In recent years, the use of geopolymer materials for ground improvement has increased with the growth in the construction sector. The lignite coal used in the thermal power plants in Turkey has low-calorie content. Thus, fly ash from these thermal power plants has high-lime content, making it unsuitable for cement production or as a cement additive material. Therefore, it is necessary to find different areas where fly ash can be utilised. In this study, the fly ashes of six power plants were tested to improve the properties of cohesive soil. The effects of fly ash along with the soil properties on improving the physicomechanical properties of the soil were examined with different mix designs. The effects of the curing period on the soil strength values were examined, and the unconfined compressive strength values of all mixtures were compared with those of the controlled specimens prepared with optimum water content. In the literature, seventeen various case studies on fly ash-soil remediation have been investigated. The data of this study and other studies were evaluated together, and multiple regression and artificial neural network analyses were performed to estimate the improved soil strength. Additionally, ternary contour diagrams were designed for assessing the fly ash-mixed soil strength using the physical and mechanical properties of the soil and fly ash.