The present work quantified the laboratory geotechnical variability by repeatability direct shear test (DS) on alluvial fine-grained soils. The effect of laboratory variability of geotechnical parameters (cohesion c' and friction angle ϕ′) on slope stability was investigated. A mixture of compacted fine sand (40%) and clayey silt (60%) taken from a quarry fines stockpile was used: these soils are commonly used to backfill exhausted quarries located in the River Paglia alluvial plain (Central Italy). As knew in the literature, the dry density achieved by a given degree of compaction controls the shear strength parameters affecting the performance of compacted soil. Four direct shear tests were conducted following the ASTM D 3080-72 procedure on samples having a dry density of about 16.5 kN/m3, corresponding to 95 % of maximum dry density. Combining four DS tests yielded 256 pairs of shear strength parameters in terms of effective stresses, ϕ′ and c' parameters show normal distribbution with ϕ′=27.0±0.8° and c′=19.22±4.08 kPa, for the stress range 100÷250 kPa. In most of the 256 combinations, the friction angle decreased as the cohesion increased. It is generally accepted that the strength parameters (c' and ϕ′) have a negative correlation, although it is appreciated this is not always the case and different cohesion values can be obtained for the same friction angle (i.e., for the same slope of Coulomb failure envelope). The effect of the uncontrolled experimental variability of shear strength parameters on the long-term stability of a single homogeneous slope whose geometry can vary was investigated. Analyzing the factor of safety obtained using all the 256 combinations of shear strength parameters, the probability of having safety factors lower than 1.30 for the different slope heights was calculated. Such analyses demonstrate not only that direct shear testing is reliable, but also that the stability of a slope can be assessed with greater accuracy.