Structural Number of Flexible Pavement

Equation: AASHTO structural number, SN = a1·D1 + a2·D2·m2 + a3·D3·m3. Each layer contributes its layer coefficient a times its thickness D in inches; the base and subbase are also multiplied by a drainage coefficient m. Values, read from each layer: asphalt D1 = 4 in, a1 = 0.44 (asphalt has no m). Base D2 = 8 in, a2 = 0.14, m2 = 1.0. Subbase D3 = 6 in, a3 = 0.11, m3 = 0.90. Step 1: asphalt, 0.44 × 4 = 1.76. Step 2: base, 0.14 × 8 × 1.0 = 1.12. Step 3: subbase, 0.11 × 6 × 0.90 = 0.59. Step 4: SN = 1.76 + 1.12 + 0.59 = 3.47. Adding a drainage factor to the asphalt layer, or leaving out m3, are the usual slips.