Drainage effects on shear behaviour and stiffness of subgrade soils and pavement drainage implications

•Stress–strain behaviour and modulus of subgrade soils under drainage were studied.•Pavement analysis was conducted to link soil testing to structural responses.•Power-function models were observed between modulus and confining pressure changes.•Subgrade drainage improvement for a fatigue-critical pavement may be uneconomic.•Single-stage and multistage loading triaxial tests were performed. Adequate drainage will generally benefit pavement structures and thus extend pavement life. In this study, drainage effects on subgrade and pavement response were quantitatively studied based on laboratory tests and simulation analysis. Single-stage and multistage loading triaxial tests were conducted, including consolidated undrained and drained triaxial tests. Stress–strain behaviour and unloading modulus were obtained for the silty sand. Particularly, pavement analysis was performed using unloading modulus to represent subgrade modulus. The results show that with drainage between loading stages, the specimen showed significant increases in peak and critical state shear strength. Due to drainage, the unloading modulus also showed a significant increase. Power-function relationships were observed between unloading modulus and effective confining pressure changes. The pavement analysis results indicate that drainage effectiveness could be impaired without considering actual engineering conditions, e.g. subgrade drainage improvement for a fatigue-critical pavement may be uneconomic. This study may facilitate the understanding of drainage effects on soil behaviour and pavement response.