This paper reports an investigation into the deformation response of geosynthetic-reinforced soil layers to evaluate the level of strain required to mobilize reinforcement mechanisms during surface loading. Specifically, an assessment of data from previous studies reporting surface settlements of reinforced and unreinforced soil layers during monotonic or cyclic loading to failure is complemented with new dynamic tests performed at small-and medium-strain magnitudes. A series of test sections with and without geosynthetic reinforcement were characterized using a Vibroseis shaker truck as a dynamic loading source. The behavior of uniform sand layers under relatively small strain magnitudes (shear strain magnitudes less than 0.2%) were characterized using embedded geophones. Shear and normal strain distributions within the test sections were measured as a function of depth during application of surface shear loading. The presence of geosynthetic reinforcement (either geogrid or geotextile) at a depth of 254 mm below the surface of the sand layer does not significantly alter the distributions of shear strain or vertical normal strain relative to an unreinforced control section for the magnitudes of shear and compressive strains applied to the soil surface. The behavior of aggregate base layers overlying sand layers under medium strain magnitudes (surface deflections up to 25 mm) were characterized using an array of LVDTs on the soil surface. Surface deflection basins were measured during application of several thousand cycles of compressive loading. The presence of a geogrid within the aggregate base layer or a woven geotextile beneath the aggregate base layer were observed not to lead to a change in the surface deflection basins up to a maximum deflection of 25 mm. These observations, together with results from the literature, indicate that reinforcement mechanisms such as lateral restraint and the tensioned-membrane effect may not be mobilized until reaching relatively large displacements in some soil layers.