The influence of a driving static shear stress on the liquefaction resistance of medium dense granular soils is investigated. A laboratory study of the behavior of five sands (percentage of silt varying between 2 and 42%, and D 50 varying between 0.55 and 0.09 mm) was undertaken. These five sands were sampled from the foundations of two dams in Quebec (Canada). Sixty cyclic direct simple shear (DSS) tests were performed with and without a driving static shear stress. Relative densities D r varied between 60 and 70%, and static shear stress ratios α= τ st/ σ′ vc varied between 0.2 and 0.6. The presence of static shear stress increased the cyclic strength of these soils. A relation between α and K( α) factor giving the effect of preshearing on cyclic shear resistance is obtained. The mechanical competence is examined as a function of D 50 and percentage of fines. The observed dilatancy yielded rather cyclic mobility than liquefaction.