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.
Helical piers were invented over 100 years ago for commercial construction, used first in tension applications to hold down cables, then later in compression applications to shore up loads in soils.
It is a simple concept in which a large helical screw called a helix and is attached to a shaft which is turned with hydraulic torque motors. The shaft drills itself down until it reaches refusal at a specific torque pressure. Foundation Supportworks helical piers are able to efficiently stabilize distressed foundations by transferring the load of the structure from unstable soil, through the piers, to competent soil strata below.
Round-shaft helical piers have a high resistance to buckling and have the torsional strength to penetrate deep into competent soil unlike other square-shaft helical alternatives. Another benefit of helical piers is that they do not use the weight of the home as resistance during installation, so they can be used on both heavy and lightly loaded structures such as sinking stoops, decks, or sunrooms.
Advantages of Helical Piles
- Can be installed year-round
- Round shaft has a high resistance to bending
- Suitable for both heavy and light loads
- Long life span galvanized steel is resistant to corrosion
- Doesn't rely on the weight of the structure for depth and capacity
- In most cases can lift foundation back to level position
- Restores Property Value
Installation of Helical Piles
First, soil is removed from the area where the helical pier will be installed. Helical pier sections are mechanically "screwed," or advanced, into the soil. The first section of the pier that is advanced into the ground has one or more helical blades (Or "bearing plates") welded to the shaft.
Additional helical pier sections do not heave bearing plates, and are instead used to extend the pier to the necessary installation depth.
Helical piers can be installed from either the exterior or interior of your home, providing an opportunity to lift your home back to its original position by the end of the installation time.
Once proper depths and capacities are achieved, heavy duty steel foundation brackets are positioned below and against the foundation footing.
The weight of the structure is then transferred through the helical piers to deep, competent soils. Lifting it back towards its original position is attempted. Then, the soil around the foundation is carefully replaced.
Determination of Helical Pile Capacity
The ultimate capacity of a helical pile may be calculated using the traditional bearing capacity equation:
Qu = ∑ [Ah (cNc + qNq)]
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.
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
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.