Local Road Management Manual 458 Structural Number The foregoing equations and nomographs determine a required structural number for the given axle loadings, reliability, overall standard deviation, change in PSI, and soil resilient modulus. In addition, the following equation is used to relate individual material types and thickness to the structural number : SN = a1D1 + a2M2D2 + a3M3D3 Where, a1, a2, and a3 = structural - layer coefficients of the wearing surface, base and subbase layers, respectively; D1, D2, and D3 = thicknesses of the wearing surface, base, and subbase layers (in inches), respectively; and M2, M3 = drainage coefficients for various types of material are used to modify the thickness of the lower pavement layers (base and subbase). There are many pavement material combinations and thicknesses that will provide a satisfactory pavement service life. Since there are many combinations of structural-layer coefficients and thicknesses that satisfy equation (4), there is a need to determine the most workable combination under existing local conditions. The economics of materials choice, dictated by haul distance, availability of sufficient amount, cost per cubic meter and material quality are the important determinants of choice