Abstract: As a special granular material, the soil sample demonstrates obvious anisotropy and inhomogeneity. The characteristics of the shear failure surface in the direct shear test and other factors lead to its different failure modes from other materials. Direct shear tests of four types of soil samples were conducted by using samples with diameters of 61.8mm and 150mm, respectively. Based on the test results, the size effect of the soil shear strength is analyzed. The results show that the change of sample size does not affect the stress-strain deformation characteristics of the sample. However, with the increase of sample size, the peak shear strength appears later and the resistance to deformation is stronger. With the increase of sample size, the proportion of effective shear area increases, resulting in the increase of theoretical shear strength. For cohesive soil, the increase of cohesion is mainly manifested while the angle of internal friction doesn't change much. On the contrary, the increase of internal friction Angle is the main performance for sandy soil.

Key words: Soil Samples, Direct Undrained Shear Test, Shear Strength, Size Effect