Helical Pier History
Helical piers where first develop around 1833 in England by Alexander Mitchell. They were originally used to support the foundation of lighthouses in Tidal Basins. Around the turn of the century helical piers gained interest and were mostly used in the Utility Power, and Petroleum Gas Industries. They were used to support and secure oil pumping rigs, power transmission towers and foundations for buildings.
What is a Helical Pier?
A helical pier is constructed from steel and is produced in either round pipe or square shaft depending upon the manufacturer. Round pipe piers are seven times stiffer than square shaft piers and provide additional stiffness allowing for greater lifting support. If too much force is applied to the square shaft piers during installation, the steel will twist or yield causing the steel to change and become more brittle. This limits the depth the pier can embed itself into bedrock. The first piece installed is called the starter section. The starter section has a helix welded to the bottom of it. The standard diameter of the helix is eight inches.
Helical Pier Installation
Installation of the pier is completed by the use of a torque head. The torque head is powered by hydraulic pressure and applies twisting force which drives or screws the pier into bedrock. Twisting force is measured in foot/pounds. After the starter section of the pier is screwed into the ground an extension section is then mechanically bolted to the starter section. By adding additional extension sections the length of the pier is unlimited and will reach bedrock regardless of the depth of that bedrock. To determine the capacity, or holding strength, of the pier the foot/pound of force used to install the pier is measured. The average torque used to install the pier is between 3,500 and 8,000 foot/pounds of force. The average capacity or load for helical piers is between 20,000 and 60,000 pounds.
The amount of force required to screw the pier into the bedrock has a direct correlation to the capacity of the pier. Through proven engineered mathematical equations, empirical data, and extensive field testing it can be determined what load the pier actually has. Because the pier is installed using a torque head, and not the house itself, the piers can be installed with a safety factor of as much as 5 or 6 times the actual load that the house needs. Any additional live load that is applied after the installation of the piers will not cause the pier the fail. The weight from your home is supported on the eight inch helix of the pier. Because the weight is concentrated on the helix, expansive soil is pushed around the pier and does not lift the pier as the soil moves upward. The average depth helical piers are installed is between 12 and 20 feet.