During the 1998 construction season, the dynamic cone penetrometer (DCP), Loadman portable falling weight deflectometer (PFWD), and Humboldt soil stiffness gauge (SSG) were used to characterize the subgrade and granular base for several projects in Minnesota. The DCP penetration index (DPI) was converted to modulus using previously established correlations between the DPI, California bearing ratio (CBR), and modulus. Standard FWD tests were also performed at some locations and the moduli backcalculated using EVERCALC. The moduli were then compared to determine the ability of each device to accurately measure in situ stiffness. Finally, thin-wall and bag samples were collected from some locations for laboratory resilient modulus testing and the results compared to the field-derived moduli.

The results show the stress dependent nature of the materials tested and that a strong correlation exists between the instruments that are designed to measure modulus. The results also show a weaker, yet still useful, correlation between the strength, as measured with the DCP, and the elastic deformation modulus, measured using the PFWD and SSG.