Geogrid-aggregate interlock provides enhanced local stiffness in the vicinity of the installed geogrid and is a responsible mechanism for the improved performance of geogrid base reinforced pavements. The objective of this study was to establish innovative approaches to quantify the local stiffness increase of geogrid-stabilized aggregate samples. Two pairs of bender elements were installed at two different heights of cylindrical specimens in a repeated load triaxial testing device. Resilient modulus testing was conducted on both geogrid reinforced and unreinforced specimens. Through the use of bender elements, the shear waves were measured during the resilient modulus tests. Experimental results show that the shear moduli estimated from the shear wave velocities increase with bulk stress, regardless of geogrid reinforcement. The shear modulus estimated in the vicinity of the geogrid was always greater than that estimated farther away from the geogrid. According to the preliminary tests conducted so far, geogrid-aggregate interlock related local stiffness increase in unbound aggregate base layers can be effectively quantified by using shear waves.