Investigation of effect of basalt fiber and mineral additives on geotechnical properties of cohesive soils

Clays are soils that display plastic behaviors, have substantially lowered strength at high water content values, and can show significant subsidence. To construct on these soils, the engineering properties of the ground need to be improved. The engineering properties of clayey soils can be improved using chemical additives such as cement, lime, silica fume and fly ash. Additionally, in recent years, the number of studies on the use of fibers in soil improvement has been increasing. In this study, the effects of basalt fiber, fly ash and silica fume additives on the strength, permeability, swelling properties and microstructure of the problematic soil were investigated. Experimental studies were designed using the Taguchi method. Experiments were conducted using the L16 design table with 4 parameters and 4 levels specific to the Taguchi method. Two separated experimental tables were created for basalt fiber-fly ash and basalt fiber-silica fume modified samples. For this purpose, varying values of basalt fiber length (6–24 mm), basalt fiber content (0–1.5%), curing time (1–56 days), fly ash and silica fume content (0–15%) were selected. To increase the pozzolanic activity, 3% lime was added to all mixtures. The effects of parameters on experiment results and the optimum parameter levels have been determined through of S/N and variance analysis. Additionally, the influence of additives on the desiccation cracking of clays has been identified in the study. Specimens that were prepared at the optimum water content and maximum density values were subjected to unconfined compressive, splitting tensile, permeability and soaked CBR tests. According to the results of the statistical analyses, the optimum parameters for the strength experiments were determined as a basalt fiber length of 18 mm, a basalt fiber content of 1%, a silica fume content of 10%, and a fly ash content of 10%. After these levels, the strength has decreased. As the curing time increased, the strength has improved. The most effective parameters on the strength were determined as curing time, silica fume/fly ash content, basalt fiber content, and basalt fiber length, respectively. As the basalt fiber length and content increased, the permeability coefficient also increased. However, with the increase in silica fume and fly ash content, the permeability coefficient decreased. The additives used were effective in reducing the amount of dessication cracks in the clay soil. With the SEM images, the state of the voids and the formation of C-S-H gels in the microstructure structure of the soil samples have been investigated and associated with the strength performance. The results obtained from experimental and statistical studies have been compatible with the literature. Keywords: Basalt fiber, Fly ash, Silica fume, Soil stabilization, Taguchi method