Example on AASHTO Flexible Method.

1. A flexible pavement is to be designed for a rural
interstate highway to carry a design ESAL of 3.0*106
It is estimate that the subbase for the pavement
structure will be exposed to moisture levels approaching
saturation approximately 5 percent of the time, and the
overall quality of drainage is “Fair”. The base will be
exposed to saturation level moisture approximately 10
percent of the time.
The following additional information is available;
Resilient Modulus for AC @ 68oF = 420000 psi
The granular Base CBR = 70 and Mr = 24000 psi
An untreated subbase has a CBR = 9 and Mr = 10000 psi
The subgrade has a CBR = 1 and Mr = 1500 psi
R = 0.85, So = 0.45, Serviceability Loss = 2 %
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2. 2.5
3.5
6.5
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7. General Procedure for selection of layer thickness
1. Using Mr, determine the structural number SN1
required to protect the base, and compute the
thickness of layer D1
D1 ≥ SN1 / a1
The thickness actually provided may be greater than
the calculated, then the values of SN actually used
may also be increased.
SN1
* = D1 a1
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8. 1. D1 ≥ SN1 / a1 = 2.5/0.42 = 5.95” Use D1
* = 6.0 “
Now SN1
* = D1
* a1 = 6 * 0.42 = 2.52
2. D2 ≥ (SN2-SN1*)/a2m2 = (3.5 – 2.52) / 0.13 (0.95)
= 7.9 Use D2* = 8.0“
Now SN2* -SN1* = D2* a2m2 = 8.0 * 0.13 *0.95 = 0.988
SN2* = 0.988 + 2.52 = 3.50
3. D3 ≥ (SN3-SN1* - SN2* )/a3m3 =(6.5–2.52–3.50)/0.075(0.90)
= 7.1” Use D3* = 7.5”
Now SN3* -SN2* -SN1* = D3* a3m3 =7.5*0.075*0.90 =0.50
SN3* = 0.50 + 2.52 +3.50 = 6.52
By the AASHTO method, the pavement has a 6” surface, 8” base,
and 7.5” subbase
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