Influence of Fiber Reinforcement on the Ultrasonic P-Wave Velocity and Unconfined Compressive Strength of Cemented Clay

In this study, the sound wave speed (SWS) variation and stress–strain behavior of fiber-reinforced cement-stabilized clay soil samples were investigated through ultrasonic and unconfined compressive strength (UCS) tests. Polypropylene (PP), polyester (PE), polyethylene terephthalate (PET), and glass fiber with several diameters (micro and macro) were used in this study. The results showed that decreasing the fiber unit weight decreases the SWS in any type of fiber. For micro fibers the effect of reinforcement on the SWS and UCS of cement-stabilized clay soil are the same specially in small fiber content, by increasing fiber content the SWS and the UCS increment changes to decrement, the decrement ratio of the SWS is more than the UCS ones. The SWS decreases with increasing fiber content and in a particular amount of fiber by decreasing the fibers diameter the SWS decreases. Single fiber initial modulus, force contribution to soil matrix, elongation at break and the number of fibers in soil matrix are the most significant parameters affecting the UCS and residual strength. By increasing fiber tensile strength and modulus at a constant diameter, the UCS and residual strength increase. Cement-stabilized clay soil’s ductility increases by increasing fiber content and elongation at break but fiber stiffness and diameter have no effect on it. For the PET fiber reinforcement, decreasing the diameter decreases the SWS and the UCS, indicating that for evaluating the effect of fiber in reinforcement the force–strain curve of fiber plays a greater role than the stress–strain curve. By examining the surfaces of the fibers using an optical microscope, it was found that increasing the roughness of the fibers surface increases its force contribution.