Resource-Efficient Fibre Reinforcement of Loess: Stiffness Compatibility, Compaction, Strength, and Failure Mechanisms

Loess is prone to brittle failure and structural collapse, motivating binder-free strategies that improve geotechnical performance while limiting material consumption. This study evaluates fibre effects on compaction, unconfined compressive strength (UCS), and stiffness of loess. Flexible polypropylene (PP) and stiff E-glass fibres, both 6 mm long, were added at 0–1.2% of the dry soil mass. Standard Proctor and unconfined compression tests, supplemented by stereo-microscopic observations, were performed. Fibre addition caused moderate changes in optimum moisture content and maximum dry density, whereas mechanical response depended strongly on fibre type. PP fibres increased UCS by up to 126.9% at 1.2%; however, the gain from 1.0% to 1.2% was comparatively small. E-glass fibres reduced UCS at all dosages, with a maximum decrease of 22.1% at 1.0%. Both fibre systems reduced the secant stiffness modulus (E50) relative to unreinforced loess. A fibre-to-soil stiffness ratio was used as an interpretative parameter, while the Fibre Reinforcement Efficiency Index (FREI) quantified relative UCS change per unit fibre dosage. For PP, maximum FREI occurred at 1.0%, showing that maximum absolute strength did not coincide with maximum dosage-normalised response. The findings support evaluating mechanical benefit relative to fibre dosage when selecting reinforcement for binder-free loess improvement.