# Experimental Study on Cyclic Shear Performance of the Four-Way Geogrid Reinforcement-Soil Interface
## 1 Introduction  
A geogrid is a type of geosynthetic material with high operational efficiency that is extensively implemented in reinforced soil structures [1; 2; 3; 4; 5; 6]. The interaction mechanism between grids and soil is a crucial topic in geogrid research. Several studies have been conducted by scholars at experimental, numerical simulation, and theoretical analysis levels. These studies have found that the mechanical properties of the grids, soil particle gradation, and particle size are the key factors that affect the geogrid-soil interface. However, most of the studies have been based on static loading conditions [7; 8; 9; 10; 11].  
During reinforced soil projects, the structure is exposed to dynamic factors such as traffic loads, wave loads, and mechanical loads resulting in cyclic shear at the geogrid-soil interface. Consequently, numerous scholars have begun emphasizing the shear characteristics of the interface under dynamic effects due to cyclic loading. [PERSON] and [PERSON] [12] employed a cyclic loading shear instrument to investigate the fundamental principles and force deformation mechanism of interfacial mechanical properties between coarse-grained soil and structural contact surfaces subjected to repeated loading. The results revealed that two physical change mechanisms, namely, granular crushing and compression of soil near the structure surface, occurred during the shear process. [PERSON] et al. [13] found the enhancement of peak shear stresses by cyclic shear through horizontal cyclic direct shear tests and pointed out that the shear displacement amplitude affects the shear shrinkage. [PERSON] et al. [14] investigated the effect of the particle size of the soil on the cycleshear behavior of the interface and found that the interfacial shrinkage increases as the particle size increases. [PERSON] et al. [15] performed cyclic direct shear tests on geogrid-gravel interfaces, simulating the effects of seismic loading on a reinforced soil structure. They found that the shear stiffness of the geogrid-gravel interface increased with the increase in normal load.  
The majority of research on the cyclic shear properties of the geogrid-soil interface focuses on uniaxial or biaxial geogrids. [PERSON] et al. [16] studied the interfacial properties of uniaxial and biaxial plastic geogrids by means of pullout and straight shear tests, and found that for unidirectional grids with reinforcement, the pullout and straight shear curves are usually strain-softened, while for bi-directional grids with reinforcement, the curves are generally strain-hardened. [PERSON] et al. [17] investigated the effect of cyclic loading on the displacement of uniaxial polyester geogrids by carrying out multistage pulling tests with a large pulling device. [PERSON] et al. [18] proposed a trilinear shear stress-displacement damage softening model for a unidirectional geogrid-soil interface based on the results of pullout tests. [PERSON] et al. [19] fabricated uniaxial, d biaxial, and triaxial polylactic acid geogrids based on 3D-printing technology, and investigated the mechanical properties of these grids through an indoor pullout test. There is little research literature on four-way geogrids. [PERSON] et al. [20] designed a new four-way geogrid and demonstrated the feasibility of the design through mechanical test and numerical simulation. In summary, experimental and theoretical research on the cyclic shear performance of the four-way geogrid-soil interface lags behind its engineering application, and the advantages and disadvantages of four-way geogrid reinforcement remain unclear. This paper investigates the cyclic shear characteristics of four-way geogrid-soil interfaces under different normal stress and shear amplitudes using an indoor large-scale dynamic straight shear test system. The purpose is to uncover the reinforcing mechanism of the interface, improve the selection of reinforced soil engineering, and promote the significant application of new materials.