Triaxial Shear Behaviour of Discrete Fibre-Reinforced Low-Plasticity Clay: A Comparative Assessment of Human Hair, Coir, Jute and Polypropylene

Abstract

Discrete fibre reinforcement is a low-carbon alternative to chemical stabilisation for weak cohesive subgrades and fills, yet the performance of waste and natural fibres relative to a synthetic benchmark under effective-stress conditions is poorly documented. This study compares human hair, coir, jute and polypropylene (PP) fibres at 0.5 and 1.0% by dry mass in a low-plasticity (CL) clay using consolidated-undrained triaxial tests with pore-pressure measurement at 50, 100 and 200 kPa. All fibres increased the peak deviatoric stress at every confinement; the largest gains were obtained with PP 1.0% (51–60%) and coir 1.0% (30–41%), while human hair 1.0% gave 19–35%. The pore pressure at peak, normalised by confining pressure, remained between 0.28 and 0.32 for every mixture, indicating that the strength gain arises from increased deviatoric resistance rather than altered pore-pressure generation. The effective cohesion intercept increased from 11.4 kPa (control) to 20.0 kPa (PP 1.0%); the accompanying rise in φ′ (31.5° to 37.0°) is sensitive to the three-point failure envelope and is reported with uncertainty bounds. The leading positions in the comparative ranking were unchanged under alternative weighting schemes and simulated measurement uncertainty. For practice, 1.0% PP or coir offers a meaningful short-term gain in strength and ductility for low-plasticity clay subgrades, and human hair is a credible waste-derived alternative, but the long-term durability of natural fibres must be verified before permanent use. Because each mixture–pressure condition was tested once, the findings are comparative trends rather than statistically replicated effects.