size. Materials with particles over 20mm are subject to the PBT (Plate Bearing Test) test. The procedure used requires a cylindrical plunder driven into the ground and a vehicle to deliver reaction load and force. Tests are done at depths ranging between 500 and 1000mm along the centreline of the construction at up to 30m intervals. At least 3 tests must be performed on every site to collect the best results and to ensure that the findings reflect the reality. Operators perform up to 10 tests during one day.

The CBR Impact in Clays

Besides dry soil, areas may contain wet clay. Before a development project starts, it's necessary to measure the soil's CBR values to identify what materials are needed for the paving. CBR values don't share consistent correlation with the strength or stiffness. The use of full load penetration is recommended with CBR to identify the type of subgrade material or capping layer needed.

About Flexible Pavement Design

The Flexible pavement design method helps identify the necessary thickness of the pavement. It requires two design methods using the CBR ratio. Several variables are necessary for the procedure including the CBR value of the subgrade, the CBR value of the base course, the CBR value of the sub-base course, and the wheel load (Kg or KN). The wheel load is chosen from the three available groups such as light traffic, medium traffic, and heavy traffic. The pavement thickness required is calculated using the total thickness (T) and the sub-base course thickness value T(sb). The recommended sub-base course thickness is the balance between T and T(sb).

The Plate Load Test and the CBR Test

The plate load test identifies the strength and bearing capacity of the ground. It's used for testing large particles or hard texture soils. The CBR test is used for roads and pavements' designs to evaluate the sub-grade's strength and it's best used in areas with particles of up to 20mm. The Plate Load test is performed according to the British Standard 1377 1990: Part 9 and involves loading a steel plate and registering the results of each load increment. The test is performed at the foundation level. A load is applied in 5 increments and readings are taken every minute until the plate's movement has stopped. During the test, the exerted pressure is added consistently. An excavator or other plant machinery is used as a kentledge. The minimum kentledge weight needed is 8 tons. To test higher loads, other plant machinery is needed. Three plate sizes are used during the test such as 300mm, 450mm, or 600mm. The results of the test include data, the modulus of subgrade reaction, and the load-settlement curve. Also, the CBR value can be extracted from the sub-grade reaction in the Plate Load Test. The Plate Load Test allows for a greater surface area to be tested compared to the CBR test.

The Dynamic Cone Penetration Test

The Dynamic Cone Penetration Test delivers the materials in-situ penetration resistance. The test is done by penetrating the ground with a metal cone by continuous strikes of an 8kg weight, which is dropped from a 575mm height. After each blow, the cone's penetration is measured and it's recorded to deliver a measure of shearing resistance up to 1 meter underground. The test's results can be correlated with the CBR results, bearing capacity, resilient modulus, and in-situ density.

The DCP test on base or sub-grade courses is performed to identify the DCP based CBR values at the field moisture content and density. The dynamic cone penometer is placed on the subgrade site. The test begins by penetrating the sub-ground soil with the hammer. The soil resistance is measured per blow. Three readings must be performed at different locations on the site to determine the average CBR values. Other field tests such as Sieve Analysis, Atterberg's limit test, and the Compaction test should be conducted on-site and in the laboratory.

The CBR value is expressed as a percentage of the actual load causing the penetrations of 2.5 mm or 5.0 mm to the standard loads.

CBR= P/Ps X 100%

Where p = measured pressure for site soils [N/mm2]

Where ps = pressure to achieve equal penetration on standard soil [N/mm2]

Advantages and Disadvantages of CBR Testing

CBR Testing provides a variety of advantages, which make it a necessary test to undertake before starting a construction project such as a road or a pavement. No technical expertise is required and it can be performed using portable equipment. It's applicable to a wide range of materials besides subgrades. Tests can be performed on samples, which are representative of future water conditions. Tests can be done on-site or in a laboratory for construction control, design, or the assessment of existing construction. CBR tests adapt pavement design better than other available tests.

Laboratory and field compaction methods are different. But you can obtain reasonable results' correlation from field materials and samples, which are compacted in the laboratory in identical conditions. Because extra strength to stabilized surfaces is neglected, the assumption of a saturated subgrade condition delivers an unreliable safety factor. Many of the procedures are arbitrary, so the test must be performed to exact standards to provide valid results.

Conclusion

The CBR test is one of the most used techniques to perform an assessment of a sub-level soil quality, the course material, and the sub-base to identify the thickness of materials needed for foundations, pavements, and roads. CBR Testing is performed for the design of housing estates and roads to identify the subgrade soil's strength and to choose the suitable pavement thickness for the expected traffic density. Foundations transfer loads from a structure to soil or rock. You'll find two types of foundations such as spread or deep. The stability of the underlying ground and the settlement influence the design of foundations. Accurate results for in-situ CBR testing can be provided by Dynamic Core Penetration or via Plate Load Testing.