Optimization of resistance of biologically improved sand by response surface methodology

Need to increase urban infrastructure and lack of access to suitable quality soil increased demand for new methods for soil improvement. Recently, improving the parameters of strength, stiffness and permeability of sand under microbial induced calcite precipitation has received much attention from geotechnical experts as innovative and environmentally friendly methods. Therefore, it is very important to study the parameters affecting the biological improvement technique and find the ideal combination to increase efficiency and reduce costs. The main purpose of this paper is to investigate the effect of the variability of cementation solution molarity, bacterial optical density and curing time (three levels of variation for each) on the parameters of deviation and effective stresses for biologically improved sand under undrained triaxial compression test. The sand used in this study is a SP soil according to the unified soil classification system, and the adopted micro-organism is Bacillus pasteurii as the urease-positive bacterium. The response surface methodology was used to optimize the stress parameters in the improved sand and to significantly reduce the number of experiments. The experiments were designed using a Box-Behnken design with five central points. Three-dimensional schemes and regression model contours were used to evaluate and compare the effect of each variable. In this method, according to the analysis of variance for the data, the effect of all variables on the responses was significant and the optimal values for effective and deviation stresses were 958.2 and 1032.4 (kPa), respectively. Due to the slope of the curves, the molarity of the cementation solution had the greatest effect on the responses, and the effect of bacterial optical density on effective stress was less than deviation stress, that may be due to the accumulation of excess bacteria (lack of nutrients) and its impact on excess pore water pressure.