Structural number
From Pavement Interactive
The structural number represents the overall structural requirement needed to sustain the design's traffic loadings. The following equation can be used to relate individual material types and thicknesses to the structural number:
SN = a1D1 + a2D2M2 + a3D3M3
where:
a1, a2, a3 = structural-layer coefficients of the wearing surface, base, and subbase layers, respectively,
D1, D2, D3 = thickness of the wearing surface, base, and subbase layers in inches, respectively, and
M2, M3 = drainage coefficients for the base and subbase, respectively.
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See Also
Principles of Highway Engineering and Traffic Analysis by Fred L. Mannering et. al.