Estimation of In-Situ Shear Strength Parameters for Subgrade Layer Using Non-destructive Testing

Abstract

The Falling Weight Deflectometer (FWD) test has been widely used for evaluating the structural condition and load-carrying capacity of asphalt pavement systems as a non-destructive testing device. Conventionally, the in situ stiffness properties of the various pavement layers are estimated from the analysis of the surface deflection measurements using a backcalculation procedure. In this pilot study, an innovative and novel approach was investigated for estimating the shear strength parameters (C and ϕ) of the subgrade layer by means of FWD testing. Such parameters become important and necessary when assessing the risk of instantaneous shear failure in asphalt pavement layers under non-standard heavy vehicles. In order to assess the applicability of proposed approach, numerically simulated FWD test as well as measured FWD field data conducted on the APT asphalt pavement sections at the National Airport Pavement Test Facility (NAPTF) were analyzed. The surface deflection measurements from the FWD testing at multiple load levels were used in conjunction with backcalculation process to capture the stress dependent behavior of unbound layers and subsequently used to determine the shear strength parameters of the subgrade layer. It was found that the proposed approach was capable of estimating the in situ shear strength parameters of the subgrade material and the results were consistent with those obtained from conventional laboratory testing. Based on the findings from this study, work is currently undergoing to extend and validate the proposed approach for different type of asphalt pavement structures and subgrade properties.