The original focus of screw pile technology was tension loads to anchor guywires and cables of power poles and the main application was in the telecom and power transmission industries. Helical screw piles, however, have been used for compressive loads as far back as 1833 to support lighthouses in tidal basins around England. It has gained wide acceptance in residential, commercial, and industrial building and retrofitting applications. The anchor consists of a plate or multiple plates that are formed into the "helix" shape with the pitch of a screw thread. The lead section with the plate(s) is installed by applying torque with a hydraucially powered torque motor. Additional extensions are coupled until a minimum required depth or torque is achieved.
The main advantages of the helical screw pile is ease and speed of installation. Installing helical screw piles does not require large cranes or augering equipment. Therefore, helical screw piles can be installed at sites with limited access and where little disturbance to the site is desired. The powering equipment and torque motors are also quiet compared to diesel driven pile hammers or large auger equipment. There is virtually no vibration created when installing helical screw piles, which is important if pile locations are near existing structures or sensitive utilities. Installation of helical screw piles does not create spoils that have to be disposed. Helical screw piles can be installed in almost all soils except for competent bedrock. The torque versus capacity ratio determined by Chance Co. engineers leads to predictable results and eliminates the need for costly load testing. Most helical piles can be loaded immediately, except for HPM's (helical pulldown micropiles) which require setup time for the grout column.
INSTALLATION TORQUE TO CAPACITY RATIO
Through its research and development, Chance Co. engineers have determined that there is direct relationship between installation torque and capacity with the helical screw piles. The ratio can be conservatively taken as 10. This means that for every foot pound of torque required to advance the screw pile, the pile has 10 pounds of ultimate capacity (tension or compression). We apply a factor of safety of 2.0 to the pile to determine the allowable pile capacity. For example, if a pile is required to carry 25 kips (1 kip = 1000 pounds), the minimum installation torque should be (25,000 x 2)/10 = 5,000 foot pounds. This ratio allows installers to predict pile capacity on an accurate basis without the need for costly load testing.
Helical pile material is typically hot-dipped galvanized to extend the life of the pile in aggressive environments. Non galvanized pile material may be used in temporary shoring/tieback locations. Corrosion protection can be increased further by using the pulldown micropile with grout column.