Mechanical response of fiber-reinforced soil
A discrete framework for design of fiber-reinforced soil structure was recently proposed. The equivalent shear strength of fiber-reinforced soil can be predicted using the discrete framework by the independent characterization of soil and of fiber properties. An experimental testing program, including series of triaxial compression tests, triaxial extension tests, fiber tensile tests, and specially designed fiber-pullout tests, was conducted in this research to validate and refine the discrete design methodology. vii The behavior of fiber-reinforced soil, which is governed by the two failure modes (fiber pullout and fiber breakage) identified in the discrete framework, was investigated using triaxial compression tests. Fibers of different tensile strength, including relatively weak fibers, were used as reinforcement in the testing program. The relationships between the equivalent shear strength and the fiber properties (e.g. fiber content, fiber aspect ratio) were evaluated for conditions representative of the various failure modes. Good agreement was found between the experimental results and the equivalent shear strength predicted by the discrete framework. The fiber-soil interface shear strength, which is quantified by the coefficient of interaction in the discrete framework, was also investigated as part of this research. The results obtained from using customized fiber pullout test setup show that a coefficient of interaction of 0.8 is adequate to define the interface shear strength between polypropylene fibers and sand for confining pressures typical of geotechnical projects. The fiber-clay interaction, investigated using a series of triaxial compression tests conducted on 5 fine-grained soils, was found to be more complex than the fibersand interaction. The coefficient of interaction was found to be influenced by factors such as volume change tendency, compaction water content, degree of saturation and strain rate. Fiber inclusions were found to improve the post-peak behavior of the soil. Evaluation of the stress-strain curve of the fiber-reinforced soil indicates that mobilization of the fiber tension generally requires a strain level higher than that corresponding to the peak strength of unreinforced soil. This observation provides viii insight on the selection of soil shear strength parameters (e.g. peak or residual) that should be used in the discrete framework. The self-weight of fibers may lead to a preferential fiber orientation during mixing procedures. The resulting shear strength anisotropy was evaluated by comparing the results obtained from triaxial extension and triaxial compression tests. This comparison showed that preferential orientation of fibers may lead to shear strength value even below that of unreinforced soil.