Comparison of Effective CBR Value between Japanese Formula and Elastic Two Layer Theories (Presentation 745)

The upper 500 mm of soil, whether in embankment or in cutting, compacted to a higher density than that of the native soil below it is considered as the subgrade of pavement. In case the existing embankment soil is weak, it is a common practice to use borrow materials of suitable quality compacted to a thickness of 500 mm/ 300 mm less or more depending on the practices of the different countries. CBR of the subgrade is often used for design of pavements. IRC: 37-2012 recommended the effective CBR considering contribution of effect of road bed soil. This paper examines the issue of selecting an effective material property, CBR or modulus value, for the combination of embankment soil and subgrade layer. Elastic two layer theories have been adopted for evaluation of effective CBR using IITPAVE program. Variation in tire pressure from 0.56MPa to 0.8MPa is considered along with variation in axle type for analysis purpose. CBR Values are compared with CBR values obtained from Japanese Formula considering damaging effect of different axle configurations. It is found that at lower embankment CBR values Japanese formula gives conservative values. Equivalent CBR obtained from elastic two layer theories is found to be more realistic as it involves the consideration of infinite depth of the embankment soil which found to be missing in Japan Roads Association formula’s for effective CBR. It is observed that effective CBR decreases with decreasing subgrade thickness. Finally on the basis equal subgrade deflection equivalent CBR values for different types of embankment soil and the subgrade layers have been suggested in form of tables.