The effectiveness of the threshold line (TL) proposed as the criterion for setting of the opening size between the shear boxes of direct shear test based on the results of several kinds of sandy soil samples in the past study was examined by the discrete element method (DEM). The conditions of the direct shear test in the DEM simulation were reproduced using four kinds of specimens with different mean particle sizes (D50) and particle size distributions, and the shearing processes were performed according to the predetermined opening size between shear boxes. As a result, it was confirmed that the maximum difference of the threshold point (T.P) obtained by the DEM simulation was about 0.17 mm compared to the TL, and it showed almost the same trend as the TL obtained from the experiments using the real soil material. Based on the results of this study and the past study results, it can be said that the TL is not affected by the mean particle size and particle size distribution of the soil sample, and could be considered to be an effective criterion for setting of the opening size between the shear boxes of direct shear test.
