Helical Anchors (also referred to as tiebacks) provide lateral stability to foundation walls and retaining walls with unbalanced earth pressures. Helical anchors can be installed with hand-held equipment, mini-excavators, skid steers, backhoes, trackhoes, or crane-supported rigs so the anchors can be installed in almost any application. This versatility, along with the ability to immediately load and test the anchors, make helicals a convenient and economical solution for a wide variety of projects.
![helical tiebacks in [state]](https://cdn.treehouseinternetgroup.com/core/images/commercial-foundation/helical-anchors-tiebacks/helical-tieback-solo.jpg)
Helical anchors are a factory-manufactured steel foundation system consisting of a central shaft with one or more helix-shaped bearing plates, commonly referred to as blades, welded to the lead section. Extension shafts, with or without additional helix plates, are used to extend the anchor into competent load-bearing soils. Helical anchors 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 foundation system loaded in axial compression, while the term "anchor" more often refers to a helical foundation system loaded in axial tension.
-- View Our Helical Anchors and Tiebacks Product Specifications document --
-- View Our Helical Anchors and Tiebacks Product Specifications document --
| Capacities Summary | |||||
|---|---|---|---|---|---|
|
Maximum Allowable Mechanical Shaft Capacities (3,5) |
|||||
|
Default Torque Correlation Factor (6) Kt (ft-1) |
Maximum Installation Torque (ft-lbs) | Maximum Ultimate Torque Correlated Soil Capacity (6,7)Qu = Kt X T (kips) |
Axial Compression (kips) | Axial Tension (kips) | |
|
HA150 |
10 |
6,500 |
65.0(8) |
26.5(1,8) |
26.5(1) |
|
HA175 |
10 |
10,000 |
100.0(8) |
65.7(8) |
53.0(1) |
|
HP287 |
9 |
5,600 |
50.4 |
46.4(4) |
23.6(2) |
|
HP288 |
9 |
7,900 |
71.1 |
65.4(4) |
34.1(2) |
|
HP349 |
7 |
13,000 |
91.0 |
88.7(4) |
50.8(2) |
|
HP350 |
7 |
16,000 |
112.0 |
107.8(4) |
62.5(5) |
The ultimate capacity of a helical anchor may be calculated using the traditional bearing capacity equation:
Qu = ∑ [Ah (cNc + qNq)]
| Where: |
|
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 anchor installation.
Like other deep foundation alternatives, there are many factors to be considered in designing a helical anchor foundation. Supportworks recommends that helical anchor design be completed by an experienced geotechnical engineer or other qualified professional.
Another well-documented and accepted method for estimating helical anchor capacity is by correlation to installation torque. In simple terms, the torsional resistance generated during helical anchor installation is a measure of soil shear strength and can be related to the bearing capacity of the anchor.
Qu = KT
| Where: |
|
The capacity to torque ratio is not a constant and varies with soil conditions and the size of the anchor shaft. Load testing using the proposed helical anchor and helix blade configuration is the best way to determine project-specific K-values. However, ICC-ES AC358 provides default K-values for varying anchor shaft sizes, which may be used conservatively for most soil conditions. The default value for the Model 150 Helical Anchor System (1.50" square shaft) is K = 10 ft-1.
-- View Our Helical Anchors and Tiebacks Product Specifications document --
Supportworks' helical anchors feature blades manufactured with a true helix shape conforming to the geometry criteria of ICC-ES AC358. The leading and trailing edges of true helix blades are within one-quarter inch of parallel to each other and any radial measurement across the blade is perpendicular to the anchor shaft. A true helix shape along with proper alignment and spacing of the blades is critical to minimize soil disturbance during installation.
Conversely, blades that are not a true helix shape are often formed to a 'duckbill' appearance. These plates create a great deal of soil disturbance and do not conform to the helix geometry requirements of ICC-ES AC358 since their torque to capacity relationships are not well documented.
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