Flexible Pavement Design Calculator

Roads — Structural Number Design Worksheet

by Shahid Pervaiz · pervaiz.shahid@gmail.com

METHOD AASHTO Guide for Design of
Pavement Structures, 1993
EQUATION Flexible Pavement — Empirical SN Model

A Project Data

Construction completion year

Projected design year

Design life (yr)

Base year ADT (veh/day)

B Design Data

Standard axle load (kips)

Reliability level R (%)

Std. normal deviate Z_R

Overall standard deviation S₀

Initial serviceability P₀

Terminal serviceability Pₜ

Serviceability loss ΔPSI

Soaked CBR of subgrade (%)

Drainage coeff. — base m₂

Drainage coeff. — subbase m₃

C Traffic & Subgrade Input for SN

Design ESAL, W₁₈ (million)

Resilient modulus M_R (psi)

log₁₀(W₁₈) — left side

M_R = 2555 × CBR^0.64 · left side = log₁₀(W₁₈ × 10⁶)

D Design Structural Number (SN)

Design SN (trial value)

Right side of equation, using Design SN

Left side vs. right side

Enter a trial SN and adjust it until the right side matches the left side (log₁₀W₁₈) — this is the Design SN used below to check against the layers provided.

E Asphalt Concrete Wearing Course

Thickness, D₁ (cm)

Layer coefficient a₁

SN₁ = a₁D₁

F Asphalt Concrete Base Course

Thickness, D₂ (cm)

Layer coefficient a₂

SN₂ = a₂D₂

G Aggregate Base Course

Thickness, D₃ (cm)

Layer coefficient a₃

SN₃ = a₃D₃m₂

H Granular Subbase Course

Thickness, D₄ (cm)

Layer coefficient a₄

SN₄ = a₄D₄m₃

Required SN

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Provided SN

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Status

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Layer contribution

Wearing course, a₁D₁

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Asphalt base, a₂D₂

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Aggregate base, a₃D₃m₂

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Granular subbase, a₄D₄m₃

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I Pavement Cross-Section — schematic, not to scale below subgrade line

Subgrade shown schematically — not to scale