Flexible pavement composition: In the layer hierarchy of a flexible pavement, which component forms the lowest (bottom-most) layer supporting the structure?

Introduction / Context:
Flexible pavements distribute wheel loads through a layered system. Understanding the function and order of layers is fundamental to pavement design, construction, and maintenance planning.

Given Data / Assumptions:

• Standard flexible pavement layers are considered.
• We identify the lowest structural layer bearing loads from above.

Concept / Approach:
The typical sequence from bottom to top is: subgrade (compacted natural soil) → sub-base (optional) → base → binder/surface courses. The subgrade provides foundational support; its strength and drainage strongly influence pavement performance and required layer thicknesses.

Step-by-Step Solution:
List layers in order from the ground up.Recognize that the subgrade is the lowest layer and must be well-compacted and drained.Select subgrade as the bottom-most component.

Verification / Alternative check:
Pavement design methods (CBR-based, mechanistic-empirical) explicitly start from the subgrade modulus/CBR to size upper layers, confirming its foundational role.

Why Other Options Are Wrong:

• Sub-base and base/base course lie above the subgrade.
• Not applicable: a clear correct layer exists.

Common Pitfalls:

• Neglecting subgrade stabilization when weak; leads to rutting and early failure.
• Ignoring drainage, which can rapidly degrade subgrade strength.

Final Answer:
Subgrade