Helical Piles and Your Foundation
Helical Piers Installed for Commercial Buildings in Illinois and Iowa
One of the most effective steel foundation systems on the market, helical piles are manufactured with a central shaft with helix-shaped bearing plates (aka flights or blades) welded to the leading section. You may have heard of these as helical piles, screw piles, helical anchors, helical piers, helix piers, or helix anchors. Technically, the term “anchor” refers to a helical pile loaded with axial tension, while the term “pier” typically refers to a helical pile loaded with axial compression.
Helical piles are advanced into the ground, similar to a screw-like motion, to reach competent soils. Sometimes, extension shafts, with or without the helical blades, are used to reach the appropriate depth and load capacity. Brackets are used to attach the tops of the piles to structures, in both new construction and retrofit applications.
Design Considerations
The design of a helical pile is such that the axial capacity comes through the bearing of the helical blades in the soil. These are usually spaced three diameters apart on the shaft to alleviate any significant stress on adjacent blades. Thus, the stress influence is contained to a “bulb” of soil within approximately two helix diameters from the bearing surface in axial directions, and one helix diameter laterally from the shaft center.
For proper installation of multiple piles, each should be spaced at least four times the diameter of the largest helix blade, center to center (ICC-ES AC358). This refers to spacing at the full helix depth, but the tops of the piles may be closer at the surface, advanced at a batter away from one another. In tension applications, the top helix blade should reach a final depth of at least 12 diameters below the surface of the ground (ICC-ES AC358).
Determination of Capacity
The traditional bearing capacity equation may be used to determine the ultimate capacity of a helical pile, as follows:
Qu = ∑ [Ah (cNc + qNq)]
Where:
Qu = Ultimate Pile Capacity (lb)
Ah = Area of Individual Helix Plate (ft2)
c = Effective Soil Cohesion (lb/ft2)
Nc = Dimensionless Bearing Capacity Factor = 9
q = Effective Vertical Overburden Pressure (lb/ft2)
Nq = Dimensionless Bearing Capacity Factor
For short-term or transient load applications, total stress parameters should be used. For long-term or permanent load applications, effective stress parameters should be used. To determine the allowable soil load-bearing capacity, a factor of safety of 2 is recommended, particularly if you are monitoring torque during the installation process.
Similar to other deep foundation support systems, several different factors should be weighed when designing a helical pier foundation. Helical pile design is best completed by a qualified geotechnical engineer or other experienced professional, as recommended by Supportworks.
For another well-documented method for calculating helical pile capacity, consider correlation to installation torque. Simply put, the torsional resistance created during the installation of the helical pile is a measure of soil shear strength, and is related to the pile’s bearing capacity.
Qu = KT
Where:
Qu = Ultimate Pile Capacity (lb)
K = Capacity to Torque Ratio (ft-1)
T = Installation Torque (ft-lb)
It is important to note that the torque-to-capacity ratio is not a constant, and varies with both the side of the pile shaft and the soil conditions. The optimal way to determine the K-values for your project is to conduct load testing with the proposed helical pile and blade configuration. For conservative estimates in most soil conditions, the ICC-ES AC358 outlines default K-values for differing shaft diameters. For example, the Model 288 Helical Pile System (2 ⅞-inch diameter) has a value of K = 9 ft-1.
Total stress parameters should be used for short-term and transient load applications and effective stress parameters should be used for long-term, permanent load applications. A factor of safety of 2 is typically used to determine the allowable soil bearing capacity, especially if torque is monitored during the helical pile installation.
Like other deep foundation alternatives, there are many factors to be considered in designing a helical pile foundation. Supportworks recommends that helical pile design be completed by an experienced geotechnical engineer or other qualified professional.
Another well-documented and accepted method for estimating helical pile capacity is by correlation to installation torque. In simple terms, the torsional resistance generated during helical pile installation is a measure of soil shear strength and can be related to the bearing capacity of the pile.
Qu = KT
Where:
Qu = Ultimate Pile Capacity (lb)
K = Capacity to Torque Ratio (ft-1)
T = Installation Torque (ft-lb)
The capacity to torque ratio is not a constant and varies with soil conditions and the size of the pile shaft. Load testing using the proposed helical pile and helix blade configuration is the best way to determine project-specific K-values. However, ICC-ES AC358 provides default K-values for varying pile shaft diameters, which may be used conservatively for most soil conditions. The default value for the Model 288 Helical Pile System (2 7/8-inch diameter) is K = 9 ft-1.
