CBR: An Effective Measuring Tool

The CBR (California bearing ratio) test is a type of penetration test used to evaluate the mechanical strength of base courses and road subgrades. It was developed before the Second World War by the California State Highway Department, thus the system's name. It was originally developed to measure soil's load bearing capacity and determining its suitability for road-building. Today, CBR is also used for measuring load bearing capacities of soils under paved airstrips and unimproved air strips.

Harder surfaces receive a higher CBR rating. Tilled farmlands, for instance, usually receive a CBR rating of 3, while moist clay or turf is rated at 4.75. Moist sand may offer a CBR of 10. The standard material used for this particular test is crushed California milestone. It has a CBR value of 100.

CBR test is used in the field as part of site investigation. It is performed as an arbitrary strength test, considered to replicate wheel loads and stress soils. It determines the thickness of materials required for the road construction.

The CBR test is done by measuring the amount of pressure it takes to penetrate a soil sample using a plunger of a standard area. The resulting pressure is then divided by the pressure required to achieve equal penetration on the standard crushed rock material. High quality sub bases usually have CBR values between 80 and 100%, maximum.

This soil test is more suitable for testing soils with maximum particle size of up to 20 mm. for material with particles greater than this, plate bearing tests are more appropriate. This technique involves driving a cylindrical plunger at a uniform rate into the tested ground with the help of a four wheel drive vehicle, which serves as the reaction load that provides the force. Newer, more portable devices can now be used to perform plate load bearing tests without the need for large vehicles that limit the capacity of the testing method to wide, open surfaces.

Both CBR testing and plate load bearing testing are carried out at surface level to ensure accurate results. They are normally performed between 500 and 1000 mm, in 20 to 30 m intervals along the proposed centerline of the construction site. Three tests are usually enough to gauge the load-bearing capacity of a particular surface. Typically (assuming that surfaces have been prepared in advance), a single operator can carry out as many as 8 o 10 CBR tests within a day, giving him the capability to churn out results instantly on site.

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