Definition of flexible pavement: A pavement is categorized as “flexible” primarily when it includes which characteristic layer?

Introduction / Context:
Pavements are classified by how they distribute loads. Flexible pavements rely on bituminous layers with granular support; rigid pavements rely on concrete slab action. Correct identification guides design methods and maintenance strategies.

Given Data / Assumptions:

• We must identify the defining characteristic of a flexible pavement.
• Focus on surfacing/binder layer composition.

Concept / Approach:
Flexible pavements have a bitumen-bound surface (and often binder) course placed over granular base and sub-base. The bituminous layer provides flexibility and distributes loads gradually to lower layers. While WBM and stabilized bases can be components, the presence of a bituminous bound layer is the hallmark of a flexible pavement.

Step-by-Step Solution:
Identify the distinguishing layer: bituminous bound stone/mixture.Recognize that WBM and stabilized bases can appear in flexible systems but are not definitive alone.Select the option that explicitly denotes a bituminous layer.

Verification / Alternative check:
Design standards classify bituminous surfacings (e.g., dense bituminous macadam, bituminous concrete) under flexible pavements; lean concrete bases are associated with rigid or composite systems.

Why Other Options Are Wrong:

• WBM/stabilized soil can exist in both flexible or semi-rigid systems.
• Lean concrete base signals semi-rigid/rigid or composite, not purely flexible.
• All the above: overinclusive and therefore incorrect.

Common Pitfalls:

• Assuming any granular or stabilized base implies flexibility; the surfacing binder governs classification.
• Overlooking composite pavements that mix rigid and flexible features.

Final Answer:
Bitumen-bound stone (bituminous) layer with graded aggregates