Abstract: A tool comprises a body having coaxial portions radially defined by soil compaction surfaces of step-down radii from the sizing portion to the tool tip. The soil-compaction surface of each portion is a cylindrical surface generated by a generatrix of a predetermined length extending parallel to the tool axis and bounded by two parallel helices of one and the same helix angle. These surfaces are successively conjugated by transition portions defined by a cylindrical surface whose generatrix is parallel to the tool axis and uniformly approaches the same axis to thereby form a smooth transition from the surface of the body portion of a greater radius to that of an adjacent body portion of a smaller radius. A method of forming a hole by the same tool is also described.