A Comparative Subgrade Evaluation Using CBR, Vane Shear, Light Weight Deflectometer, and Resilient Modulus Tests

California Bearing Ratio (CBR) is generally used in airport pavement thickness design procedures to characterize subgrade soils. Alternative thickness design procedure based on layered elastic analysis has also been available (LEDFAA) since 1995 which accepts either subgrade CBR or resilient modulus. The CBR value is converted to resilient modulus in psi by multiplying by 1,500. The relationship is based on a correlation between dynamic modulus and CBR measured for soils and aggregates. A strong trend is apparent in the correlation but there is a lot of scatter. Resilient modulus, in combination with a measure of strength such as shear, could displace CBR as a means of characterizing subgrade soils. Therefore, it is necessary to understand and document the relationships between CBR and other methods of characterizing soils. This study summarizes the preliminary results obtained from subgrade characterization testing (CBR, vane shear, light weight deflectometer, and repeated load triaxial tests) at the FAA NAPTF.