Effect of relative density on plate loading tests on fibre-reinforced sand

Plate load tests were carried out on un-reinforced sand and sand reinforced with fibres, compacted at relative densities (DR) of 30%, 50% and 90%. For the reinforced sand, 24 mm long polypropylene fibres were added, a concentration of 0.5% by dry weight. The analysis of the results indicates that the soil load-settlement behaviour is significantly influenced by the fibre inclusion, changing the kinematics of failure. The best performance was obtained for the densest (DR = 90%) fibre-sand mixture, where a significant change in the load-settlement behaviour was observed at very small (almost zero) displacements. However, for the loose to medium dense sand (DR = 30% and 50%), significant settlements (50 mm and 30 mm respectively) were required for the differences in the load-settlement responses to appear. The settlement required for this divergence to occur could best be represented using a logarithmic relationship between settlement and relative density. The overall behaviour seems to support the argument that inclusion of fibres increases strength of sandy soil by a mechanism that involves the partial suppression of dilation (and hence produces an increase in effective confining pressure, and a consequent increase in shear strength).