EFFECTIVE GRANULAR MODULUS TO MODEL PAVEMENT RESPONSES

This report presents the results of a research study to further investigate the initial findings of an earlier research project concerning the prediction of pavement deflections. The principal objective of this study was to explore the consistent lack of agreement between field-measured deflections and those computed by using elastic-layered theory coupled with nonlinear dynamic modulus tests. During several periods of the year, surface deflections were measured with the Thumper testing vehicles on the same three Maryland flexible pavement sections that were previously studied. These field deflections were predicted mathematically for a large number of specific test conditions. Although these predicted deflections failed to match the measured values, a consistent trend in the ratio of the corresponding deflections was detected. As suggested in a previous study, an adjustment factor was applied to the granular base modulus to cause the deflection ratio to approach one. Linear log-log relationships were derived between this factor and increasing measured deflection values. From this analysis, it was surmised that current laboratory methods of granular material characterization appear to be inadequate for modeling in situ behavior, regardless of the measuring device. Based on the findings of recent seismic research, further analysis was made to determine whether a relationship exists between the adjustment factor and the induced shear strain in the granular layer. A clear curvilinear plot was produced, which indicated that the adjustment to the granular modulus is definitely related to the shear strain that develops in response to the surface loading. As a result, a procedure was presented for correcting for the effective in situ granular base modulus. (Author)