Rainfall causes an increase in water in the soil. In the early stages of rainfall, the soil surface absorbs water, causing the water content to increase rapidly. Short-term rainfall may only affect the top layer of the soil; however, when rainfall is heavy or of longer duration, water gradually infiltrates into the deeper layers of the soil, resulting in an increase in the deep soil water content, especially in the case of good soil structure and strong drainage capacity. In actual engineering, earth dams, road embankments, and other fill slopes as well as natural soil slopes may undergo strength damage due to reduced soil stability under overloading, seepage or even heavy rainfall, which may cause natural disasters such as landslides [1,2,3,4].

Reinforced soil structure has good shear strength and seismic performance, and plays an important role in many fields such as retaining walls, foundation treatment, slope protection, and so on [5,6,7,8]. The working principle of reinforced soil is to use the friction between the soil and the reinforcement to transfer the tensile stress in the soil to the reinforcement, which will bear the tensile force, and the soil will bear the soil pressure and shear stress, so that both the reinforcement and the soil in the reinforced soil can play their respective roles well. The shear strength of the geogrid–soil interface is a crucial indicator, and the problem of soil strength is essentially the problem of shear strength. The direct shear test is a common test methods for assessing the shear strength of the geogrid–soil interface.

There are many studies on the mechanical behavior of the geogrid–soil interface, mainly focusing on the type and compactness of the soil, the characteristics and arrangements of the reinforcement, and the methods and conditions of the test. Morsy et al. [9,10] studied the interaction between reinforcement and soil under different vertical spacing, normal stress, and tensile strain of reinforcement through experiments. It was found that the influence of normal stress on the active load transfer can be neglected, but when the active load is large, the decrease in normal stress will reduce the transfer amount. In addition, the change in lateral earth pressure is proportional to the vertical spacing and the tensile strain of reinforcement, and is inversely proportional to the vertical stress, and the normal stress will affect the thickness of the shear zone. Sayão et al. [11] found that the inclination angle of the reinforcement had a significant effect on the strength of the soil body, and the strength of the soil body was at its maximum when the inclination angle of the geogrid arrangement was at 60°. Nhema et al. [12] analyzed the effects of discrete fiber strips on the mechanical properties of reinforced sandy soils by means of a series of isotropic direct shear tests in which the initial orientation of the fiber strips was strictly controlled. Infante et al. [13] found that the shear strength of geogrid-reinforced specimens was generally better than that of geotextile-reinforced specimens. Makkar et al. [14] compared and analyzed the effect of triangular and rectangular forms of reinforcement on interfacial shear strength, and found that 3D geogrid-reinforced soils possessed higher interfacial shear strength than un-reinforced and planar geogrid-reinforced soils. Linhares et al. [15] conducted a parametric study on different combinations of additional load width, wall height, compaction-induced stress (CIS), and wall inclination. The results show that the maximum reinforcement load and lateral displacement of the GRS wall are different under different combinations. Liu et al. investigated the effects of particle shape, particle size ratio, concrete surface roughness, moisture content and displacement amplitudes on the cyclic shear characteristics of reinforced soil interface [16,17,18,19,20].

The widespread use of reinforcement in engineering practice has shown that they are essential for improving the soil properties under actual working conditions. The effect of rainfall on soil is mainly reflected in the change in moisture content. Therefore, an in-depth study of soil moisture content is important for soil prevention and control under rainfall. Ferreira et al. [21] found that an increase in soil moisture content can significantly reduce the shear strength of the reinforced soil interface. Vieira et al. [22] studied the interface characteristics of building demolition waste as a filler material for reinforced structures. It was found that the interfacial shear strength between it and geosynthetics after proper compaction was affected by water content, and high-water content would lead to a decrease in strength. Ensani et al. [23] studied the interfacial shear strength of five geosynthetics in marginal clay sand under different water contents through large-scale direct shear tests. The results show that compared with the optimal water content, the cohesion and internal friction angle decrease significantly when the water content increases by 6%. Namjoo et al. [24] found that the interfacial friction angle and friction force of the specimens increased with decreasing moisture content by using the direct shear test and pullout test, respectively.