Screw Anchor & Soil Nailing
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.
Advantages
- Predictable capacity
- Helix blade configuration selected to achieve design embedment and capacity
- All-weather installation
- Can be installed in areas of limited or tight access
- Installation does not generate spoils
- Clean installation with no messy grout
- Load tests can be performed immediately following installation
- Available with optional hot-dip galvanizing for added corrosion protection
Design Considerations
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.
- Model 150
- Model 175
- Capacities Summary
- Determination of Capacity
Model 150 Helical Anchor System
- Outer Dimensions = 1.50" x 1.50"
- Anchor Shaft Yield Strength = 90 ksi (min.)
- Coupling Hardware: ¾" Grade 8 Bolt with Nut
- Available Helix Blade Diameters = 6", 8", 10", 12" and 14"
- Helix Blade Thickness = 0.375"
- Termination Hardware: 1" Threaded Rod, Tensile Strength = 120 ksi (min.)
Model 175 Helical Anchor System
- Outer Dimensions = 1.75" Round Corner Square Bar
- Anchor Shaft Yield Strength = 90 ksi (min.)
- Coupling Hardware: (2) ¾" Grade 8 Bolt with Nut
- Available Helix Blade Diameters = 6", 8", 10", 12" and 14"
- Helix Blade Thickness = 0.375"
- Termination Hardware: 1" Threaded Rod, Tensile Strength = 120 ksi (min.)
- Governed by AISC allowable capacity of single Ø3/4" (HA150) or (2) Ø3/4" (HA175) Grade 8 bolt(s) in double shear.
- Governed by bearing at the bolt holes.
- Capacities include a scheduled loss in steel thickness due to corrosion for black, uncoated steel. Scheduled thickness losses are for a period of 50 years and are in accordance with ICC-ES AC358.
- Allowable compression capacities are based on continuous lateral soil confinement in soils with SPT blow counts ≥ 4.Piles with exposed unbraced lengths or piles placed in weaker or fluid soils should be evaluated on a case-by-case basis by the project engineer.
- Listed mechanical capacities are for the shaft only. System capacities should also not exceed the installed torque correlated capacity or those listed in the respective bracket capacity tables.
- Listed default Kt factors are widely accepted industry standards. They are generally conservative and are consistent with those listed in ICC-ES AC358. Site-specific K t factors can be determined for a given project with full-scale load testing.
- Soil capacities listed are ultimate values at maximum installation torque. Allowable soil capacity values are obtained by dividing the ultimate values by the appropriate factor of safety (FOS). FOS is most commonly taken as 2.0, although a higher or lower FOS may be considered at the discretion of the helical pile designer or as dictated by local code requirements.
- Square shaft piles may be considered for compression applications in soil profiles that offer sufficient continuous lateral support; e.g., in soils with SPT blow counts ≥ 10. Even in these higher strength soil conditions, buckling analysis should be considered, taking into account discontinuities and potential eccentricities created by the couplers.
Determination of Capacity
The ultimate capacity of a helical anchor may be calculated using the traditional bearing capacity equation:
Qu = ∑ [Ah (cNc + qNq)]
Where:
|
Qu |
= |
Ultimate Anchor 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 |
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.
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.
Helical Blade Geometry
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.