This paper presents a field case study of in situ automated plate load testing (APLT) to evaluate the mechanistic properties of two foundation layer test sections constructed on Illinois Tollway pavement research test sections. One section consisted of nominal 102 mm thick aggregate subbase underlain by nominal 254 mm thick lime stabilized subgrade (LSS) over natural subgrade. The other section consisted of nominal 102 mm thick aggregate subbase over compacted subgrade. The foundation layers were designed to support a cement treated base (CTB) layer and continuously reinforced concrete pavement (CRCP). Testing was conducted to determine in situ composite resilient modulus (Mr) on top of the aggregate subbase layer, the individual layer Mr values, modulus of subgrade reaction (k) values on unstabilized and LSS layers, and permanent deformation (dp) characteristics on the aggregate subbase and subgrade layers. Results show that the pavement foundation conditions were highly variable within and between test sections [coefficient of variation (COV) values ranged between 39% to 63%]. Permanent deformation models were developed to estimate deformations under future trafficking. Stress-dependent universal Mr model parameter values were developed for comparison to typical laboratory Mr values. Falling weight deflectometer (FWD) testing was conducted after the pavement was constructed, and the backcalculated results suggest that the variability of the foundation layers is dampened by the stiffer CRCP and CTB structure.