Optimization of carpet waste fibers and steel slag particles to reinforce expansive soil using response surface methodology

Abstract Expansive soils exhibit great volume change with any moisture variation. In this paper, using carpet waste fibers and steel slag as an environmental friendly additives were taken into account to overcome the swelling and weakness of the expansive soil. Response surface methodology (RSM) was employed to design the experiments, evaluate the results and finally optimize the content of slag-fiber mixture for achieving appropriate values of swell percent, swelling pressure and unconfined compressive strength (UCS). Three independent variables including slag content (0–25% by dry weight of the soil), carpet waste fiber (0.2–3% by dry weight of the soil) and aspect ratio of fibers L/d (5–45) were considered and transformed to coded value, and a second-order polynomial regression equation was then derived to predict responses. The percentage contribution, validity of model and significance of independent variables and their interaction were assessed by analysis of variance (ANOVA). The optimum values of three independent variables was suggested as 14% slag and 0.78% fiber with fiber aspect ratio of 45 to improve swell percent, swelling pressure and UCS 89%, 84%, and 111%, respectively. The results showed that both chemical modification and mechanical reinforcement by adding fibers and slag are efficient methods to improve general properties of expansive clayey soil.