Deep Soil Mixing (DSM) is one of the effective methods widely used in the stabilization of soft and weak soils. For DSM applications, an industrial by-product based geopolymer binder was recommended as a sustainable alternative to cement. The addition of fibers to treated soil arrests crack propagation thus further improving its strength and ductility. In the present investigation, ground granulated blast furnace slag (GGBS) reacted with 8 M NaOH was used as a geopolymer (GP) binder to treat soft clay at high water content. Polypropylene (PP) fibers of 12 mm length were used as reinforcement in 0.25 to 1.0% proportions. Unconfined compressive strength (UCS) and durability (wetting – drying) tests were conducted on the prepared specimens. From the study, the advantages of fiber inclusion in improving the mechanical behaviour and durability of the treated soft soil were highlighted. The GP treated soil mixes with binder content of 30% and Activator/Binder (A/B) ratio of 0.75 reinforced with 1% PP fibers by weight have exhibited superior strength and durability characteristics and thus can be used as sustainable alternative to the traditional binders in deep soil mixing applications. © 2019 Elsevier Ltd. All rights reserved.