Study on the Static Characteristics of Lignin-Fiber-Reinforced Sand

In this study, compression, rebound, and triaxial tests were conducted to investigate the strength and deformation behavior of lignin-fiber-reinforced sandy soil under various conditions, with a focus on the influence of fiber content (FC) on its mechanical properties. Based on the experimental results, a modified Duncan–Chang model suitable for lignin-fiber-reinforced sandy soil was established. The results indicate that the addition of lignin fibers increases the compressive deformation of sandy soil. Under saturated conditions, the fibers suppress compressive deformation while enhancing rebound deformation, with the minimum compressive deformation observed at an FC of 0.5%. Quantitative analysis shows that as FC increases, the effect of dry and saturated states on compression and rebound indicators gradually diminishes. When the FC reaches 5%, these indicators are no longer significantly affected by moisture conditions. The inclusion of fibers also improves the shear strength of sandy soil. With increasing FC and confining pressure, the stress–strain curves gradually transition to a strain-hardening type. At an FC of 5% and under confining pressures of 100 kPa and 200 kPa, the stress–strain curves exhibit a more pronounced hardening trend compared to those at other fiber contents; under a confining pressure of 300 kPa, the curve exhibits a strain-hardening type. As FC increases, the specimens initially show dilatancy followed by contraction. The curves calculated using the modified Duncan–Chang model are in good agreement with the experimental data, validating the model’s feasibility in capturing softening-type stress–strain behavior.