Flexible Pavement Thickness Calculator: Thickness From Wheel Load, Subgrade Pressure and Contact Radius
Pavement thickness from the cone pressure distribution.
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How this calculator works
Enter the wheel load and the allowable bearing pressure of the subgrade.
Enter the radius of the contact area, or the tyre pressure and the radius is worked out from it.
The calculator gives the pavement thickness in metres and millimetres.
Formulas used
- t = 0.564\sqrt{\frac{W}{f_1}} - r
- r = \sqrt{\frac{W}{\pi p}}
Worked example
With the default numbers in the calculator above:
- Radius of the contact area: 165.00 mm
- Radius of the load spread at the subgrade: 0.65 m
- Thickness of the pavement in metres: 0.49 m
- Thickness of the pavement in millimetres: 486.25 mm
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: Wheel load 53.5 kN; Allowable bearing pressure of the subgrade 0.14 MPa; How is the contact area given? As a tyre pressure (the radius is worked out from it); Tyre pressure (if the contact area comes from it) 0.62 MPa.
- Radius of the contact area from the tyre pressure: 165.73 mm
- Radius of the load spread at the subgrade: 0.35 m
- Thickness of the pavement in metres: 0.18 m
- Thickness of the pavement in millimetres: 182.92 mm
Common questions
Where does 0.564 come from?
It is one divided by the square root of pi. The load spreads to a circle of area W divided by the bearing pressure at the subgrade, and the radius of that circle is 0.564 times the square root of W divided by f1.
How do I find the contact radius?
The contact area is the wheel load divided by the tyre pressure, and the radius is the square root of that area divided by pi. The calculator does this when you choose the tyre pressure.
What if the answer is negative?
The subgrade can carry the wheel load on its own at that contact area, so no cover is needed for this check.
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Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: Any highway or pavement engineering text (flexible pavement, cone pressure distribution). Use the road authority method (for example Austroads) for a real pavement.
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