Helical piles are a factory-manufactured steel foundation system consisting of a central shaft with one or more helix-shaped bearing plates and a bracket that allows attachment to a structure. The helix plates are commonly referred to as blades or flights and are welded to the lead section.
Extension shafts, with or without additional helix plates, are used to extend the pile to competent load bearing soil and to achieve design depth and capacity. Brackets are used at the tops of the piles for attachment to structures, either for new construction or retrofit applications. Helical piles are advanced (screwed) into the ground with the application of torque.
The terms helical piles, screw piles, helical piers, helical anchors, helix piers, and helix anchors are often used interchangeably by specifiers. However, the term “pier” more often refers to a helical pile loaded in axial compression, while the term “anchor” more often refers to a helical pile loaded in axial tension. The term “pile” traditionally describes a deep foundation that can resist both tension and compression loads.
Did You Know?
The use of helical piles and anchors in construction dates back nearly 200 years. In the 1830’s, the earliest versions of today’s helical piles were used in England for moorings and for the foundations of lighthouse structures.
Today, helical piles are used in both tension and compression load applications and are gaining worldwide acceptance throughout the construction industry and engineering community due to the versatility of both the product and the installation equipment. In 2007, the International Code Council Evaluation Service (ICC-ES) approved AC358, Acceptance Criteria for Helical Foundation Systems and Devices. Helical piles are also now included in sections of the 2009 International Building Code.
Helical piles are designed such that most of the axial capacity of the pile is generated through bearing of the helix plates against the soil. The helix plates are typically spaced three diameters apart along the pile shaft to prevent one plate from contributing significant stress to the bearing soil of the adjacent plate. Significant stress influence is limited to a “bulb” of soil within about two helix diameters from the bearing surface in the axial direction and one helix diameter from the center of the pile shaft in the lateral direction. Each helix plate therefore acts independently in bearing along the pile shaft.
Multiple piles shall have a center to center spacing at the helix depth of at least four (4) times the diameter of the largest helix plate (ICC-ES AC358). The tops of the piles may be closer at the ground surface but installed at a batter away from each other in order to meet the spacing criteria at the helix depth.
For tension applications, the uppermost helix plate shall be installed to a depth at least twelve (12) diameters below the ground surface (ICC-ES AC358). The actual depth will vary depending upon soil conditions and capacity requirements, but should not be less than 12 diameters.
The uppermost helix plate shall be embedded in the ground to a depth of at least five (5) diameters to create a deep foundation bearing condition.
The upper helix plate shall also be located below the depth of seasonal frost penetration and below the “active zone”; i.e., the depth of soil that undergoes seasonal volume changes with changes in moisture content. The depth of the helix plates should therefore be determined from the greatest of these values.
The initial installation of a helical pile is performed by applying a downward force (crowd) and rotating the pile into the earth via the helix plates. Once the helix plates penetrate to a depth of about 2 to 3 feet, the piles generally require less crowd and installation is accomplished mostly by the downward force generated from the helix plates, similar to the effect of turning a screw into a block of wood. Therefore, the helix plate performs a vital role in providing the downward force or thrust needed to advance the pile to the bearing depth. The helix plate geometry further affects the rate of penetration, soil disturbance and torque to capacity correlation.
The consequences of a poorly-formed helix are twofold; (1) the helix plate severely disturbs the soil with an augering effect which (2) directly results in more movement upon loading than a pile with well-formed helices. The differences between a well-formed helix and poorly-formed helix are visually obvious and are shown in the figure above.
ICC-ES AC358 establishes design and testing criteria for helical piles evaluated in accordance with the International Building Code. AC358 further provides criteria for helix plates in order to be considered as a “conforming system”. Foundation Supportworks helical piles feature plates manufactured with a helix shape conforming to the geometry criteria of ICC-ES AC358. Conversely, blades that are not a helix shape are often formed to a “duckbill” appearance. These plates create a great deal of soil disturbance, do not conform to the helix geometry requirements of ICC-ES AC358, and their torque to capacity relationships are not well documented.
