Sandy soils may become unstable under loading which causes large deformations and consequently, impose damages to existing structures. In this research, Discrete Element Method (DEM) is used to simulate 2D drained and undrained behavior of polygonal granular materials to investigate instability occurrence. The so-called cylinder method is applied to simulate the undrained behavior that a pipe is assumed between neighboring pores to allow fluid flow. The applicability of this method was evaluated with constant-volume approach. The advantage of the cylinder method is to assess the distribution of fluid pressure. Comparison of the results indicate good match. Based on both drained and undrained simulations, a unique critical state line was obtained; however, the position of instability line was influenced by the initial void ratio. At the onset of potential instability, the central pore pressure increases highly; however, it reduces afterwards and the fluid pressure is almost distributed uniformly throughout the sample. (c) 2020 Elsevier B.V. All rights reserved.