Geogrid (uniaxial or biaxial) is one type of geosynthetics that has been successfully used in slopes, walls, roads, and other applications. The main application of biaxial geogrid is to stabilize soft subgrade and reinforce weak base courses by providing lateral confinement. The confinement due to the interaction between aggregates and the ribs of biaxial geogrid depends on the geometry properties of the geogrid, such as rib shape and apertures size, the stiffness of the ribs, and the properties of aggregates. Research has shown that biaxial geogrid cannot provide uniform tensile resistance in all directions. To overcome this problem, a geogrid product with triangular apertures was developed and introduced into the market. Recent studies showed that the triangular aperture geogrid can provide nearly uniform tensile resistance in all directions and is more efficient in improving the performance of reinforced bases as compared with biaxial geogrid. However, the performance of triangular aperture geogrid-reinforced bases under dynamic loading and the influence of base course thickness on the confinement effect of triangular aperture geogrids have not been well evaluated. In this study, unreinforced and triangular aperture geogrid-reinforced bases at different thicknesses over a weak subgrade were constructed in a large geotechnical testing box (2 × 2.2 × 2 m) at the University of Kansas and tested under cyclic loading. During the tests, surface deformations and vertical stresses at the interface between the base and the subgrade were monitored. The test results indicated that triangular aperture geogrids reduced permanent deformation and maximum vertical stress at the interface as compared with the unreinforced bases. The geogrids improved the performance of the aggregate bases at different thicknesses. The benefit became more pronounced when a heavier-duty geogrid was used. The back-calculations from the test data showed that the stress distribution angle and the modulus ratio of base course to subgrade decreased with an increase of the load cycles. The stress distribution angle increased with the increase of the base thickness. The vertical stress distributions were compared with the computed distribution by the layered linear elastic theory. These test data provide the basis for the development of a design method for triangular aperture geogrid-reinforced bases over weak subgrade in the future.