Fibre-reinforced clayey soil stabilisation: Mechanical performance, ductility and multi-criteria sustainability ranking of natural, waste-derived and synthetic reinforcements

Soil–fibre composites are emerging as sustainable composite materials, now recognised as a viable, sustainable and cost-effective approach to improve the geotechnical behaviour of problematic and weak soils. The present work investigates the engineering response of a clayey soil (CL) reinforced with four different fibre types (human hair, coir, jute and polypropylene) at two levels of reinforcement (0.5% and 1.0% by dry weight of the soil). The Standard Proctor Compaction Test, Unconfined Compressive Strength (UCS) Test, California Bearing Ratio (CBR) Test and an empirical determination of Resilient Modulus (Mr) were conducted on each sample in triplicate and the data analysed by two-way ANOVA. Fibre inclusion resulted in significant improvements in strength, ductility and load-carrying capacity. UCS gains ranged from 25% to 75% and CBR gains from 43% to 128%, depending on fibre type and dosage, relative to the unreinforced control. Polypropylene fibres provided the best mechanical performance, but natural fibres – specifically coir and human hair – achieved similar mechanical strength with significantly better sustainability and cost profiles. Within the tested range, 1.0% was the most effective dosage; dosages above 1.0% were not evaluated in this study. Fibre inclusion caused a slight reduction in Maximum Dry Density (MDD) and a slight increase in Optimum Moisture Content (OMC), both within field-acceptable limits. Human hair (OI = 0.85) and coir (OI = 0.86) emerged as the most balanced choices when a weighted Optimization Index (OI) integrating quantified mechanical, cost and embodied-carbon criteria was applied. Fibre-reinforced stabilization is presented as a lower-embodied-carbon alternative to cement stabilization, based on published material carbon factors, offering lower material cost and greater post-peak ductility. The results indicate the potential of natural and waste fibres in subgrades and embankments, subject to further validation through direct cyclic-triaxial and long-term durability testing.