The helical pier is a deep foundation system used to support or resist any load or application. Installed by mobile equipment ranging in size from lightweight units to heavier units depending on the load requirements, it can be loaded immediately. The helical pier’s elegant simplicity is its greatest asset. Its mechanical design and manufacture balance the capacities of its three basic parts and maximize the efficient use of their material.
Chance Civil Construction is the international leader in earth anchoring and piering. Chance Helical Piers are used worldwide to secure residential and commercial buildings, tower foundations, heavy equipment foundations and many other deep foundation applications.
Components of a Helical Pier
1. At least one bearing plate (helix)
Dies form each steel bearing plate into a true helix. The plates are formed in a true helical shape to minimize soil disturbance during installation (as opposed to the inclined plane of an auger which mixes soil as it excavates). Properly formed helical plates do not measurably disturb the soil. The helical bearing plates transfer the load to the soil bearing stratum deep below the ground surface. “Deep” is considered five helix diameters vertically below the surface where the helical plate can develop full capacity of the plate-to-soil interaction.
2. A central shaft
During installation, the central steel shaft transmits torque to the helical plate(s). The shaft transfers the axial load to the helical plate(s) and on to the soil bearing stratum. In addition to having the torsional strength to resist the torque required to install the helical plates into load bearing soils, the shaft must have the structural capacity to exceed the applied stresses from the building or structure that it will be supporting.
3. A termination
The termination connects the structure to the top of the helical pile/anchor transferring the load down the shaft to the helical plate(s) to the bearing soil. To evenly distribute the structure load to the helical piles/anchors, the termination may be a manufactured bracket or an attachment produced on site as designed by the structural engineer. MDTI and AB Chance design and manufacture specialized termination brackets for a wide variety of loading conditions and applications including compression, tension, seismic and lateral types of loading, among others.
Helical Pier Applications
In its simplest form, the helical pier is a deep foundation element, i.e., it transfers a structure’s dead and live loads to competent soil strata deep below grade. This is the same for any deep foundation element such as driven piles, drilled shafts, grouted tendons, auger-cast piles, belled piers, etc.
Therefore, helical piers can be used as an alternative method to drilled shafts and driven piles. Practical constraints,
primarily related to installation, currently limit the maximum design load per helical pier to 100 kips in tension and 200 kips in compression, which means helical piers can resist relatively light to medium loads on a per pier basis, and much heavier loading when used in pile groups. But as is the case with virtually all engineering problems, more than one solution exists. It is the responsibility of the engineer to evaluate all possible alternatives, and to select the most cost effective solution.
Today, helical piers are commonly used for residential and light commercial and heavy commercial construction, machinery/equipment foundations, telecommunication and transmission towers, tie-downs for wind and/or seismic forces, and virtually any application where site access is limited or remote. They have become the deep foundation of choice for walkways and boardwalks in environmentally sensitive areas, such as wetlands and protected forestland. In expansive soil areas, helical piles can save money and time when compared to expensive over-excavation and fill options.
Helical piles/anchors do have several advantages that make them the foundation of choice for many applications including
these general categories:
- Machinery/Equipment Foundations
- Limited Access Sites
- Wind and Seismic Loading
- Replacement for Drilled/Driven Piles
Helical Pier Advantages
- No need for concrete to cure
- Quick, easy turnkey installation
- Immediate loading
- Small installation equipment
- Pre-engineered system
- Easily field modified
- Torque-to-capacity relationship for production control
- Install in any weather
- Environmentally friendly
- No vibration
- No spoils to remove
- Restricted access sites
- High water table
- Weak surface soils
Helical Pier History
In 1833, the helical pier was originally patented as a “screw pile” by English inventor Alexander Mitchell. Soon after, he installed screw piles to support lighthouses in tidal basins of England. The concept also was used for lighthouses off the coasts of Maryland, Delaware and Florida.
Innovations of the helical pier have been advanced by both its academic and commercial advocates. Considerable research has been performed by public and private organizations to further advance the design and analysis of helical piles and anchors.
Today, readily available hydraulic equipment, either small or large, can install helical pile/anchors almost anywhere. Backhoes, skid-steer loaders and mini-excavators are easily fitted with hydraulically driven torque motors to install helical pile/anchors in construction sites inaccessible by the larger equipment required for other deep foundation methods. According to site conditions, installation equipment can include guided-head and articulated-head torquehead machinery, self-propelled, carrier-mounted, tracked, wheeled or floating.
PISA (Power Installed Screw Anchors)
In the late 1950’s, the A.B. Chance Company introduced the patented PISA® system. This coincided with the invention of truck-mounted hole-digging equipment following World War II. The PISA® system has become the worldwide method of choice for guying pole lines of electric and telephone utilities.
The PISA® system’s all-steel components include one or two helix plates welded to a square hub, a rod threaded on both ends, a forged guy wire eye nut, and a special installing wrench. The square-tube anchor wrench attaches to the kelly bar of a digger truck, fits over the rod, engages the helical hub and typically installs a PISA® anchor in 8 to 10 minutes. Rod and wrench extensions may be added to reach soil layers which develop enough resistance to achieve capacity. PISA® rods come in 5/8”, 3/4” and 1” diameters.
