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Helical anchors are designed such that most of the axial capacity of the anchor is generated through bearing of the helix blades against the soil. The helix blades are typically spaced three diameters apart along the anchor shaft to prevent one blade from contributing significant stress to the bearing soil of the adjacent blade. Significant stress influence is limited to a 'bulb' of soil within about two helix diameters from the bearing surface in the axial direction and one helix diameter from the center of the anchor shaft in the lateral direction. Each helix blade therefore acts independently in bearing along the anchor shaft.
Multiple anchors shall have a center to center spacing at the helix depth of at least four (4) times the diameter of the largest helix blade (ICC-ES AC358). The tops of the anchors may be closer at the ground surface but installed at a batter or angle away from each other in order to meet the spacing criteria at the helix depth. Helix blades shall have an embedment depth of at least twelve (12) diameters in the direction of loading (ICC-ES AC358).
The cross section of a square shaft is very compact which can allow the anchor to penetrate more easily through the soil. This compact shape also reduces the stiffness of the cross section and introduces more potential for buckling. These two factors make square shaft helical anchors better suited for tension loads. Foundation Supportworks, Inc. therefore recommends their use mainly for these types of applications. Square shaft helical anchors (piles) used in compression should be evaluated on a case by case basis by the project engineer.
Mechanical Axial Capacity (see note):
Allowable Tension = 26.5 kips*
*The mechanical tensile capacity of the Model 150 Helical Anchor System is limited by the allowable stress levels dictated by AISC for a high strength bolt in double shear. The allowable tensile capacity of the shaft is actually much higher than this Allowable Tension value.
Torque Limited Axial Design Capacities based on Ultimate Torsional
Resistance of Anchor Shaft = 6,340 ft-lbs**:
Ultimate Soil Capacity = 63.4 kips** (with K = 10 ft-1, see note)
Allowable Soil Capacity = 31.7 kips (FOS = 2, Allowable System Capacity therefore governed by mechanical capacity = 26.5 kips*)
**This Ultimate Torsional Resistance and its corresponding Torque Limited Capacities are based on laboratory test results from an IAS accredited facility and may only be approached in idealized conditions. Plastic torsional deformations can begin in the anchor shaft near 4,600 ft-lbs. This value may be reached and exceeded in the field by maintaining alignment between the anchor and the drive head, limiting impact forces and torque reversal, and reducing the tendency to "crowd" (push down on) the anchor. Installation through soils with obstructions or high variability may result in impact loading on the anchor. In these cases, achieving high torque values becomes more difficult and a further reduction in the Design Torque Limit may be appropriate.
Note:
K = 10 ft-1 is a default value as published in ICC-ES AC358 which can, in many cases, be considered conservative. Higher capacities can often be achieved with site-specific load testing. Allowable capacities based on site testing shall not exceed the Mechanical Axial Capacity.
The extensions for the Model 150 Helical Anchor System are manufactured with forged, upset ends that are drilled to create a socket and bolt coupling. Direct, sufficient contact of the anchor shaft is much more difficult to achieve within upset couplings than couplings for shafts with machine cut ends. The coupling bolt then acts in double shear for both tension and compression applications. Since FSI recommends that square shaft anchors be used mainly to resist tension loads, this level of precision is not justified.
The Model 150 Helical Anchor System provides 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 installation equipment can be sized for the project.
Foundation Supportworks recommends that all helical anchors be pre-tensioned following installation. Pre-tensioning to the design load minimizes deflection of the anchors and structure as the anchors are put into service. Anchors can also be "pull tested" or load tested to typically two (2) times the design load or more to identify the ultimate system capacity, better assess soil conditions and soil/anchor interaction, and validate design procedures.
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