Flexible pavement design philosophy: which statement best describes the basis on which flexible pavements are designed in highway engineering?

Introduction / Context:
Flexible pavements are multilayered systems that distribute wheel loads to the subgrade. Because materials (bituminous mixes, granular bases, subgrade soils) are nonlinear, stress-dependent, and climate-sensitive, design cannot rely on pure theory alone. Instead, modern practice blends mechanics with calibration to performance data.

Given Data / Assumptions:

• Design must handle traffic repetitions, seasonal moisture/temperature, and material variability.
• Mechanistic models estimate critical responses (strains, stresses, deflections).
• Empirical transfer functions link responses to observed distresses (rutting, fatigue, roughness).

Concept / Approach:
Mechanistic–empirical (M–E) design uses layered elastic or viscoelastic models to compute tensile strain at the bottom of bituminous layers and compressive strain on top of the subgrade, then applies empirically derived damage relationships to determine layer thicknesses meeting a specified reliability over design traffic (e.g., cumulative standard axles). Historical methods like CBR are empirical but are often interpreted within a mechanistic framework for rational checks.

Step-by-Step Solution:
Model load responses using mechanistic tools (e.g., layered analysis). Compute critical strains and compare to allowable limits via empirical transfer functions. Iterate layer thicknesses until performance criteria (rutting, fatigue) are satisfied.

Verification / Alternative check:
Calibration sections and long-term pavement performance databases confirm that predictions align when empirical models are tuned to local climate/materials, validating the blended approach over purely theoretical or purely empirical methods alone.

Why Other Options Are Wrong:

• Pure theory: Ignores material nonlinearity and site-specific performance.
• Pure empirical: Lacks generality beyond the original calibration domain.
• Exact plasticity: Impractical for routine layered systems and typical inputs.

Common Pitfalls:

• Using uncalibrated empirical models for climates or materials very different from the source database.

Final Answer:
A compromise of pure theory and pure empirical formula (mechanistic–empirical approach).