How important is accurate layer thickness information?
It is usually important to know the thickness of any structural AC layer, if the adopted method of analysis calculates tensile strains at the base of that layer. It is less important if the method uses only curvature functions. If thicknesses of the granular layers are not known, then sensitivity anlyses may be carried for a series of possible thicknesses to find out what differences in overlay requirements are indicated, and also decide on layer thicknesses that result in moduli consistent with the values typically achieved in subbase and basecourse materials. Comparisons with moduli found in the layers of other pavements in the same area are also used to arrive at likely layer thicknesses. Preferably, some test pits information or as-builts are desirable, but much fewer than are needed if FWD bowls have not been measured. In some instances, test pit information from old roads may not fit closely with the back-analysed model. Reasons for this are that the test pit may relate only to a
