Impact of Geogrid on Cross-Anisotropy and Permanent Deformation of Unbound Granular Materials

This study evaluated the benefits of geogrid reinforcement on unbound granular materials (UGMs) in terms of cross-anisotropy and stress-dependent permanent deformation by using repeated-load triaxial (RLT) tests. One type of crushed granite material and three types of geogrid were selected for the RLT tests. The influence of the aperture type, the sheet stiffness, and the location of the geogrid was quantified in terms of the increase of resilient modulus and the reduction of permanent deformation of the UGMs. The RLT test results indicated that the geogrid reinforced both the vertical and horizontal resilient moduli of the UGM but did not affect its anisotropic ratio. The geogrid with triangular apertures and high sheet stiffness placed in the middle of a UGM specimen provided the most benefits in reinforcing the cross-anisotropic resilient modulus and reducing the permanent deformation. To characterize the stress-dependent permanent deformation of geogrid-reinforced UGMs, a mechanistic–empirical rutting model was proposed by incorporating a softening stress term and a hardening stress term into the Tseng–Lytton model. A new permanent deformation test protocol was developed to determine the model coefficients and to examine the model’s prediction accuracy. The comparison of the model-predicted permanent strain curves with those measured in laboratory tests confirmed that the developed rutting model accurately captured the stress dependences of the permanent deformation for the geogrid-reinforced UGM. The determined rutting model coefficients can be used to predict the permanent deformation of UGMs at any stress level and number of load repetitions.