During the construction of pavements, gravelly soil is used as the main material in the base and sub-base layers. Predicting the deformation behaviour of gravelly soil has been increasingly important, and this may require comprehensive knowledge about both the mechanical and microstructural behaviours of gravelly materials. This study evaluates the saturated and unsaturated behaviours of 200-mm-high remoulded gravelly soil samples under a large-scale consolidated drained triaxial test. The laboratory tests included physical, mechanical, and microstructural testing on gravelly soil and different sizes of gravel. A series of drained triaxial tests were conducted on gravelly soil under both saturated and unsaturated conditions and on 5 mm, 10 mm, 14 mm, 20 mm, and 25 mm sizes of gravel. The results indicated an increase in the mechanical properties with increasing gravel size. The shear strength parameters were also improved with increasing gravel size. When comparing the results of the gravelly soil under both saturated and unsaturated conditions, the soil had an apparent shear strength that decreased beyond the residual suction during the unsaturated condition. The deviator stress and axial and volumetric strains were significantly dependent on the confining pressure. The microstructural studies confirmed the presence of calcite minerals with granular structures and crystalline textures as the main mineral in the gravelly soil. Therefore, the particle breakage when confining pressure is applied will be minimal, which enables the use of this soil in the base and sub-base layers of the pavement. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.