Transportation agencies commonly use large size aggregates, often referred to as rock cap or aggregate subgrade, e.g., by Illinois Department of Transportation (IDOT), for stabilizing weak subgrades at wet of optimum moisture states. Adequate characterization of these large rocks is not possible in the laboratory with the use of standard tests. Accordingly, a cone penetration based strength index is the best field assessment tool since shear strength profile is closely linked to unbound aggregate or aggregate subgrade layer performance. To this end, an innovative variable energy dynamic cone penetration (DCP) device, popularly known as PANDA in France, was utilized in a recent Illinois Center for Transportation (ICT) research study involving the performance assessment of large size aggregates over soft subgrades. Twelve full scale working platform sections were constructed with six different types of virgin and recycled large size aggregate materials. Accelerated pavement testing (APT) was carried out on these sections to monitor the rutting progression with number of passes of a certain wheel load assembly. To evaluate layer properties and adequately relate them to rutting performance, PANDA tests were conducted along with traditional DCP soundings on the loading applied pavement test section centerlines. A geo-endoscopic probe was also used in the holes opened by the PANDA tests to identify layer interfaces and visually document subsurface moisture conditions. The PANDA and geo-endoscopy testing has proven very beneficial in the performance assessment of the large size aggregate subgrade materials under simulated traffic loading. This paper presents current detailed technical knowledge on the PANDA and geo-endoscopy test equipment and highlights field results associated with the recent ICT project soundings conducted in the pavement working platform test sections.