CBR Method – Governing Layer for Total Pavement Thickness In the California Bearing Ratio (CBR) method for flexible pavement design, the required total pavement thickness is primarily governed by the CBR value of which layer?

Introduction / Context:
The CBR method relates the pavement thickness to the bearing capacity of the supporting soil. The weakest supporting medium dictates how thick the overlying pavement must be to distribute loads and keep subgrade stresses within permissible limits. Understanding which layer’s CBR controls the thickness is fundamental in preliminary design and evaluation.

Given Data / Assumptions:

• Pavement type: flexible pavement.
• Design approach: empirical CBR charts or equations.
• Traffic category and reliability handled separately by charts/adjustments.

Concept / Approach:

CBR design charts index total thickness against the CBR of the subgrade soil because the pavement structure’s role is to protect the subgrade from excessive stresses. While base and surface layers have their own strength/CBR, these values are used to apportion the total thickness into layer-wise requirements. The total thickness, however, is set primarily by the subgrade support level.

Step-by-Step Solution:

Verification / Alternative check:

Poor subgrades (low CBR) produce large total thicknesses on charts; improving subgrade CBR through stabilization or thicker sub-base demonstrably reduces the required total thickness.

Why Other Options Are Wrong:

• Base or surface course: Their CBR affects internal layer distribution, not the total thickness driver.
• All layers: While layer qualities matter, the controlling parameter for total thickness in CBR design is the subgrade CBR.

Common Pitfalls:

Using unsoaked CBR where soaked values are required; ignoring seasonal moisture variation; failing to apply minimum layer thickness constraints.

Final Answer:

Subgrade