Mechanical Properties of Subgrade Soil Reinforced with Basalt Fiber and Cement under Freeze–Thaw Cycles

The stability of soil is an essential requirement for various geotechnical engineering projects. The application of composite materials made from cemented soil has become prevalent in road subgrade engineering and foundation treatment due to their affordability, quick construction, and ability to withstand high compression forces. However, the mechanism about the incorporating fibers into cemented soil to enhance strength characteristics, mitigate the formation of microcracks in the soil matrix, and increase frost resistance is still unclear. In this study, a composite improvement method of adding basalt fiber (BF) to cemented soil is proposed, which is to select a single subgrade filling material with most significant freeze–thaw (FT) durability on the basis of traditional cement improvement methods. A series of static/dynamic triaxial compression tests were performed with cemented soil samples reinforced by three BF contents (0, 0.25%, 0.50%, and 0.75%) after FT cycles. The physical properties of these samples were studied, such as the optimal ratio of fiber content, the stress–strain relationship, failure strength, shear strength, and shear modulus, among others. The results revealed that both the shear modulus and failure strength of cemented subgrade soil reinforced with BF showed a significant increase. Compared with cemented soil, fiber-cemented soil exhibited a lower reduction rate in its mechanical properties after 15 FT cycles. The cohesion of the reinforced soil exhibited a gradual decrease as the number of FT cycles increased. Conversely, the friction angle initially decreased but later exhibited an increase. Compared with the reinforcement effects of BF at 0.25% and 0.75%, fiber-reinforced cemented soil with BF content of 0.5% demonstrated the highest strength and performed well in minimizing the effect of FT cycles. It is therefore recommended that ratio of 6% cement and 0.5% BF should be used to enhance the integrity of subgrade filling materials on silty clay.