Mechanical stabilization of unbound aggregate layers involves creating a geogrid-aggregate stiffened zone with higher confinement. This paper presents findings from a laboratory study with the objective to quantify stiffness increases in geogrid-stabilized aggregate systems. A bender element (BE) field sensor was utilized to evaluate the local stiffness of unbound aggregates in the vicinity of an installed geogrid. A dense-graded crushed stone aggregate material was compacted in a large-scale laboratory testbed over a soft but uniform support, with and without a geogrid placed at the bottom of the aggregate layer. Static surcharge loading providing different confinement levels was applied to all tests including geogrid stabilization and control section. The BE field sensor measured shear wave velocities were then used to estimate the local stiffness and the extent of the stiffened zone at three different locations above the geogrid. Two punched and drawn geogrids with different aperture sizes were evaluated under the static surcharge loading scenarios. The small strain moduli and the extents of the stiffened zones on top of the two geogrids with different aperture sizes were quantified. The extents of the stiffened zone generated by geogrid stabilization were between 15.2 cm (6 in.) and 25.4 cm (10 in.) above the geogrid, and geogrid with a smaller aperture size was a better match and therefore more effective for the dense-graded aggregate material. The effectiveness of geogrid stabilization depends on the level of confinement of the aggregate layer, geogrid aperture size, and gravel to sand ratio, which was also observed from the stiffened zone characteristics from repeated load triaxial tests in a previous study.