## 1 Introduction
This process eventually results in a decline in the mechanical properties and consequently damage to the material. Chemical attack can be launched directly in acidic and alkaline soils or indirectly by active waste, which is present in landfills. Depending on the chemical compound, a change in the structure of the polymer can be obtained by oxidation, chain scission, cross-linking, swelling or dissolution of the polymers. Furthermore, the effect of chemical degradation can be accelerated by temperature [24]. The predominant mechanism of degradation for most polymeric materials (geosynthetics) is chain cutting that occurs as a result of reactions of the polymer, which leads to the breakage of bonds in the backbone of the polymer chain, reduction in the chain length, and thus reduction in the molecular weight [25]. This phenomenon significantly alters the properties of polymeric materials, such as strength and elongation, and consequently affects the geotechnical structures. The oxidation process is initiated by heat (from temperature or UV (ultraviolet) radiation), mechanical stress, catalyst residues derived from the production of the geosynthetics, or reaction with impurities [26]. High density polyethylene (HDPE) geogrids are very resistant to chemical substances and do not easily deteriorate when exposed to alkaline and acid agents (except oxidizing acids), salt solutions, or microbes, because they are non-polar in nature [22]. However, in general there is more than one degradation mechanism operating at a given time, and synergistic effects (changes in pH, changes in temperature and changes in mechanical stress) can accelerate degradation [21]. Geosynthetics made from high density polyethylene have a potential for stress cracking, which is material failure caused by tensile stresses lower than the short-term mechanical strength. Cracking is usually formed in areas or regions with concentrated stress in the microstructure of local inhomogeneities. This phenomenon consists of two phases: crack initiation and crack growth. Environmental stress cracking (ESC) is the bursting of the polymer under tension when exposed to a chemical environment [26]. This failure mode can reduce the life of PE (polyethylene) used for critical applications such as reinforcement applications in a landfill. The first reinforced soil structures in the world were built in France in 1970 and 1971 [27; 28]. In subsequent years, the technique began to be used throughout the world; for example, in the United States, where structures of this type have been built since 1974 [29]. The first permanent geogrid reinforced structure constructed in the U.S. was built in 1982 to support roadway access to the Devils Punch Bowl State Park along the central Oregon coast [30]. This paper focuses on the durability of geogrids used for the reinforcement of a slope in an old sanitary landfill. The analyzed geogrid was installed more than 20 years ago in an old sanitary landfill located in Warsaw, Poland. The geogrid was excavated in October 2013 and then tested by several methods. The geosynthetics were installed in the landfill and exposed to mechanical and chemical factors (e.g., mechanical stress by load and trucks, a wide range of pH values and high temperatures) and were also subjected to changing weather conditions.