Automated Dynamic Cone Penetrometer for Subgrade Resilient Modulus Characterization

Abstract

Subgrade soil characterization, expressed as resilient modulus ( M _R), is crucial for pavement design. For a new design, M _R is generally obtained by conducting repeated triaxial tests on reconstituted and undisturbed cylindrical specimens. Because of the test’s complexities, in situ tests are desirable, if reliable correlation is established. This study investigated the viability of using the automated dynamic cone penetrometer (ADCP, abbreviated as DCP) for subgrade characterization through correlation between DCP index (penetration per blow) and M _R. The sensitivity of DCP results to changes in subgrade stiffness, expressed in modulus values, was also investigated. Twelve as-built subgrade sections, reflecting a range of typical Mississippi subgrade materials, were selected and tested with DCP. Undisturbed samples were extracted with a Shelby tube and tested in a repeated triaxial machine for M _R. After other soil physical properties were determined, the soil was classified according to AASHTO procedure. DCP tests were repeated atop the subgrade through drilled holes after construction of pavement layers. Falling weight deflectometer (FWD) testing was conducted near the DCP test locations. Results suggest two relationships, for fine-grain and for coarse-grain soils, in correlating the DCP index to laboratory M _R. Other physical properties helped improve the robustness of the regression models. For model verification, the DCP and M _R tests were repeated and produced good agreement between predicted and actual M _R values. The DCP index before and after pavement layer’s emplacement suggests subgrade stiffness enhancement, owing to pavement overburden, which agrees with FWD-backcalculated moduli.