This paper describes the influence of joint spacing and joint orientation on the penetration rate of a Tunnel Boring Machine(TBM) disc cutter as modeled by the Discrete Element Method(DEM).The input data for the simulations were obtained from the sandstone along the Alborz tunnel that is currently being excavated in Iran using a 5.2 m diameter open TBM.Three joint spacings,150,200,and 300 mm,were modeled together with seven values of joint orientation;0°,15°,30°,45°,60°,75°,and 90°.The results show that the penetration increases when joint orientation increases from 0° to 75°,but it decreases as the joint orientation increases further from 75° to 90°.This is true for each joint spacing.In addition,for a given joint orientation increasing the joint spacing causes the TBM penetration to decrease.The optimum joint orientation,from the viewpoint of TBM penetration,is about 60-75°. This paper describes the influence of joint spacing and joint orientation on the penetration rate of a Tunnel Boring Machine (TBM) disc cutter as modeled by the Discrete Element Method (DEM). The input data for the simulations were obtained from the sandstone along the Alborz tunnel that is currently being excavated in Iran using a 5.2 m diameter open TBM.Three joint spacings, 150,200, and 300 mm, were modeled together with seven values ​​of joint orientation; 0 °, 15 °, 30 °, 45 °, 60 ° , 75 °, and 90 °. The results show that the penetration increases when joint orientation increases from 0 ° to 75 °, but it decreases as the joint orientation increases further from 75 ° to 90 °. This is true for each joint spacing. In addition, for a given joint orientation increasing the joint spacing causes the TBM penetration to decrease. Optimum joint orientation, from the viewpoint of TBM penetration, is about 60-75 °.