Optimization of polypropylene fiber reinforced geopolymer deep mixing column by response surface methodology and investigation of the temperature effect

Deep mixing columns up to 30m in length are one of the deep soil improvement methods frequently used in cohesive and non-cohesive soils. Deep mixing columns, which operate completely buried in the soil, take their cure from the soil environment in which they are formed. In this case, the soil temperature that changes throughout the soil depth is also the curing temperature of the deep mixing columns. The parameters such as annual mean air temperature, surface air temperature, and thermal conductivity of the soil play vital role in the variation of soil temperature along the soil depth. Therefore, mostly the temperature along the soil depth differs significantly from the surface temperature. This causes subjecting the different curing temperatures of the deep mix columns along the soil depth. This affects the bearing capacity and settlement parameters of the columns. Within the scope of the study, soil samples of 50 mm diameter and 100 mm height were prepared considering the recommendations of Federal Highway Administration (FHWA) for deep mixing columns. This thesis study aimed to determine the effect of temperature on the mechanical properties of polypropylene fiber-reinforced geopolymer-based deep mixing column mortar to stabilize high plasticity clayey soils. The study used F-class fly ash as an alkali-activated binder in the geopolymerization process. The mixture of 10 M NaOH solution and Na2SiO3 solutions was used as an alkali activator. The study comprises three stages. In the first stage, alkali-activated precursor and alkali activator amounts were optimized using Response Surface Methodology (RSM) to maximize unconfined compressive strength. In the second stage of the study, the unconfined compressive strengths of the samples obtained by adding different amounts of 6 mm long polypropylene fiber reinforcement to the optimum amount of activator and binder were determined. Thus the reinforcement content that gives the maximum strength was obtained. In the third step of the study, soil samples were prepared considering optimum amounts of binder, activator, and reinforcement additives. The prepared samples were subjected to 5°C, 10°C, 20°C, and 30°C curing temperatures for 7 days and 28 days. The unconfined and triaxial compression tests were performed on the prepared samples. Thus, the effects of curing temperature on the parameters such as unconfined compressive strength, modulus of elasticity, cohesion, and internal friction angle were determined. As a result of the study, it was observed that the mechanical properties of the samples increased significantly with the increasing temperature up to the curing temperature of 20°C, and there was no great change in the mechanical properties above this temperature value.