History   The earliest known use of an anchor foundation was for the support of lighthouses in tidal basins around England. A blind English brickmaker, Alexander Mitchell, is credited with design of a "screw pile" for this purpose in 1833. The use of the "screw pile" was apparently successful, but advancement of the helix-plate foundation did not progress. In the 1950s, the A.B. Chance Company introduced the (PISA®) Power-Installed Screw Anchor for resisting tension loads. The anchor found favorable, widespread acceptance. This anchor consists of a plate or plates, formed into the shape of a helix or one pitch of a screw thread. The plate is attached to a central shaft. The helix plate has its characteristic shape to facilitate installation. Installation is accomplished by applying torque to the anchor and screwing it into the soil. The effort to install the anchor is supplied by a torque motor. Research & Development   With the development of the tension screw anchor, came the use of the same or similar devices to resist compression loads. Thus, screw pile foundations came into greater use. Various sizes and numbers of helices have been used with shafts of varying sections to provide foundations for different applications. In the past 40 years, projects that have utilized screw pile foundations include electric utility transmission structures, Federal Aviation Administration flight guidance structures, pipeline supports, building foundations, remedial underpinning, streetlights, walkways in environmentally sensitive areas and many others. Torque capacities of available installation equipment have increased over the past years. Hydraulic torque motors in the 3,000 to 5,000 ft.-lb. (4.0 to 6.8 kN-m) range have increased to the 12,000 to 15,000 ft.-lb. (16 to 20 kN-m) range. Mechanical diggers now extend the upper range to 50,000 ft.-LB (68 kN-m) or more. "Hand-held" installers have expanded the available equipment in the lower range of torque, with a capacity up to 2,500 ft.-LB (3.4 kN-m). Though called "hand-held," these installers are hand-guided while a torque bar or other device is used to resist the torque being applied to the screw pile foundation. As suggested earlier, the screw pile foundation may be utilized in various forms. The lead section (i.e., the first part to enter the ground) may be used with one or more helices (generally, four is the maximum) with varying diameters in the range from 6 to 14 inches (15 to 36 cm). Extensions, either plain or with additional helices, may be used to reach deep load-bearing strata. Generally, eight is the maximum number of helices used on a single screw pile foundation. The shaft size may vary from 11/2 " (3.8 cm) square solid bar material to 10" (25 cm) diameter pipe material. The number and size of helices and the diameter and length of shaft for a given application are generally selected based on the in-situ soil conditions and the loads that are to be applied. Advantages   The screw pile foundation system is known for its ease and speed of installation. Installation generally requires no removal of soil, so there are no spoils to dispose of. Installation causes a displacement of soils for the most part. However, in the case of a foundation with a pipe shaft, some soil will enter the interior of the pipe until it becomes plugged. Installation equipment can be mounted on vehicles when required. The installation of a screw pile foundation is for practical purposes vibration free. These features make the screw pile foundation attractive on sites that are environmentally sensitive. Installations near existing foundations or footings generally cause no problems. However, the screw pile foundation generally cannot be installed into competent rock or concrete. Penetration will cease when materials of this nature are encountered.