Numerical triaxial simulations using the discrete element method (DEM) were performed to examine how the adopted contact models and the associated parameters affect the response. The increase in the shear modulus G in the Hertz-Mindlin’s contact model can enhance the small-strain Young’s modulus and reduce the initial volumetric contraction but the influence from the Poisson’s ratio, ν, used in the same model can be neglected. Samples with a higher interparticle friction coefficient, fs, give rise to higher shear strength and greater volumetric dilation. However, the associated peak and critical-state friction angles, ∅p and ∅cs, have a non-proportional relationship with increasing fs. The addition of rolling resistance can render a similar effect to increasing fs but enable the overall response, including the resulting ∅p, ∅cs, εp (the strain at the peak strength), and dilatancy behavior, closer to the experimental observations. As the rolling coefficient, Jn, and the coefficient of rotational sliding, η, increase, the ∅p and ∅cs also increase until they both reach a limit and become saturated. Moreover, the resulting ∅p and ∅cs from the samples with a fixed rolling resistance but different fs also show a similar non-proportional response as fs increases. When fs reaches a certain value, the failure is mainly by rotational sliding and not by frictional sliding because rotational sliding can occur more easily. Hence, the frictional resistance cannot be fully developed and ∅p (or ∅cs) ceases to increase no matter how much fs increase. Similarly, when η reaches a certain value, the failure switches into a frictional sliding mode so ∅p (or ∅cs) also stops increasing even as η continues to increase. In this study, DEM simulations on triaxial creep tests of dense and loose samples were carried out to examine the involved micromechanics during creep in sand. The simulated creep responses fairly reproduce the published experimental results. During the primary creep, the creep rate continuously decreases. This is due to ...