Abstract. Controlling the swelling of expansive clay soils is always challenging for
geotechnicians. In this study, the effect of hollow polyester fiber as an additive on the free
swelling percentage and pressure reduction of bentonite clay is investigated experimentally.
The fibers are employed in two forms of spread and concentrated. Fiber dosages of 0.2, 0.5, 1,
and 1.5 are employed for the spread form and fiber columns with diameters of 1.2, 2 and 3.2
cm, are considered for the concentrated form with fiber column densities of 30 and 60 kg/m3.
The obtained results show a significant improvement in the swelling behaviour of bentonite for
both the spread and concentrated forms, with better performance for the later form. In which,
higher densities and larger diameters of the fiber columns generally decreases the swelling
percentage and pressure. The best improvement was observed in the concentrated form sample,
with 60 kg/cm3 density and 3.2 cm diameter, with 19 and 57 percent reduction in swelling
percentage and pressure, respectively.