The coupler detail for a helical foundation system is yet another extremely important feature when considering helical piles and when selecting or specifying a product manufacturer. Manufacturers may advertise that they carry the same or equivalent helical shaft. However, shaft and coupler details are not consistent between manufacturers and these differences may not be readily apparent by simply reviewing product capacity tables.
Some manufacturers rate their products based upon the capacities of the gross section of the shaft, thereby ignoring any limitations caused by the coupled connections. For these “equivalent” products, there can be dramatic differences in material properties, tolerances, spacing of bolt holes, oversize of bolt holes, general fit-up, weld quality, etc.
Some of the more common coupler details for round shaft include external welded, external detached, internal detached, and forged and upset. External couplers utilize tube or pipe sections with an internal diameter slightly larger than the outside diameter of the central shaft material (See Figure 4 and Figure 5). These couplers can be sized to provide tight connections that reduce angular deformation and variances from straightness. Such displacements at the couplers introduce eccentricities to the system which can significantly reduce the allowable compressive capacity of the pile, especially considering the slenderness of the more widely used shaft material (typically 3.5-inch outside diameter and smaller).
Internal detached couplers are made from solid round stock or tube or pipe material but with an outside diameter smaller than the inside diameter of the central shaft material.Internal coupler diameters may be significantly undersized to prevent interferences with internal weld beads of the central shaft or due to the variations that are typical in wall thicknesses and inside diameters of pipe sections. Larger gaps between the inside diameter of the shaft and the outside diameter of the coupler can result in a connection with more potential for angular displacements.
Forged and upset couplers are formed by heating one end of the shaft, placing this end in a form and then enlarging the end with a hammer-like tool or press (See Figure 6).
With this method of manufacturing, it is difficult to create tight connections to strict tolerances.
It is not uncommon to have 1/8-inch or more difference between the outside diameter of the shaft and the inside diameter of the upset coupler of the round shaft (See Figure 7).
Again, the greater the freedom allowed in the connection, the greater the potential variance from straightness and the higher the potential for bending or buckling of the pile under high compressive loads (See Figure 8)
The risk of pile buckling further increases with unsupported lengths above the ground surface, or if the pile extends through soil strata consisting of soft clays or very loose sand.
FSI round shaft helical piles are manufactured with external welded or detached couplers.
These systems are manufactured to strict tolerances to allow the pile shafts to be in direct contact inside the coupling, similar to Figure 9.
Why is this important? The load path for piles under compression is then directly through the shafts of the extensions and lead section without having to pass through welds and bolts at each connection.
The annular space between the pile shaft and coupler is also kept as tight as practical to maintain pile rigidity while also providing connections that are easily joined in the field (See Figure 10 and Figure 11).
The most common coupler detail for solid square shaft utilizes a forged and upset end .
Cast detached couplers have also been used in lieu of the upsetting process.
The upset end of square shaft is created in a similar manner as for the round shaft, except for forming a square socket connection.(See Figure 12)
Figure 13 clearly shows a comparison of coupling rigidity between an FSI external welder coupler for round shaft and a typical upset coupler for square shaft.
A similar draping effect is typical for round shaft helical piles with upset couplers.
FSI recommends that the design engineer request product drawings and review coupling details, tolerances and general fit-up prior to product selection.
Round vs. Square
Coupler Rigidity Comparison FSI round shaft external weided coupler vs typical upset coupler for square shaft
Solid square shaft helical piles have been used successfully for decades in tension applications; i.e., as anchors, tiebacks and soil nails, and have proven to be a suitable and reliable support alternative for such projects.
Not surprisingly, manufacturers then adapted the use of square shaft helical products to be installed vertically for the support of compression loads.
Hollow round shaft piles have also been used in both tension and compression applications.
In general, FSI believes that hollow round shafts are better suited for compression applications whereas solid square shaft may provide some advantages in tension applications.
That said, project and site-specific soil conditions vary which may push the merits and advantages of one system over the other, and the design professional should select the product best suited for the project.
Hollow round shaft helical piles are particularly suited to compression loading applications and offer the following advantages over comparably sized square shaft piles.
Solid square shaft helical piles do offer some advantages over their round shaft counterparts.
Helical Foundation Systems – Advantages