Subgrade characterization employing the falling weight deflectometer

Subgrade soil characterization expressed in terms of resilient modulus (M-r) has become crucial for pavement design. For new pavement design, M-r values are generally obtained by conducting repeated load triaxial tests, employing TP46/P46 protocol. Because of the complexities encountered with the test, in situ tests would be desirable if reliable correlations could be established. This study investigates the viability of using the falling weight deflectometer (FWD) for direct testing of subgrades with the objective of deriving resilient moduli by means of a correlation between FWD moduli and TP46 M-r values. Ten as-built subgrade sections, reflecting typical subgrade soil materials in Mississippi, were selected and tested with the FWD. Undisturbed samples were extracted by using a Shelby tube and tested in a repeated-load triaxial machine for M-r, employing the TP46 protocol. In addition, other routine laboratory tests were conducted to classify the soils. In the suggested approach making use of seven FWD sensor deflections, elastic moduli, E, to E are calculated through forward calculation equations (assuming an elastic half-space). The first sensor modulus, E-1, and an average offset sensor modulus, E3-5 (i.e., the average of E-3, E-4, and E-5), are regressed against M-r, advancing two models for M-r prediction. By employing E-1 and E3-5, two distinct resilient moduli are derived with the lesser of the two serving as the "design resilient modulus." A feature of the approach is that the entry, into the process of both the center sensor modulus and the offset sensor modulus ensures proper subgrade characterization, even when significant nonhomogeneity in a subgrade profile is being encountered.