Investigation of the usability of bio-based natural fibers in improving soils

Soils used as subgrade materials in transportation infrastructures may lose their strength over time due to traffic loads and atmospheric conditions. As a result, these soils tend to settle, exhibit low shear strength, and undergo significant compaction, eventually leading to the functional failure of the structures built upon them. Various methods are currently employed to enhance the engineering properties and improve the performance of such problematic soils. In this study, reinforcement using bio-based natural fibrous materials is investigated as an alternative soil improvement technique. These materials are distinguished by their low environmental impact and ease of production. The stabilization of soils reinforced with bio-fibers such as wool, human hair, flax, and wood shavings was examined through compaction parameters (optimum moisture content and maximum dry density) and strength characteristics. Compaction and California Bearing Ratio (CBR) test results were evaluated comparatively before and after reinforcement. CBR was employed as a measure of the strength of subgrade soils and construction materials used in road foundations. To this end, bio-based materials were added to the soil in specified proportions—wood shavings at 5%, 10%, and 20%, and human hair, wool, or flax fibers at 0.5%, 1.0%, and 2.0%. Compaction parameters and strength characteristics of the fiber-soil mixtures were determined using Standard Proctor and CBR tests, and the outcomes were comparatively analyzed. Furthermore, the study discussed the general properties of bio-materials and the practical implications of their usage, especially in regions exposed to extreme environmental conditions (e.g., heavy rainfall and freeze–thaw cycles), by assessing the advantages and limitations of bio-fiber–reinforced soils as subbase and fill materials in transportation infrastructures. Accordingly, the usability of bio-fiber–soil composites as subgrade materials in areas exposed to adverse climatic conditions was evaluated. It was concluded that such additives offer substantial potential in terms of environmental sustainability, low cost, and geotechnical performance. Based on the experimental findings, 1% hair+wool addition improved CBR values by 22% in Soil-1 (SM), 15% in Soil-2 (SC), and 28% in Soil-3 (GC). Moreover, the highest improvement was observed with 2% flax addition in Soil-3 (GC), where the CBR value reached 33%. These results reveal that each type of bio-additive provides notable reinforcement at different proportions, demonstrating their effectiveness in soil stabilization applications. Keywords: California Bearing Ratio (CBR), Compaction, Maximum dry density (γd), Optimum moisture content (Wopt)