Geogrid-stabilised soil has been widely adopted to enhance performance in transportation and infrastructure sectors. In a study, the performance of unstabilised and triaxial-geogrid-stabilised sandy soils in terms of dynamic deformation modulus and elastic modulus was measured using lightweight deflectometer test and plate load tests respectively. The objective of the study was to evaluate: (a) determination of dynamic deformation modulus of sandy soils with different relative compactions from deflectometer testing and compare with the elastic modulus of sandy soils from plate load tests for a given relative compaction of 97%; (b) determination of modulus improvement factors of triaxial-geogrid-stabilised sandy soils through extensive laboratory testing; and (c) demonstration of compaction quality control of well-graded gravel using deflectometer testing through a field study. The triaxial-geogrid-stabilised sandy soil layer showed modulus improvement factors of 1.51 and 1.37 obtained from deflectometer and plate tests corresponding to an embedded depth of 50 mm respectively. The percentage differences in these values are in the range of 4% to 9% for all the embedded depths. Hence a lightweight deflectometer can be used instead of a plate load test as a quick and low-cost alternative for determining the modulus improvement factor of geogrid-stabilised sand soil, both in the laboratory and in the field. © 2023 ICE Publishing: All rights reserved.