## 1 Introduction  
Geogrids are widely used as reinforcements in slopes, walls, roads, and foundations where they are subjected to constant stress throughout their service life [1; 2; 3]. Uniaxial high density polyethylene (HDPE) geogrids are designed to be used in geotechnical structures where soil particles need support over long time periods. HDPE geogrids, due to their high strength and durability, are commonly used for the construction of steep slopes, where the rigid nodes of the geogrids are used to wedge soil in the mesh of the geogrids. Grain aggregates or soil particles pass through the geogrid mesh, partly clogging the spaces between the ribs. The strength and stiffness of the ribs prevent the displacement of soils on the sides but may lead to mechanical damage of the material [3; 4; 5]. The grid interaction with soil is a complex phenomenon and depends on several factors, such as soil type and density, grid geometry and mechanical properties, surface roughness, stress levels, and boundary and loading conditions. Geogrids embedded in soil both during the construction phase and during service life, used as reinforcements, can be exposed to different load conditions. The applied tensile loads on the geosynthetics can be permanent loads (dead weight of soil), repetitive loads (traffic loads), and loads applied during construction [6]. Viscosity is recognized as a significant property of polymeric materials. The loading rate effects, creep deformation, and stress relaxation of polymeric geosynthetics are inherent responses to their viscous properties. The viscous properties of polymeric geosynthetics have been investigated by tensile tests with respect to the loading rate, where the deformation characteristics of polymeric geosynthetic reinforcements are more or less viscous, and the peak strength decreases noticeably with a decrease in the strain rate at failure [7; 8]. These features have been studied by various experimental and theoretical methods [6; 7; 8; 9; 10; 11; 12]. However, these materials are exposed not only to mechanical effects but also to influences of the environment in which they are used, as well as the ageing process. Current standards claim up to 120 ears of design life for the structures; therefore, geogrids as components of such structures must also fulfil this age criterion [13; 14; 15]. The properties of geosynthetic material, including geogrids, generally depend on time. The decrease in the allowable tensile strength depends on short-term effects, such as installation damage, which reduces the maximum tensile strength but does not further affect long-term properties, as well as on effects such as creep and ageing by oxidation and abrasion, which result in a loss in long-term strength [16]. The service life of a structure with geosynthetic material largely depends on its durability over time. The ageing process of geosynthetic materials can be envisioned as the simultaneous combination of physical and chemical ageing [17; 18; 19; 20]. As a result of ageing or degradation, several detrimental effects occur in the polymer: loss of additives and plasticizers, change in molecular weight, formation of free radicals and brittleness [21]. Physical ageing is related to degradation, which does not involve modifications in the molecular structure of the polymer chains. Some of these mechanisms involve mass transfer with the environment surrounding the material (extraction of additive, absorption of solvent, etc.). Others involve modifications of the organization of the internal chain in the material, i.e., change in morphology (chain orientation, crystallinity, etc.) [22]. In physical aging, the material attempts to establish an equilibrium from its as-manufactured non-equilibrium state. As a consequence, no primary (covalent) bonds are broken, and for semi-crystalline polymers such as HDPE, an increase in the material crystallinity occurs [19; 23]. The mechanism of chemical ageing leads to changes in the molecular structure of the polymer chains [14].