Input data
Example data table
| Case | CBR (%) | Wheel load (kN) | Pressure (kPa) | Passes | Safety | Total thickness (mm) |
|---|---|---|---|---|---|---|
| A | 6 | 160 | 1050 | 5000 | 1.10 | ≈ 900–1200 |
| B | 10 | 180 | 1100 | 8000 | 1.10 | ≈ 800–1100 |
| C | 15 | 140 | 950 | 3000 | 1.05 | ≈ 600–900 |
Formula used
Step 1 — Unit conversion
- Convert wheel load per tire to lbf.
- Convert tire pressure to psi.
Step 2 — Contact area (gross)
A = W / p where A is in², W is lbf, and p is psi.
Step 3 — CBR ratio and thickness
Define r = CBR / p (dimensionless). Thickness t is computed in inches using one of these curve fits:
- Cubic fit (default):
t = a · √A · P(log10(r))(empirical polynomial). - Eq. 1/2 blend: uses a threshold on
rand a quadratic adjustment.
Step 4 — Safety factor
t_final = t · SF, then convert to millimeters with mm = in · 25.4.
How to use this calculator
- Enter the subgrade CBR from lab or field data.
- Enter wheel load per tire and select units.
- Enter tire pressure and select units.
- Enter expected passes/coverages for the design traffic.
- Choose a design method and a safety factor.
- Adjust minimum layer thickness values if needed.
- Click Calculate thickness to view results above the form.
- Use Download CSV or Download PDF for reporting.
Technical overview
1) Purpose of preliminary runway thickness checks
Runway flexible pavement thickness is often screened early to validate earthworks levels, drainage concepts, and budget allowances. This calculator estimates total thickness using a CBR-based approach with wheel load, tire pressure, traffic repetitions, and a selectable safety factor. It is most useful for feasibility, tender comparisons, and early design option studies.
2) Inputs that control the result
Subgrade strength is captured by CBR, commonly 3–15% for many sites after treatment. Wheel load is entered per tire (not aircraft gross weight) and is converted internally to lbf. Tire pressure is converted to psi; higher pressure reduces the contact area and typically increases required thickness. Passes/coverages influence the repetition factor, reflecting damage accumulation over traffic.
3) What the intermediate values mean
The contact area is computed as A = W/p. For example, a 180 kN wheel load is approximately 40,465 lbf, and at 1,050 kPa (≈152 psi) the contact area is about 266 in². The ratio r = CBR/p positions the design within the selected curve-fit used to estimate thickness.
4) Layer split and minimum thickness limits
The total thickness is converted to millimeters and distributed into surface, base, and subbase shares. Minimum layer limits are then applied to keep the section practical for construction and performance. If minimums govern, the displayed total becomes the adjusted sum of layers, which is appropriate for take-offs and preliminary section drawings.
5) Professional use notes
Use project-specific traffic, wander, drainage, and materials whenever available. Always confirm with applicable airfield standards and detailed pavement design methods for final sizing. Consider climate, frost, and subgrade variability; a modest safety factor (1.05–1.20) is typical for early-stage checks.
FAQs
1) Is this suitable for final runway pavement design?
No. It is a screening estimator for early planning. Final runway design should follow the governing airfield standards, calibrated traffic assumptions, material properties, drainage, and detailed mechanistic or empirical procedures.
2) What does “wheel load per tire” mean?
It is the load carried by a single tire within the landing gear. If you only know total gear load, divide by the number of tires sharing that load before entering the value.
3) Why does tire pressure change the thickness?
Pressure controls contact area. Higher pressure reduces contact area for the same wheel load, increasing contact stress and usually increasing the computed thickness. The method uses pressure in the CBR/pressure ratio.
4) How should I choose passes/coverages?
Use a reasonable estimate of design traffic repetitions for the critical aircraft family. If you have traffic from an airfield study, use it. Otherwise, test multiple scenarios and compare thickness sensitivity.
5) What safety factor should I apply?
For early design checks, 1.05–1.20 is common. Use higher values when subgrade data is uncertain, drainage is challenging, or construction variability is expected. Keep it consistent across options for comparison.
6) Why does the “adjusted” thickness sometimes exceed the computed thickness?
Minimum layer limits can govern. When surface, base, or subbase minimums are enforced, the sum of layers may exceed the computed total. The calculator reports the adjusted total for a buildable section.
7) Can I use this for rigid pavement?
No. This tool estimates flexible pavement thickness from CBR-based relationships. Rigid pavement design depends on concrete properties, slab dimensions, load transfer, and different stress models.