Select Layer Material and Thickness. Once the structural coefficients are known, the
thickness of the individual layers is determined by varying D1
, D2
, and D3 until the calculated
SN is equal to or greater than the required SN. Unbound bases are commonly specified
to the nearest 1 in (25 mm), and asphalt concrete is normally specified to the nearest
1/4 in (6 mm). The procedure shown in Fig. 3.21 illustrates one method recommended for
determining layer thickness. This procedure designs the upper layers to protect the lower
layers