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Commercial Helical Anchors / Tiebacks

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

Commercial Helical Tieback Installation

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.)

-- View Our Helical Anchors and Tiebacks Product Specifications document --

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.)

-- 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)

  1. Governed by AISC allowable capacity of single Ø3/4" (HA150) or (2) Ø3/4" (HA175) Grade 8 bolt(s) in double shear.
  2. Governed by bearing at the bolt holes.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. 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.

Determiniation of Capacity

Like other deep foundation alternatives, there are many factors to be considered in designing a helical anchor foundation. 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:
Qu = Ultimate anchor 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 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 --

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