Evaluation of factors affecting the shear strength of the biologically treated sandy soil using Taguchi method

Population and civil infrastructure continue to expand at unprecedented rates. On the one hand, the growing needs for the development and, on the other hand, the environmental crisis, stress the importance of finding methods not harming the environment while they are able to meet the requirements for development. Infrastructure demands are even more severe in other countries, particularly in developing ones. Infrastructure is insufficient in countries such as China, where 10 million people immigrate to major cities each year. Population growth is particularly acute for historic cities and regions where expansion is limited by geographical boundaries and inadequate soil conditions. The confluence of these factors necessitates the exploration and development of new alternative soil improvement methods and associated reliable monitoring techniques. Bio-mediated soil improvement is an innovative, and interdisciplinary technique with the approach of being environmentally friendly, which utilizes some bacteria utilizing some bacteria to precipitate calcite on soil particles. In addition, this system broadly refers to a chemical reaction network that is managed and controlled within soil through biological activity and whose byproducts alter the engineering properties of soil. Therefore, Microbial carbonate precipitation (MCP) has experienced an increased level of interest in recent years for applications such as restoration of calcareous stone materials , bioremediation, wastewater treatment, strengthening of concrete and selective plugging for enhanced oil recovery. In this research, to attain the highest number of experiments without repeating the unnecessary ones, Taguchi design method was utilized. The Taguchi method was developed to improve the implementation of total quality control. The effect of factors on characteristic properties (response) and the optimal conditions of factors can be determined using the Taguchi design. It is feasible to find out the optimal experimental conditions with the least variability. Taguchi analysis is based on choosing the best run by analyzing signal-to-noise ratio (S/N), whose form depends on the experiment objective. A standard L9 orthogonal array with four parameters consisting of bacterial cell concentration, molar concentration ratio of nutrient solution, curing time, and inoculum ratio, each was assigned three levels, was selected. In this regard, soil samples were stabilized in sandy soil columns. Two-phase stabilization were conducted by adding the bacterium Sporosarcina pasteurii PTCC 1642 in the first phase and nutrient in the second phase. Specimens were subjected to direct shear stress test with the normal stress of 12.5, 40, 68 kPa. ANOVA suggested that the effect of each parameter on the direct shear stress. The most effective parameter was curing time with 45.97% of the overall variance of the experimental data followed by bacterial cell concentration (22%), molar concentration ratio of nutrient solution (20%), and inoculum ratio (12%). The direct shear strength increased from 6, 18, 31 kPa for the normal stress of 12.5, 40, 68 kPa to 470, 491, 512 kPa in optimally treated specimens.