HISTORY OF HELICAL PIERS
Originally Helical Piers or screw piles were used for the stabilization and support of lighthouses in tidal basins around England. Alexander Mitchell, an English brick maker, is credited with the design of the “screw pile” in 1833. The “screw pile” method worked extremely well, however, development of the helix- plate foundation did not take off immediately.
When the Helical Tension Anchor was produced, the use of the same or similar devices to resist compres- sion loads was also developed.
Around the globe helical piers and anchors are used on a vast array of projects. Helical Applications in- clude: pole bases and communication towers, utility and pipeline tie downs, tiebacks and wall anchors, deep foundation Piers for new construction, underpinning commercial and residential structures, board- walks, bulkheads, sea walls, temporary and reusable wall anchors, slope stabilization, concrete slab lifting and stabilization, shoring, and concrete-less foundations. A helical pile can be used in nearly any situation and where driven or cast in place piles are currently used.
BEARING CAPACITY USING TORQUE
Pullout and bearing capacities of helical piers placed vertically are essentially the same provided a depth to blade diameter ratio is greater than 5.
Three methods exist for determining bearing and pullout capacity: “cylindrical shear”, “individual bearing”, and “installation torque” (Ghaly and Clemence, 1998, A.B. Chance, 1993b). The installation torque method has been proven to have the most reliable results (Hoyt and Clemence, 1989).
Given that the instillation torque of helical piers also supplies an indication of soil strength at the depth of the helices, a lower factor of safety is permitted for acceptable bearing and pullout capacity calculation. In general a factor of safety of 2 is used in helical pier design. The engineer should select a factor of safety that is steady with the trustworthiness of subsurface conditions, availability and accuracy of installation torque measurements, likelihood that live loads will be applied, and other typically considered factors.
Torque measurements acquired during installation of helical piers are an indication of soil shear strength at the depth through which the helical blades are passing. Because of the intricate interaction of the blades with the soil, it is complicated to relate torque measurements directly with angle of internal friction and cohesion of the soil. To circumvent this trouble, Perko (2000) proposed a model in which the capacity of a helix pier is directly related to the installation torque by energy equivalence.