Among the most important parameters affecting the shear strength of soils in mechanized excavation work with earth balance systems are soil gradation, especially the percentage of fine particles, moisture content, an‎d foam injection percentage. In this research, by sampling different sections of Isfahan Metro Line 2 an‎d conducting gradation studies an‎d foam injection in the laboratory, the changes in soil shear strength under the influence of changes in gradation, moisture, an‎d foam injection coefficient were investigated. Based on economic an‎d technical considerations, the optimal foam consumption was determined. Regarding gradation changes, three gradation curves with three different fine particle values (passing through sieve 200 equal to 20, 30, an‎d 40 percent) an‎d three gradation curves with three different coarse particle values (D60 equal to 4.05, 1.68, an‎d 0.68 mm) were used. This range of variations for the above parameters was selec‎ted based on the values of these parameters in most materials along Isfahan Metro Line 2, which collectively produces nine gradation curves. Each test was repeated three times, an‎d the average of the test results was chosen as the criterion. Three values were also considered for moisture (0, 5, an‎d 7.5 percent) an‎d three values for the foam injection coefficient (10, 20, an‎d 40 percent). The values related to the foam concentration coefficient an‎d foam expansion coefficient were kept constant, an‎d to increase accuracy an‎d reliability, six shear strength determination tests (V.S.T) were performed on each sample. To analyze the test results (combination an‎d number of tests), the design of experiments method using Minitab software, which is based on the Taguchi method, was utilized. The results indicate that with an increase in fine particle content up to 40 percent, shear strength decreases by up to 84 percent, an‎d with an increase in soil particle size, shear strength increases to a certain extent an‎d then remains constant. at 5 percent moisture, we encounter the lowest shear strength, an‎d with an increase in water content to 7.5 percent, shear strength increases, which confirms the theory of maximum soil strength at 7.5 percent moisture. Given the linear relationship between machine power consumption an‎d soil shear strength inside the machineʹs pressure cell, changes in soil shear strength under foam injection are proportional to changes in machine power. Therefore, optimal economic conditions, including reduced torque, reduced power, an‎d reduced energy consumption, directly occur within the 40 percent foam injection coefficient range. Torque an‎d power consumption along the path decrease by an average of 59 percent. This reduction in power, on average, will result in an economic saving of 465 million Rials for each excavation step (1.4 meters) along the entire excavation path of Isfahan Metro Line 2.

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حميد هاشم الحسيني , ابراهيم فرخ