The Effect of Discrete and Strand Fibres on The Subgrade and Fill Applications of a Compacted Residual Soil

The use of fibre for the improvement of residual clay soils for road backfill and infrastructure rehabilitation applications was investigated through a series of laboratory model tests. A series of standard tests to evaluate the California Bearing Ratio (CBR) and Indirect Tensile Strength (ITS) of residual clayey soil that was reinforced with strand and discrete fibres (0.6-3.0%) was conducted. The results show improvement in CBR of 25.05% and 19.13% for discrete and strand fibres, respectively, under unsoaked conditions, as well as an increase in ITS with a maximum tensile strength of 37.45 kPa at 1.8% for strand fibres and 30.8 kPa at 2.4% for discrete fibres. A series of laboratory model testing was conducted on fibre reinforced samples that were prepared in a steel reinforced box model (460 mm x 410 mm x 1.0 m). The result revealed that the static bearing capacity of the soil, associated with settlement of 25mm – 50mm, can be improved by the incorporation of 1.8–2.4% strand and 3.0% discrete. Dynamic loading tests, however, revealed that for up to 250 loading cycles associated with a cumulative settlement of 50mm, unreinforced soil and lime stabilized soils exhibited higher stiffness than fibre reinforced soil. Beyond 250 loading cycles, the relatively greater capacity of fibre reinforced soil is associated with the ductility of the soil fibre composite. 1.8% discrete fibre reinforcement exhibited minimal cumulative settlement, while fibre-reinforced and fibre-lime composites demonstrated exceptional resistance to dynamic loading. Notably, strand fibres mobilized greater stiffness compared to discrete fibres. The laboratory model tests provide some insight into the capacity and stability of fibre reinforced clay soil under static and dynamic load conditions in relation to other stabilization protocols, thus offering some guidance to field engineers on ground improvements.