Pavements & traffic · Pavement design
Flexible pavement: mechanistic check (CIRCLY-style) calculator
Mechanistic check of a flexible pavement under the Austroads Standard Axle. Layers are transformed by Odemark's equivalent thickness and the critical strains are computed in an elastic half-space under the two tyres of the dual wheel (each tyre integrated as a 750 kPa circular load): vertical compressive strain at the top of the subgrade and horizontal tensile strain at the bottom of the asphalt, checked against the Austroads transfer functions for the design traffic. A chart shows the cumulative damage as the asphalt thickness varies.
Standard: Austroads AGPT02-17 Sec 8 mechanistic procedure: Standard Axle (80 kN dual tyres, 750 kPa, 330 mm), subgrade N = (9300/με)^7, asphalt fatigue · Odemark–Boussinesq layered approximation
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Layers above subgrade: thickness (mm), modulus E (MPa), Poisson's ratio — asphalt first | 150, 2500, 0.40 250, 350, 0.35 150, 210, 0.35 | |
| Subgrade CBR | 5 | % |
| Subgrade Poisson's ratio | 0.45 | |
| Design traffic DESA | 5000000 | ESA |
| SAR7/ESA (subgrade) | 1.6 | |
| SAR5/ESA (asphalt) | 1.1 | |
| Asphalt binder volume Vb | 11 | % |
| Asphalt reliability factor RF (95 % = 1.0, 90 % = 1.5) | 1 | |
| Load per tyre | 20 | kN |
| Tyre pressure | 750 | kPa |
| Dual tyre spacing (centres) | 330 | mm |
| Odemark correction factor f | 0.9 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Tyre contact radius | 92.1 | mm |
| Equivalent depth to the subgrade (Odemark) | 1,146 | mm |
| Vertical strain at the top of the subgrade | 276 | µε |
| Equivalent asphalt thickness on layer 2 | 260 | mm |
| Tensile strain at the bottom of the asphalt | 161 | µε |
| Allowable repetitions, subgrade (9300/με)^7 | 49,588,129,438 | SAR7 |
| Allowable repetitions, asphalt fatigue | 14,447,018 | SAR5 |
| Cumulative damage subgrade / asphalt | 0.00 / 0.38 |
- ✓ Subgrade rutting CDF ≤ 1 — 0.00
- ✓ Asphalt fatigue CDF ≤ 1 — 0.38
- ✗ Simplified method (verify with CIRCLY for final design) — Odemark–Boussinesq
Method, formulas and assumptions
- Standard Axle: 80 kN single axle with dual tyres, 20 kN per tyre at 750 kPa, 330 mm between tyre centres. Strains are taken under a tyre and midway between the tyres; the larger governs.
- Odemark: he = f·Σhi(Ei/Esub)^(1/3) to the subgrade; the asphalt is transformed onto layer 2 (he1 = f·h1(E1/E2)^(1/3)) and the horizontal strain at that depth is the strain at the bottom of the asphalt (strain continuity).
- Austroads transfer functions: subgrade N = (9300/µε)^7; asphalt N = RF[6918(0.856Vb + 1.08)/(Smix^0.36·µε)]^5. Damage = DESA × SAR/ESA ÷ N.
- This is a preliminary approximation of the layered-elastic (CIRCLY) analysis; granular sublayering and anisotropy are not modelled.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.