Helical Foundation Terms & Definitions
Explore definitions for common terms used in helical piles, helical piers, screw piles, and deep foundation systems used in construction and foundation support.
A
Allowable Load – Geotechnical (ALG)
The maximum allowable load that may be transferred into a helical screw pile foundation as determined by the geotechnical properties of the soil. Usually determined by load test behavior or engineering analysis. ALG is typically limited to 30% to 50% of the ultimate geotechnical capacity of the pile, but this may vary, depending on project parameters and other factors.
Allowable Load – Mechanical (ALM)
The maximum allowable load on a structural element as specified by its manufacturer. ALM is usually limited to 50% of the rated ultimate mechanical capacity.
B
Bearing Stratum
The undisturbed soil layer at the helical pile location, which provides a significant portion of the axial resistance of an installed helical pile bearing on one or more of the pile helices.
C
Coupling
Central steel shaft connection means formed as an integral part of the plain extension shaft material. Couplings are internal or external sleeves or hot-upset forged sockets.
Coupling Bolt(s)
High strength, structural steel fasteners used to connect helical pile segments together. Coupling bolts should only transfer axial forces.
Creep
The continuous deflection (movement) of a pile while subjected to a constant load.
Crowd
Axial compressive force applied to the helical anchor as needed to ensure that the anchor progresses into the ground a distance equal to the helix pitch (approximately 3 inches) for each revolution.
D
Dead Load
Loads resulting from the weight of the structure plus all material (equipment) permanently fastened thereto or supported thereby.
Design Load (DL)
The maximum load that will be transferred to the pile. The design load is expressed in terms of magnitude and direction. Design loads can be tensile, compressive, or lateral. Also known as service load and working load.
E
Effective Torsional Resistance
Effective Torsional Resistance is used to estimate ultimate geotechnical capacity of a pile during construction. Helical pile foundations are typically installed to a predetermined specified installation torque.
F
Failure – Geotechnical
Unless otherwise noted, geotechnical failure is achieved when continuous deflection (movement) of a pile occurs while subjected to a constant load.
Foundation
An element that connects a structure to the earth. Loads are transferred from the structure to the foundation, to the earth. These loads can be compression, tension, lateral or dynamic.
Foundation Capacity
The bearing capacity of the helical pile (helices and skin friction) interacts with the specified bearing soil.
G
Geotechnical Capacity
The maximum load that can be resisted via the bearing of helices on the soil and skin friction or adhesion between the soil and shaft in which the pile is embedded.
H
Helical Extension
Same as a plain extension, but with one or more helices welded to it. A helical extension may be utilized to increase the pile bearing capacity. May be used in areas of very weak soil or as an onsite modification when unexpected weak soil is encountered. The helical extension is attached directly above the helical lead section or sometimes above other helical extensions.
Helical Lead Section
A central shaft with one or more helices welded to it. The first component of a helical pile foundation that enters the soil. Extensions are used in conjunction with the helical lead section to achieve the specified depth and to enter competent soil.
Helical Pile
A bearing-type foundation element consisting of a lead or starter section, helical extension (if required by site conditions), plain extension section(s), and a pile cap or bracket. A.K.A. helical screw pile, screw pile, helical screw foundation, helical anchor. Their purpose is to transfer structural loads (tension and/or compression) to a load bearing stratum.
Helical Pile Contractor (contractor)
Installer of the helical foundation system.
Helix Driver
A high torque hydraulic motor used to advance (rotate) a helical pile into the soil to the specified bearing depth. Depending on the capacity of the helix driver, it may be either handheld or machine operated.
Helix or Helices
Generally, a rounded steel plate is formed into a helical flight. When rotated in the ground, the helical shape provides thrust along its longitudinal axis, aiding anchor installation, and the plate transfers axial load to the soil through bearing after installation.
I
Installation Torque
The resistance generated by a helical pile when installed into soil. The installation resistance is a function of the soil type, and size and shape of the various components of the helical pile.
L
Live Load
Loads resulting from vehicles, people, snow, ice, wind, and impact. Other forces resulting from earthquakes and other extraordinary conditions.
Load Test
Conducted in accordance with ASTM D1143-07 to determine capacity and relation of load to movement by applying incremental loads to the helical pile.
P
Pile Cap
Connection means by which structural loads are transferred to the helical pile. The type of connection varies depending upon the requirements of the project and type of helical pile material used.
Pitch
The distance between the upper and lower split edges of the helices. The distance a lead should travel in one rotation.
Plain Extension
A central shaft (with no helices) that is attached directly above the helical lead section. Extensions are coupled together and are used to extend the helical lead section to a required depth and into a soil stratum of suitable strength.
Project Documents
Documentation that is considered part of the official project record. This includes but is not limited to project drawings, specifications, submittals, etc.
Proof Test
Load testing a helical pile foundation by applying loads in predetermined increments, maintaining each load for a period of time, and recording deflection at the beginning and end of each increment. Engineers, to determine allowable load, often apply the results of proof tests.
R
Rated or Ultimate Mechanical Capacity
The ultimate mechanical capacity of a structural element as specified by its manufacturer. Determined by load tests and/or engineering analysis. Also referred to as mechanical strength.
Rated or Ultimate Torsional Strength
The maximum torque that can be safely applied to a helical screw pile foundation during installation.
Reveal
The distance from the ground surface to the upper end of the last installed extension of an anchor, measured along the anchor’s longitudinal axis.
S
Safety Factor – Geotechnical (SFG)
The ratio of the Ultimate Geotechnical Capacity (UCG) of a helical screw pile foundation to the design load.
Safety Factor – Mechanical (SFM)
The ratio of the rated ultimate mechanical capacity of a structural element to the design load.
Shaft
The central shaft of a helical pile used to transfer load from the structure to the helices. Shafts may be solid round corner square steel bar or hollow steel pipe sections. Shaft sizes typically range from 1.25” to 2.00” round corner square and from 2.875” to 10.75” diameter hollow round.
Specified Installation Torque
The effective torsional resistance to be achieved – as specified by the project engineer of record.
U
Ultimate Pile Capacity – Geotechnical (UCG)
The maximum load that can be applied to a helical pile foundation prior to soil bearing failure. At this point no additional capacity can be justified.
Ultimate Pullout Resistance
Limit state based on the lesser of mechanical strength or global capacity of the helical anchor – defined as the point at which no additional axial tension load can be justified.
Ultimate or Rated Pile Capacity – Mechanical (UCM)
The manufacturers’ rated ultimate mechanical capacity of the helical pile foundation.