The earliest known use of a helical pier foundation was for the support of lighthouses in tidal basins around England. An English brick maker, Alexander Mitchell, is credited with design of a “pier” for this purpose in 1833. The use of the “pier” was successful, but advancement of the helix-plate foundation did not progress.
In the 1950s, a power installed screw anchor for resisting tension loads was introduced. The anchor found favorable, widespread acceptance, and consisted of a plate or plates, formed into the shape of a helix or one pitch of a screw thread. The plate was attached to a central shaft and has its characteristic shape to facilitate installation. Installation was accomplished by applying torque to the anchor and screwing it into the soil. The effort to install the anchor was supplied by a torque motor.
Research and Development
Along with the development of the tension screw anchor, similar devices began to be developed to resist compression loads. Various sizes and numbers of helices have been used with shafts of varying size and lengths to provide foundations for different applications. Experience has shown a square shafted helical pier works best. In the past 60 years, projects that have utilized pier 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 20,000 ft.-lb. (16 to 27 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 5,500 ft.-lbs. (7.5 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 pier foundation. As suggested earlier, the pier 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, three is the maximum) with varying diameters ranging 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. The shaft size may vary from 1 1/2 “ (3.8 cm) square solid bar material to 3” structural tube. The number and size of helices and the size 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.
The helical pier 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. The installation of a pier foundation is for practical purposes vibration free. These features make the helical pier foundation attractive on sites that are environmentally sensitive. Installations near existing foundations or footings generally cause no problems.