Investigation of the Shear Strength Behavior of Clay Soil Reinforced with Basalt Fiber Using Ring Shear Tests

The limited availability of competent foundation soils in rapidly urbanizing regions makes construction on weak clayey deposits increasingly unavoidable. Such soils typically exhibit low shear strength, high compressibility, and pronounced deformation under undrained conditions, posing significant risks to structural safety and long-term serviceability. In this study, the effect of basalt fiber inclusion on the undrained shear behavior of clay soil obtained from the Kızılyer region of Eskişehir, Türkiye, was experimentally investigated using a ring shear apparatus. Initially, soil classification and index property tests were performed to characterize the material. Subsequently, clay specimens were reinforced with varying basalt fiber contents and subjected to large-strain shearing conditions. The evolution of peak and residual shear strength with increasing fiber dosage was systematically evaluated. The results indicate that basalt fiber reinforcement leads to a substantial enhancement in both peak and residual shear strength and contributes to improved post-peak ductility. The observed improvements are primarily attributed to fiber–soil interaction mechanisms, including tensile bridging and crack-arrest effects, which modify the failure process and delay shear localization. Overall, the findings demonstrate that basalt fiber represents an environmentally compatible and mechanically effective alternative for sustainable soil improvement applications, particularly in clayey soils subjected to undrained loading conditions.