Improvement of Cohesion of Silty Sands by Biological Microbiology (Case Study: Firouzkooh Silty Sands, Iran)

Nowadays, construction on problematic soils is inevitable owing to the growing scarcity of land worldwide. Problematic soils are commonly characterized by low strength and high compressibility. Thus, the mechanical properties of these soils should be improved. In recent years the researchers have attempted to present approaches based on environmental measurements. One of the methods recently considered in the improvement of sandy soils, is Microbial-Induced Calcite Precipitation (MICP). In order to accelerate the MICP process, it is possible to improve the soil structure cementation and soil engineering properties by creating Urea microorganism. In order to do this, a solution containing bacterial cell and cementation is required which includes urea and one of the Calcium ion sources such as CaSO4.2H2O, CaCl2. 2H2O, CaCl2. In this paper, in order to accelerate the calcite precipitation, a Bacillus bacterium called Bacillus Pasteurii is used as catalyzer of this chemical reaction. In order to investigate the effectiveness of time factor on improving cohesion of silty sand from Firouzkooh city, Iran, an experimental study consisting of 5 direct shear tests were performed. The time taken for the experiments was 3, 7, 14, 21 and 28 days from MICP. The results of this research demonstrated that, over time, cohesion of improved silty sandy soils increased by bio-microbial method. This indicates the role of the growth of bacterial cells and accelerates the progress of the MICP process and the precipitation of calcite crystals in the pores. The results from scanning electron microscope (SEM) confirmed this finding. SEM images indicated that the rate of calcite precipitation increases with time. However, the rate of growth of bacteria depends on their amount of optimal growth or volume of residual pores of the soil that can be decreasing or increasing.