Dynamic compaction induced by heavy impact or vibratory rollers is acknowledged as the most powerful process to increase the shear resistance and to reduce the deformability of granular materials. Ground improvement techniques based on this principle are currently adopted to build artificial embankments and to enhance the mechanical response of in-situ soils. In an attempt to clarify the role of grain size distribution on the compactability of granular materials, a systematic laboratory experimental investigation was undertaken on a variety of quartz-limestone sands. Samples were prepared with ten different grain size distributions, by mixing three selected uniform sands with variable percentage. Each sample was subjected to different standard and modified Proctor compaction tests where the number of blows per layer was systematically varied (i.e. impact energy). The effectiveness of each "Proctor" technique was evaluated in terms of relative density. Test results indicate that compaction effectiveness is noticeably affected by the heterogeneity of soil composition, while a limited dependency on the water content is observed. A single correlation to predict the dry unit weight of sands combining coefficient of uniformity, specific gravity and compaction energy is proposed. It was found that the proposed correlation can predict the dry unit weight under an arbitrary level of compaction energy with an error of less that 5%.