Through A.B. Chance Company testing and close contact with utilities, the PISA® anchor family soon expanded to develop higher strengths capable of penetrating harder soils including glacial till. This quickly gave rise to the development of CHANCE® Helical Piles/Anchors with higher capacities and larger dimensions.
More recent developments include the SQUARE ONE® (1980) and the TOUGH ONE® (1989) patented guy anchor families with 10,000 and 15,000 ft-lb installing torque capacities. Unlike previous PISA® designs, these anchor designs are driven by a wrench that engages inside, rather than over, their welded socket hubs. Both use the PISA® extension rods with threaded couplings.
Round Rod (RR) Anchors
In 1961, the A.B. Chance Company developed extendable Type RR multihelix anchors, originally for use as tiedowns for underground pipelines in poor soil conditions on the Gulf of Mexico coast. These anchors are not driven by a wrench; instead, installing torque is applied directly to their 1-1/4” diameter shafts. Type RR anchors worked well in weak surficial soils, but their shaft (although extendable by plain shafts with bolted upset couplings) did not provide enough torque strength to penetrate very far into firm bearing soils.
Square Shaft (SS) Anchors
Development of a high-torque, shaft-driven, multi-helix anchor began in 1963, culminating in the introduction of CHANCE® Type SS 1½” Square Shaft multi-helix anchors in 1964-65. The SS anchor family since has expanded to include higher-strength 1-3/4”, 2” and 2-1/4” square shafts. With the acquisition of Atlas Systems, Inc., in 2005, the Type SS product line has been expanded to include 1-1/4” square shafts. Extension shafts with upset sockets for the 1-1/4”, 1-1/2”, 1-3/4”, 2” and 2-1/4” square shafts also lengthen these anchors to penetrate most soils at significant depths for many civil construction applications including guying, foundations, tiebacks and more recently, soil nails.
High Strength (HS) Anchors/Piles [now called Round Shaft (RS) Piles]
Later in the 1960’s, Type HS anchors developed first for high-torque guying requirements later were applied as foundation helical piles for utility substations and transmission towers. The HS anchor family has 3-1/2” pipe shafts which may be lengthened by extensions with swaged couplings. HS anchors now are used for a wide array of foundation applications. The Type HS Piles/Anchors are now referred to as Type RS Piles/anchors.
Power Installed Foundation (PIF) Anchors/Piles
Also launched in the 1960’s were non-extendable anchors termed Power Installed Foundations. PIF sizes and load capacities support requirements for foundations that support a broad range of equipment, platforms and field enclosures. Most versatile are the 5-ft to 10-ft-long PIFs with pipe shafts of 3-1/2”, 4”, 6-5/8”, 8-5/8” and 10-3/4” diameters, each with a single helix of 10”, 12”, 14” or 16” diameter. Integral base plates permit direct bolt-up connections on either fixed or variable bolt-circle patterns.
Bumper post anchors are similar to the 3½”-shaft PIF, but with fence-type caps instead of base plates, to serve as traffic barriers around booths, cabinets, doorways, etc. One with a 2-3/8” pipe shaft 69” long is called a Square Drive Foundation for its 2”- square drive head. The solid head is internally threaded for adding a straight stud or adjustable leveling pad after installation.
Street Light Foundation (SLF) Anchors/Piles
In 1972, CHANCE® Street Light Foundations (SLF) were introduced. Anchors with pipe shaft diameters of 6-5/8”, 8-5/8” and 10-3/4” in fixed lengths of 5, 8 and 10 feet. Complete with an internal cableway, these foundations with bolt-up base plates deliver the quick solution their name implies and now are used to support similar loads for a variety of applications.
Helical Pier Foundation Systems/Piles
In 1985, Chance ® patented products for repairing foundations of all residential and commercial buildings were introduced. Originally based on Type SS helical anchors, its special foundation repair brackets transfer structural loads to stable soil strata below weak surface conditions. Since then, the product also has been used to deepen foundations for new construction by installing the helical piles at intervals between footing forms prior to pouring reinforced concrete.
CHANCE HELICAL PULLDOWN® Micropiles
Developed in 1997, for sites with especially weak surface soils, this patented innovative application of the helical pile integrates portland-cement-based grout to stiffen the shaft. By “pulling down” a special flowable grout as the foundation is screwed into the soil, the result is a pile with both a friction-bearing central shaft and end-bearing helical plates in competent substrata. Where needed for poor surface conditions, this performance combination converts sites previously deemed as “non-buildable” to usable sites suited for not only building construction but also telecom tower foundations in areas inaccessible by equipment utilized for other deep foundation methods. It employs SS, RS and combinations of these two types of helical piles.
Large Diameter Pipe Piles (LDPP)
To meet an industry need for helical piles with higher tension/compression capacities and larger bending resistance, the large diameter pipe pile research project was initiated in 2007. The research culminated in product offerings including extendable large diameter piles with a box coupling system capable of installation torques as high as 60,000 ft-lbs and compression capacities of 300 kips.