Multi-scale degradation of polypropylene geostrips retrieved from a 21-year failed geosynthetic-reinforced soil retaining wall

The long-term performance prediction of geosynthetic-reinforced soil (GRS) retaining walls remains challenging due to limited understanding of in-service degradation mechanisms of geosynthetic materials and the scarcity of field-obtained data. To address this issue, the study investigates the long-term degradation behavior of polypropylene (PP) geostrips retrieved from a failed GRS wall after 21 years of service. A multi-scale analytical approach was employed to characterize the degradation of the geostrips, integrating dimensional measurements, tensile testing, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier-transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The results reveal pronounced losses in tensile strength and elongation. At the molecular scale, oxidative chain scission was identified as the dominant degradation pathway, whereas microstructural analyses highlight surface oxidation, cracking, and fibrillation. Importantly, these characteristics vary systematically with burial depth, reflecting distinct stress–environment interactions between shallow and deep layers. Near-surface layers were primarily governed by environmental factors such as moisture fluctuations and salt ingress, whereas deeper layers were mainly dominated by stress–chemical interactions under sustained overburden loads. This depth-dependent degradation profile provides rare long-term field evidence, underscores that current design reduction factors may underestimate actual strength loss, and highlights the need for refining durability-oriented design approaches for GRS walls in diverse environmental settings.