Mahmoud Malakouti Olounabadi

This research investigated the effect of using polypropylene fibers and nano-silica on the mechanical properties of silty sandy soil, such as the California bearing ratio (CBR) and shear strength tests. Different laboratory samples were prepared with varying percentages of nano-silica (1, 2, and 3%) and polypropylene fibers of different lengths (6 and 12 mm). The fibers were mixed with soil at weight percentages of 0.1, 0.5, and 1%. The microscopic characteristics of soils were examined using SEM. The results showed that the addition of polypropylene fibers significantly increased the CBR value, and the use of 1% fibers by weight of the soil improved the interlocking and bonding of soil particles, creating a more unified soil texture. Increasing the length of polypropylene fibers from 6 to 12 mm also increased the CBR. Nano-silica, at 1, 2, and 3%, increased the CBR value by 2.2, 3.3, and 4.2, respectively. The highest load-bearing and shear strength were observed with the use of 1% polypropylene fibers and 3% nano-silica. The shear strength test revealed that polypropylene fibers did not change the soil cohesion but increased the angle of internal friction, while nano-silica improved the soil cohesion but did not affect the internal friction angle. The combination of these two materials significantly increased the shear strength of the studied soil.