The accuracy of five different backcalculation procedures for in situ characterization of granular/subgrade layers is studied by utilizing deflection data, both Dynaflect and Falling Weight Deflectometer. ELSDEF, BISDEF, MODCOMP2, MODULUS V.4 and FPEDD1/RPEDD1 are the programs evaluated with the authors' data as well as those from the literature. Included in this study is the gyratory resilient moduli of laboratory compacted samples from five sites in Mississippi. Another aspect of backcalculation procedure, lacking total consensus, is how to account for nonlinear response of granular and subgrade materials in a backcalculation algorithm. Patterned after the results of two decades of soil dynamic studies, the authors propose strain-softening models to effect correction of nonlinear behavior. The correction curve, otherwise known as shear strain attenuation curve is implemented in the FPEDD1/RPEDD1 programs. Backcalculated moduli with the FPEDD1 program show satisfactory agreement with laboratory moduli, validating the reasonableness of the strain-softening correction in light load devices. The accuracy of five different backcalculation procedures for in situ characterization of granular/subgrade layers is studied by utilizing deflection data, both Dynaflect and Falling Weight Deflectometer. ELSDEF, BISDEF, MODCOMP2, MODULUS V.4 and FPEDD1/RPEDD1 are the programs evaluated with the authors' data as well as those from the literature. Included in this study is the gyratory resilient moduli of laboratory compacted samples from five sites in Mississippi. Another aspect of backcalculation procedure, lacking total consensus, is how to account for nonlinear response of granular and subgrade materials in a backcalculation algorithm. Patterned after the results of two decades of soil dynamic studies, the authors propose strain-softening models to effect correction of nonlinear behavior. The correction curve, otherwise known as shear strain attenuation curve is implemented in the FPEDD1/RPEDD1 programs. Backcalculated moduli with the FPEDD1 program show satisfactory agreement with laboratory moduli, validating the reasonableness of the strain-softening correction in light load devices.
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