Design subgrade CBR for flexible pavements: comparison of predictive methods

Subgrade performance is a function of a soil's strength and its behaviour under traffic loading. The subgrade should be sufficiently stable to prevent excessive rutting and shoving during construction, provide good support for placement and compaction of pavement layers, limit pavement rebound deflections to acceptable limits, restrict the development of excessive permanent deformation (rutting) in the subgrade during the service life of the pavement and minimise effect of changes in moisture level. When the subgrade does not possess these attributes, corrective action in the form of a subgrade treatment is needed. The method of excavation and replacement is commonly adopted in situations where the subgrade soaked CBR is less than the assumed design soaked CBR. This paper discusses various methods used to obtain the design (effective) subgrade CBR for use in a mechanistic design procedure for flexible pavements. The results from the Odemark Transformation Method, both with and without a correction factor, are compared with the results from multi-layered elastic analyses for both isotropic and anisotropic conditions. A new method for calculating the effective subgrade CBR is proposed and validated based on the performance of a number of typical pavement structures.