The highest CBR number is required for

Explanation:

The California Bearing Ratio (CBR) is a test used to determine the strength of subgrade and base materials for road and pavement design. A higher CBR value indicates better load-bearing capacity. Among the layers in a pavement system, the subgrade must possess the highest CBR because it serves as the foundation for all upper layers.

Pavement

• Refers to the entire road structure, including surface, base, sub base, and subgrade.

• It is not a single layer, hence not directly assigned a CBR value.

• CBR testing is conducted for individual layers like subgrade or sub base.

Sub grade

• This is the bottom-most layer of the pavement structure.

• It bears the entire load transmitted from the upper layers (sub base, base, and pavement surface).

• A weak subgrade leads to settlement and failure of the road structure.

• Requires the highest CBR to ensure long-term pavement performance.

Sub base

• Lies directly above the subgrade.

• Distributes the load from the base course to the subgrade.

• Requires moderate CBR, but not as high as the subgrade.

• Helps in drainage and frost protection.

Base

• Sits above the sub base and below the surface course.

• Made of high-quality material (e.g., crushed stones).

• Provides structural support and improves load distribution.

• Has high CBR requirements but still relies on a strong subgrade.