Effect of Fiber Reinforcement on the Strength of Geopolymerised Soil: An Experimental Investigation and Numerical Modeling

Volumetric variations in expansive black cotton soil (BCS) lead to low strength bearing ratio with a high degree of compressibility. This vulnerable behavior can be controlled by adding cementitious additives to the soil. In the present study, an experimental program is undertaken to investigate the effects of fiber reinforcement in alkali-activated binder (AAB) treated expansive soil. Two different types of fibers—artificial polypropylene fiber (PF) and chemically treated naturally available coir fiber (TCF) are used. Two different lengths (12.0 and 25.0 mm) and two fiber dosages (i.e., 0.5 and 1.0% by dry weight of soil) are considered for comparison. AAB is prepared by combining an alkaline solution (a mixture of sodium silicate and sodium hydroxide) with aluminosilicate precursors (Class F fly ash) at a 0.4 water to solids ratio. This study analyzes the mechanism of bonding interaction for both fibers reinforced AAB treated soil using stereomicroscopic, Fourier-transform infrared (FTIR) spectroscopy, and scanning electron microscope (SEM) analysis. The unconfined compressive strength (UCS), split tensile strength (STS), and flexural tests for PF-AAB-soil and TCF-AAB-soil are carried out. A 3-point bending test is conducted to investigate the flexural behavior of AAB-treated soil reinforced with PF and TCF after 28 days of curing. The influences of varying fiber dosages in AAB show a significant improvement in shear, tensile, and flexural strength properties of both fiber-reinforced AAB treated soil. The geomechanical results also show that PF reinforced soil attains a higher compressive shear and bonding resistance compared to TCF reinforced soil. Furthermore, numerical analysis using commercially available finite element software is carried out to compare the PF’s flexural failure patterns, and the TCF reinforced soil beam with the experimental results.