As a special soil widely distributed in the southeast coastal areas of China, granite residual soil is greatly influenced by water, leading to significant changes in its mechanical properties and greater susceptibility to disasters. To investigate the shear characteristics of the granite residual soil-geotextile interface, the effects of different moisture contents (12%, 16%, 20%, 24%) and vertical stresses (50, 100, 150, 200 kPa) were analyzed through large-scale direct shear tests. Based on the test results, the PFC2D model was established to reveal the meso-mechanism of the graniteresidual soil-geotextile interface under varying moisture content conditions during the shearing process. The results show that the interfacial shear strength and friction angle decrease with increasing moisture content, while the apparent cohesion increases first and then decreases, reaching the maximum value at a moisture content of 16%. Numerical simulation also shows the morphology of the shear zone and the displacement of soil particles. The presence of geotextile prevents the penetration of force chains between the upper and lower sections, and the primary directions of normal and tangential contact forces remain consistent under different moisture contents. Energy dissipation mainly occurs during the sliding between soil and soil particles.

Water content has a great influence on the mechanical properties of granite residual soil reinforced with geogrids. In order to study the cyclic shear characteristics of granite residual soil-geogrid interface under different water contents, a series of cyclic shear tests were carried out by large-scale indoor direct shear apparatus. The shear stress-shear displacement curves, shear strength, shear stiffness and volume of the soil-reinforced interface were analyzed under four values of water content (13%, 19%, 25%, 32%), three values of normal stress (50, 100, 150 kPa), four shear frequencies (0.2, 0.5, 1, 2 Hz) and four shear amplitudes (5, 10, 15, 20 mm). The test results show that: When the soil is saturated with water content of 32%, the peak shear stress and shear stiffness of the interface increase first and then decrease during the cyclic shearing process. The initial increase of the peak shear stress under the normal stress of 50, 100 and 150 kPa is 6.2%, 22.3% and 33.0%, respectively, indicating that the increase of the normal stress is greater than that of the interface at the initial stage. The interface of soil-reinforced unsaturated soil shows shear softening characteristics. Under different normal stresses, the cyclic shear strength of the interface is negatively correlated with the water content. When the water content is 13%, 19%, 25% and 32%, the final shear shrinkage of the interface is 4.6, 7.7, 8.6 and 7.2 mm, respectively, indicating that the shear shrinkage increases first and then decreases with water content. At each water content, the maximum shear stiffness of the interface decreases first and then increases with the increase of shear frequency, and decreases with the increase of shear amplitude. The shear frequency of 0.5 Hz has the strongest weakening effect on the interface shear stiffness of the interface.

The dynamic shear characteristics of the reinforcement–soil interface affect the stability and durability of reinforced soil–rock mixture subgrades. A series of static and dynamic direct shear tests was conducted on the soil–rock mixture–geotextile interface using a large dynamic direct shear apparatus under different rock contents (0%, 25%, 50%, 75% and 100%). The effects of normal stress amplitude (10, 20, 30, 40 and 60 kPa) and normal loading frequency (0.5, 1.0 and 2.0 Hz) on the shear response of the interface were analyzed. The test results indicate that the shear strength of the upper and lower boundaries of the interface first increases and then decreases with the increase in rock content. This is positively correlated with the normal stress amplitude, while negatively correlated with the normal loading frequency. An increase in rock content amplifies the interface dilatancy effect, while increases in stress amplitude and loading frequency reduce the interface dilatancy effect. The enhancement of the interface friction effect can be attributed to increased rock content and stress amplitude. An empirical formula for the interface friction coefficient, as a function of rock content, stress amplitude, and loading frequency, has been established. This formula coincides well with the test results.

To study the shear characteristics of rubber–sand mixtures, the effects of four rubber–sand mixture gradations (one type of gap gradation, two types of continuous gradations, and one type of open gradation), three rubber contents (10%, 30%, and 60%), and three vertical stresses (30 kPa, 60 kPa, and 90 kPa) on the strength and volumetric change characteristics of rubber–sand mixtures were investigated by using a large-scale laboratory direct shear apparatus. Then, the discrete element models of pure sand and rubber–sand mixtures were established according to the same gradation and rubber content. The intrinsic mechanism of rubber–sand mixtures was explored from the perspective of particle contact state and displacement. The results show that the shear stress curve of rubber–sand mixtures is the same as that of pure sand at low rubber content, but its shear strength is lower than that of pure sand. The shear stress of rubber–sand mixtures increases with the increase in vertical stress, and the shear strength of continuous gradation SR2 is the largest among the four gradations of rubber–sand mixtures. The addition of rubber particles can effectively inhibit the dilatancy of sandy soil, among which the gap gradation SR1 has the best effect on inhibiting soil dilatancy, and the dilatancy is reduced by 37.6% compared with that of pure sand. The internal friction angle of rubber–sand mixtures decreases with the increase of rubber content, and the internal friction angle of continuous gradation SR2 is the largest under the same rubber content. Rubber particles mainly participate in the formation of weak force chain in the force chain network of rubber–sand mixtures, and the shear zone width of rubber–sand mixtures is smaller than that of pure sand.