Structural Number with Drainage Factors

Equation: SN = a1·D1 + a2·D2·m2 + a3·D3·m3. The drainage coefficient m reduces a granular layer's contribution when it drains poorly (m < 1). Values, read from each layer: asphalt D1 = 4 in, a1 = 0.44 (no m for asphalt). Base D2 = 8 in, a2 = 0.14, m2 = 0.80. Subbase D3 = 6 in, a3 = 0.11, m3 = 1.0. Step 1: asphalt, 0.44 × 4 = 1.76. Step 2: base, 0.14 × 8 × 0.80 = 0.90. Step 3: subbase, 0.11 × 6 × 1.0 = 0.66. Step 4: SN = 1.76 + 0.90 + 0.66 = 3.32. Ignoring m2 gives 3.54; the poorly draining base contributes 20% less than it would with m2 = 1.0.