CBR (California Bearing Ratio) is a measure used in pavement design to evaluate the strength and suitability of subgrade soils. It is an empirical test that determines the ratio of the force required to penetrate a soil sample with a standard circular plunger to the force required to penetrate a standard material (usually crushed stone) with the same plunger under similar conditions. The CBR value is expressed as a percentage.


The CBR value is commonly used in the design of flexible pavements. Flexible pavements are constructed using layers of asphalt or bitumen mixtures that can flex under traffic loads. These pavements distribute the load over a wider area, reducing the stress on the underlying soil. The CBR value helps engineers determine the thickness of each pavement layer and the overall design of the pavement structure.


On the other hand, rigid pavements, such as concrete pavements, are designed using different methods that consider the stiffness and strength of the concrete. The CBR value is not directly used in the design of rigid pavements.


The CBR test provides valuable information about the strength and characteristics of the subgrade soil. It helps engineers assess the load-bearing capacity of the soil and determine its suitability for supporting the pavement structure. The CBR value can vary depending on factors such as moisture content, compaction, and type of soil. Soils with higher CBR values are considered stronger and more suitable for supporting pavements.


In conclusion, the correct answer is option 'B' - CBR is a flexible pavement design method. It is used to evaluate the strength and suitability of subgrade soils and helps engineers determine the thickness and design of flexible pavements. It is not directly used in the design of rigid pavements. Additionally, the CBR test provides valuable information about the soil characteristics and its load-bearing capacity.