# Anisotropic Shear Strength Behavior of Soil-Geogrid Interfaces
## 1 Introduction  
Geogrids have high tensile strength, and are widely used as reinforcements in pavements, embankments, slopes, retaining walls and bridge abutments [1; 2; 3; 4; 5; 6]. In geogrid-reinforced soil, the geogrid interacts with the surrounding soil through interlocking and friction, causing the tensile stresses to be mobilized in the geogrid, leading to the improved shear strength of geogrid-reinforced soil. Therefore, the shear strength of the soil-geogrid interface plays an important role in the stability of reinforced soil structures and is key to the design of internal stability [7; 8].  
Extensive experimental research has been conducted to investigate the behavior of soil-geosynthetic interfaces using the pullout test and the direct shear test [9; 10; 11; 12; 13; 14; 15]. Most of the above research focuses on the influences of different soil properties (e.g., type, friction angle, cohesion, gradation, and particle size) and geosynthetic properties (e.g., type, tensile strength, and aperture size) [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. Research has also been conducted on the influences of testing conditions on the shear strength of soil-geosynthetic interfaces, such as loading rate, cyclic loading frequency and displacement. Corresponding experimental results indicate that testing conditions have significant effects on the shear strength of soil-geosynthetic interfaces [27; 28; 11; 29].  
The above experimental studies typically considered the shear strength behavior of soil-geogrid interfaces with shear loading applied along a specific direction, such as the machine direction (MD) or cross-machine direction (CMD) for biaxial geogrids and thediagonal direction for triaxial geogrids. However, the loading direction of soil-geogrid interfaces in some field situations may be different from those tested in the standard [30], such as geogrids at the corners of retaining walls or bridge abutments, and may even be uncertain during service life, e.g., geogrids in the pavements under traffic loading; for example, the soil-geogrid interaction for shear loading applied in the MD and CMD of the biaxial geogrid are different [31; 32; 33]. Therefore, it is important to investigate the influence of shear direction on the shear strength of soil-geogrid interfaces to ensure appropriate and safe design of these reinforced soil structures.  
In addition, soil particle size also has a significant effect on the soil-geosynthetic interaction [34; 35]. The shear strength of the soil-geosynthetic interface largely depends on the interlocking mechanism between the soil and geosynthetic material, and this interlocking mechanism is significantly affected by particle size [24]. Many studies have analyzed the importance of particle size on the shear strength of soil-geosynthetic interfaces, which is related not only to the type of geosynthetics [35], but also to the ratio of aperture size to average particle size [24; 34]. However, the influence of particle size on the shear strength of soil-geogrid interfaces under different shear directions has not been studied. Therefore, the influence of particle size on the anisotropic shear behavior of the soil-geogrid interface remains to be investigated.  
The goal of this study is to investigate the influence of shear direction on the shear strength behavior of soil-geogrid interfaces, which could be important for the design of reinforced soil structures. Meanwhile, a new type of interface shear test device that can apply shear loading in different directions was developed to investigate the anisotropic shear strength behavior of soil-geogrid interfaces. To the best of the authors' knowledge, this is the first apparatus developed for this type of test. A series of interface shear tests were carried out to investigate the influences of shear direction and particle size on the shear strength of the soil-geogrid interface. The experimental results for the shear strength behavior are presented and discussed to provide insights into the design of geogrid-reinforced soil structures.