Estimation of Resilient Modulus of Subgrade Soils Using Falling Weight Deflectometer

Field and laboratory testing programs were conducted to develop models that predict the resilient modulus (M-r) of subgrade soils from the falling weight deflectometer (FWD) test results. M-r is used in the design and evaluation of pavement structures. The field testing program included testing various sections in 10 pavement projects within Louisiana. The tested sections covered the common subgrade soil types found in Louisiana. Three backcalculation software packages were used to interpret the FWD data: ELMOD 5.1.69, MODULUS 6.0, and EVERCALC 5.0. In addition, the AASHTO and Florida backcalculation equations were used for comparison. The laboratory testing program consisted of conducting repeated load triaxial M-r tests, physical properties, and compaction tests on soil samples obtained from tested sections. Statistical models were developed that can relate the laboratory measured M-r to FWD moduli backcalculated (E-FWD) using each of the methods considered. The results of this study showed that the E-FWD/M-r ratio ranged from 0.51 to 8.1 for the tested subgrade soils. Furthermore, this ratio was higher at lower M-r values and hence weaker subgrade soils. The E-FWD/M-r ratio was also found to be significantly affected by the backcalculation method. The results of the regression analysis showed that among all backcalculated FWD moduli, those backcalculated using ELMOD 5.1.69 software had the best correlation with M-r. Furthermore, the results showed that the M-r prediction was significantly enhanced when soil properties were included as variables within the regression model.