There are various methods for soil improvement, among which the injection of chemical and biological materials can be mentioned. The injection of chemical materials such as cement and lime, in addition to toxic effects, is often costly. Due to environmental compatibility, biological materials have recently been extensively used and have had favorable results. In the present study, the microbial induced calcite precipitation (MICP) process, which is one of the best methods of biological soil improvement, has been utilized. In this method, after injecting the bacterial solution, the cementation and fixation solutions are injected into the soil, which results in calcite precipitation, thus greatly improving the soil properties. In this study, the effect of biological improvement on bearing capacity and shear modulus was eva‎luated. For this research, high plasticity clay and fine sand were used. The bacteria used in this study is Bacillus pasteurii. In this research, tests were performed on biologically improved sand columns to eva‎luate bearing capacity. The first variable is the column length to diameter ratio (L/D), which was specified with three values of 2, 2.5 and 3. The second variable is the injection method, which was considered in three ways: injecting all solutions at one hour intervals, mixing the bacterial solution and injecting other solutions, and injecting the cementation solution 16 hours after the stabilization solution. The third variable is the concentration of the cementation solution, which was considered to be 0.5, 1 and 1.5 molar. A circular rigid foundation with a diameter of 5 cm was used to load the single columns. The test design was Taguchi design of experimental(TDOE) method and finally the optimum condition is obtained by Taguchi analysis. According to the test results, it can be stated that mixing bacteria causes the bacteria to escape from the column and reduces the bearing capacity. The optimum condition according to Taguchi analysis belongs to L/D ratio of 3, injection of cementation solution with 16 hours time interval and 1.5 molar concentration of cementation solution. Considering the optimum condition in single columns, group columns with square and triangular arrangements were eva‎luated. According to previous studies, the distance between the columns was kept constant at 2.5D. A square rigid foundation 18×18 cm wide was used for loading and finally the optimum condition of the column group with a square arrangement was obtained. To eva‎luate shear modulus, unconfined compressive strength testing was conducted. The untreated soil sample had a shear modulus of 0.096 MPa. The treated soil sample had a shear modulus of 31.65 MPa. This represents a 38 times increase in the shear modulus of the treated sample compared to the untreated sample. It can be concluded that the use of biologic injection leads to an increase in shear modulus.

حميد هاشم الحسيني , عليرضا باغبانان

محمدرضا خان محمدي , رسول اجل لوئيان