Unbound aggregate layers are widely used in pavement base or subbase applications for both conventional flexible and rigid pavements. Hydraulic properties, i.e., hydraulic conductivity and moisture-suction characteristics, of these layers dictate their effectiveness in providing adequate drainage, which is essential for long-term pavement performance. Ideally, determining in-situ hydraulic conductivity of an unbound aggregate layer would be most desirable for reliable mechanistic-based pavement design and construction practices. This paper presents findings from a recent field study undertaken at the University of Illinois utilizing an innovative gas permeameter test (GPT) device to measure in-situ hydraulic properties of unsurfaced pavement test sections. The test pavements were constructed using both crushed and uncrushed granular layers on a weak subgrade of controlled strength to study effects of various unbound aggregate material properties (or qualities) on subgrade rutting performance. The repeatability of GPT measurements was quite satisfactory and saturated hydraulic conductivity (Ksat) values obtained were closely linked to field-measured moisture and density properties. The GPT-measured Ksat values were statistically correlated to fines contents in the aggregate matrix, void ratios, and imaging-based quantifiable aggregate shape properties (flat and elongated ratio and angularity index). The different unbound aggregate Ksat values estimated from grain-size distributions using empirical Hazen's and Chapuis' models were also compared quite favorably with the in-situ GPT measurements to verify their applicability and potential for predicting field-permeability properties of pavement base or subbase materials.