Investigation of the effect of hollow polyester fibers on the swell behavior of Expansive soil Frzaneh Aghili f.aghili@cv.iut.ac.ir September 20, 2020 Department of Civil Engineering Isfahan University of Technology, Isfahan 84156-83111, Iran Degree: M.Sc Language: Farsi Supervisors: M. Abtahi, Prof. Email address: mabtahi@iut.ac.ir H. Share Isfahani , Prof. Email address: hajarshare@iut.ac.ir Clay Soil swelling control has become a great concern for geotechnical engineers since the first U.S Bureau of Reclamation report in 1938 about the damages caused by clay swelling. This phenomenon is mostly observed in cohesive clay soils. This type of soil is sensitive to water content fluctuations, which can be due to water table level variation, flood, and sewage pipe leakage and etc. This leads to shrinkage in the case of losing water and swelling during the absorption process. These soil types are widely distributed around the world, causing severe damages and injuries. The swelling and shrinkage behavior of soils can be observed by the uplift pressure, applied stress, and created cracks on the surface structures .While the uplift pressure is not able to be converted to expansion due to the high surface structure weight, it will cause crack generation and propagation in the lightweight structure. The swelling intensity significantly related to the electric and chemical attraction and distraction magnitude, mineral types, and the moleculeond of the soil particle. Controlling the swelling of expansive clay soils is always challenging for geotechnicians. In this study, the effect of hollow polyester fiber as additives are experimentally investigated to reduce the free swelling amount and pressure of bentonite. The fibers are employed in spread and concentrated forms. For the spread form fiber percentages of 0.2, 0.4, 0.5, 0.6, 1, and 1.5 are employed 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/m 3 . The obtained results show a significant improvement in the swelling behavior of bentonite for both spread and concentrated forms. Where the column form with 60 and 30 kg/cm 3 density and diameters of 2 and 3.2 cm have the highest reduction in swelling percentage and pressure, 22 and 61 percent, respectively. Keywords Expansive clay soil, Hollow Polyester Fiber, Swelling Pressure, Free Swelling