Estimating Subgrade Stiffness and Bedrock Depth: Combined Falling Weight Deflectometer and Simplified Spectral Analysis of Surface Waves Measurements

Abstract

The spectral analysis of surface waves (SASW) method is an in situ seismic technique that is used for evaluation of the stiffnesses of pavement systems at low strain levels. The stiffness of the surface layer can be determined by direct measurements in the field. Stiffnesses of the layers beneath the surface layer require forward modeling of SASW field data in order for the pavement profile to be obtained. The forward modeling process can be time consuming, especially if a three-dimensional model is used. A simplified procedure is proposed for determining an average value of the subgrade stiffness without performing forward modeling. Additionally, the simplified procedure can be used for determining the depth of shallow bedrock beneath pavement sites. The recommended procedure is based on SASW tests performed at 24 flexible pavement sections in the state of Texas. Parametric studies were also conducted with idealized rigid and flexible pavement profiles for the purpose of verifying the accuracy of the procedure and evaluating its limitations. An important point is that SASW measurements associated with the simplified procedure can be performed in conjunction with falling weight deflectometer (FWD) measurements using the drop weight as a source. The drop-weight source generates the frequency range required for SASW receiver spacings of 3, 6, and 9 m, which are recommended in the simplified procedure. The subgrade stiffness and depth to bedrock (if it exists) determined by the simplified procedure can be used as input parameters for enhancement of the backcalculation procedure associated with FWD measurements.

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