# Effect of Boundary Conditions on the Mechanical Behavior of the Geogrid-Soil Interface
## 1 Introduction  
Geosynthetic-reinforced soil structures are extensively adopted in several technical domains, such as civil engineering [1; 2; 3], transportation engineering [4; 5; 6] and hydraulic engineering [7; 8], owing to their noticeable economic advantages and successful performance. Geogrids, one of the important reinforcements in geosynthetics, are known for high strength, low elongation, and durability. Geogrids can prevent soil movement and spread soil stress in the reinforced constructions [9]. The longitudinal ribs, transverse ribs and aperture of geogrid can provide better confinement for infill materials, such as friction resistance and passive bearing capacity [10; 11; 12], as compared to the geotextiles. Hence, geogrids are widely constructed in various reinforced soil structures, such as reinforced soil retaining walls [13; 14], reinforced soil slope [15; 16], and especially steep slopes [1]. The reinforcement effect offered by geogrids is mainly generated by the interaction of the geogrid-soil interface. Meanwhile, the friction properties of the geogrid reinforcement-soil interface has a great impact on the stability of reinforced soil structures [17; 18; 19; 20]. Thus, many researchers focused on the mechanical response of the geogrid reinforcement-soil interface and performed considerable studies. At present, the pullout test is thought to be a good way to explore the geogrid-soil interaction [21; 22]. Simultaneously, some useful results on pullout testing were acquired through laboratory experiments and numerical simulations. [PERSON] [23] investigated the pullout behavior of triaxial geogrids in the apparent soil and analyzed the failure pattern of triaxial geogrids under different normal loading. Under high normal loads, the reinforcing effect of a biaxial geogrid was superior to that of a triaxial geogrid, according to the test findings. [PERSON] [24] conducted a series of pullout experiments to examine the impact of grain size and distribution on the mechanical behavior of geogrid reinforcement-soil interface. In addition, the influence of grain on the shear zones of the interface was also studied by using the finite element software ABAQUS. [PERSON] [25] studied the influence of the aperture ratio (geogrid aperture size/medium grain size) on the mechanical characteristics of the interface. [PERSON] [26] performed a large-scale pullout trial to explore the interaction between the maximum particle size of tire-derived aggregate (TDA) and geosynthesis and observed combined tensile-pullout failure subjected to higher normal loading. [PERSON] [27] presented the findings of lab pullout experiments on geogrid-reinforced soil under static and dynamic stress, as well as a proposal for simplifying dynamic loading research. [PERSON] [28] looked at how cyclic loading history affected post-cyclic pullout resistance. The impacts of numerous parameters on friction characteristics, including the kind of reinforcement, qualities of infill materials, and loading scenarios, have been extensively explored in the aforementioned research. However, few research has been conducted on the influence of boundary conditions on the geogrid-soil interfacial friction characteristics. In addition, the above pullout tests were both subjected to rigid boundary conditions. At the same time, the current geogrid-reinforced soil structure design approach was based on the pullout findings under rigid boundary circumstances. However, the effect of flexible boundary condition was different from that of rigid boundary condition. There was a paucity of study on the influence of boundary conditions on the friction characteristics of the geogrid-soil interface. Therefore, for the precise and safe design of reinforced structures, it is critical to investigate the impact of boundary conditions on the mechanical response of the geogrid-soil interface. Boundary conditions, including top boundary and positive boundary, are mainly related to the pullout apparatus.