Correlation between Subgrade Reaction Modulus and CBR for Airport Pavement Subgrades

Abstract

Subgrade characterization plays a key role in the thickness design of airport pavements. From a mechanical point of view, a subgrade soil represents a highly diverse heterogeneous structure. The usual practical way to deal with heterogeneous composites is to average their mechanical properties over the material volume. In the current study, the particular soil parameters under consideration are Young's modulus E, subgrade reaction modulus k (k-value), and California Bearing Ratio (CBR). The Federal Aviation Administration's (FAA) FAARFIELD thickness design program embeds a simple mathematical correlation between k-value and CBR. This correlation is conservative in the sense of over-estimating the subgrade reaction for a given CBR value, which encourages designers to unnecessarily increase the design thickness of rigid pavements. On the other hand, the commonly used PCA conversion formula overestimates k-value. Both formulas are not reliable for low-strength soils. The objective of this work is to present an updated conversion formula relating the k-values and CBR for lower CBR soils. A new conversion formula is proposed based on the analysis of available plate load and CBR tests for a range of subgrade soils. Results are shown to be consistent with linear regression of experimental data obtained during tests in the FAA's National Airport Pavement Test Facility.