# Effect of Boundary Conditions on the Mechanical Behavior of the Geogrid-Soil Interface
## 1 Introduction
With regard to the top boundary condition, it was generally determined by the loading plate. The positive boundary condition, closed to the clamp in the pullout direction, was determined by the pullout box. At present, to simulate the strength and ductility characteristics of the geosynthetics reinforcement under different normal loading, especially higher normal loading, the test box of the pullout apparatus had to be made of steel to prevent deformation for the duration of the test. In addition, the function of the loading plate was to transfer the vertical load, the airbag or water bag may not be able to withstand too high a normal loading, at the same time, the failure of the air bag or water bag may not ensure the accurate normal loading in the process of test; thus, the current loading plate was also made of steel. [PERSON] [29] and [PERSON] [30] investigated the influence of positive boundary condition and normal loading on the displacement distribution of the geogrid and soil. [PERSON] [31] analyzed the effect of the rigidity of the loading plate on the friction characteristics between quartz sand and HDPE geotextiles. The test results showed that the flexible loading plate could apply uniform pressure distribution compared to the rigid loading plate. The above-mentioned literature mainly studied the effect of a single boundary condition (positive or top) on the friction properties of geogrid reinforcement and soil. It was concluded that the effect of the boundary condition influenced the test results. However, the reinforced zones are generally subjected to both top and positive boundary conditions in the actual reinforced soil structures. In particular, a single boundary condition may not be able to accurately capture the mechanical response of the reinforcement-soil interface. For example, in recent years, the reinforced soil structures with different forms of facing panel, mainly including rigid and flexible, were constructed more frequently. In the pullout test, the rigid or flexible facing panel can be equivalent to positive boundary conditions, especially at the lower portion of the structure. Meanwhile, the top boundary of the structures also include rigid and flexible according to the purpose or materials of the top structure. Examples of boundary conditions commonly used in practical engineering are as follows. The schematic representation of reinforced soil structure widely used at present is shown in Figure 1. It can be observed that the boundary conditions are (a) top rigid and positive rigid, (b) top flexible and positive flexible, respectively. The coupling effect of top boundary and positive boundary conditions on the mechanical properties of the geogrid reinforcement-soil interface, safety, and reliability of the reinforced soil structure lacked detailed research. In addition, the growth pattern pullout resistance offered by geogrid-soil interface subjected to different combinations of boundary conditions lack study. Lastly, the variation law of friction characteristics of the interface under certain the combinations of boundary conditions with the position varying from region A to B and C or from D to E and F, as shown in Figure 1, lack further study. The design uncertainties and unknown performance of geosynthetics such as geogrids in the construction of reinforced structures have hampered their diverse applications and economic performance. As a result, in order to investigate the mechanical behavior of the geogrid-soil interface subjected to different boundary conditions, this study performed a series of laboratory pullout tests on HDPE geogrid-reinforced coarse sand using the large-scale pullout equipment and flexible rubber employed to simulate the top and positive flexible boundary condition. The mechanical response of the geogrid-soil interface was investigated in terms of top and positive boundary conditions. Meanwhile, the geogrid-soil interface's growth pattern and shear strength characteristics under four boundary circumstances were addressed.