Optimization of mixed culture for bacterial stabilization of sands Vahid Eftekhari far Department of Civil Engineering Isfahan University of Technology, Isfahan 84156-83111, Iran Degree: M.Sc Language: Farsi Supervisor: Dr. M. A. Rowshanzamir mohamali@cc.iut.ac.ir ecause of global environmental problems due to desert expansion and dust storms as well as constructional requirements, looking for more efficient and comprehensive methods to stabilize dune sands and weak grounds seems to be an essential necessity. MICP (Microbial-induced CaCO3 precipitation) is an innovative technique in which bacterial activities within soil mass are harnessed to modify the physical and mechanical properties of soils. This method produces calcium carbonate precipitation in the soil pores by fracturing urea in the presence of calcium ions. An important factor in this method is the selection of the desired bacterium or bacteria for hydrolysis of urea and control of the biological process and in case of using several types of bacteria, the mixing percentage of the desired bacteria. In this research study, an urease microorganism (i.e. B.Pasteurii) was prepared at single and mixed cultures (with B.Subtilis) in the laboratory and for optimizing the mixing percentage in mixed cultivation method, Different percentages of bacteria were used. Bacterial solution was injected into the soil under the mentioned conditions. After required and appropriate curing time, the samples were subjected under unconfined compression, direct shear, triaxial, falling-head permeability, acid washing and SEM tests. The test results showed significant strength improvement and reduction of permeability of the treated samples in comparison with those of the untreated soil, while the performance of the mixed culture was much better than that of the single culture in the stabilization outcome. In this research study, the percentage of optimal mixing of bacteria is 70% of B.Pasteurii and 30% of B.Subtilis.The unconfined strength of the soil samples treated with the mixed culture with optimal mixing percentage was 1.33 times that of samples treated with the single culture. The amount of permeability reduction from mixed culture method with optimal mixing percentage was also 79% which was 10% more than the use of equal mixing percentage. The results of the direct shear test also showed a 40% increase in the friction angle in the use of optimal mixing percentage. Which was 20% higher than the use of equal mixing percentage and also soil cohesion in using the optimal mixing percentage was 1.15 times that of samples treated with Equal mixing percentage of bacteria. Finally the amount of precipitated calcite and its shape were shown in acid washing and SEM tests, respectively. The research results verified the capability of the application of biological treatment, particularly with the mixed culture, on the dune sand which may be regarded as a potential technique to control desert expansion, and dust storms through the stabilization of the weak grounds. Keywords: Soil improvement, Dune Sand, Mixed culture, Single culture, Unconfined Compressive strength test, Direct Shear test, Falling head test