What is the correlation between FWD subgrade modulus and CBR?
The CBR test imposes high strain, plastic deformation, in marked contrast to the loading applied to the subgrade to determine resilient modulus which imposes low strain, elastic conditions. There is hence little reason to expect good correlations between CBR and resilient modulus and any values inferred from a mean value relationship could be in error by a factor of 2 or more (AUSTROADS, 1992). It is important to note that the E-CBR relationship for the subgrade used by AUSTROADS, is given by: Ev = 10 CBR, Eh = 5 CBR (because modular anisotropy of 2 is adopted). It follows from the earlier discussion on anisotropy, that the equivalent isotropic modulus of the subgrade, implied by AUSTROADS is: E isotropic = 6.7 CBR This assumes that the subgrade is at a depth of about 300 mm and has a Poisson’s Ratio of 0.45 although there is very little sensitivity to these parameters. The above relationship is clearly more conservative than relationships adopted by other organisations for estimating
