Commercial Helical Piles

Commercial helical piles, also known as screw piles, are the primary components of a factory-manufactured heavy steel support system designed to stabilize commercial building foundations. 

Foundation problems occur in commercial applications for many of the same reasons they occur in residential applications: the soil beneath the structure is unable to bear its weight. This can be due to one or more of several factors, several of which relate to an excess or lack of moisture.

When there is too much moisture in the soil, it can soften or even wash away. When there is too little moisture, the soil can shrink, leaving voids into which a foundation can sink. 

A third possible reason for foundation settlement is that it may have been built upon soil that was poorly compacted to begin with. As part of the initial construction process, the location was graded and landscaped — soil from hilltops was moved into valleys to create a level surface. If that fill soil isn’t compacted properly, it can compress over time under the weight of the newly constructed building. 

Regardless of the case, the solution to foundation settlement is to connect the structure to bedrock or load-bearing competent soils. One of the best ways to do this is via the installation of a helical pile system that can push past the poor soil and find these load-bearing strata.

Commercial Helical Installation

Installing commercial helical piles

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The Importance of Foundation Stabilization

There is a common misconception that it is normal for a building to “settle” over time. Unfortunately, when people refer to a building “settling,” what they typically mean is that the structure’s foundation is sinking into the soil beneath it. Unfortunately, when a foundation sinks, it rarely does so in a uniform fashion.

This uneven settlement results in a structure that is itself uneven — which means that everything that is related to it is affected by this unevenness. Floors sag, ceilings gap, doors and windows stick. Therefore, before a building is completed or before renovations can be made, its foundation must be stabilized to prevent further settlement. Again, helical piles are a simple, reliable and cost-effective solution to this problem.

Design

Helical piles consist of a central shaft with one or more helix-shaped bearing plates, commonly referred to as blades or flights, welded to the main section.

Extension shafts, with or without additional helix blades, are used to extend the pile to competent load-bearing soils and to achieve design depth and capacity. Brackets are used at the tops of the piles for attachment to structures, either for new construction or retrofit applications. Helical piles are advanced (screwed) into the ground via the application of torque.

The terms helical piles, screw piles, helical piers, helical anchors, helix piers, and helix anchors are often used interchangeably. However, the term pier typically refers to a helical pile loaded in axial compression, while the term anchor more often refers to a helical pile loaded in axial tension.

How They Work

Helical piles are designed so that most of the axial capacity of the pile is generated through the bearing of the helix blades against the soil. The helix blades are typically spaced three diameters apart along the pile 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 pile shaft in the lateral direction. Each helix blade, therefore, acts independently in bearing along the pile shaft (see diagram).

Multiple piles 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 piles may be closer to the ground surface but installed at a batter away from each other to meet the spacing criteria at the helix depth. For tension applications, the uppermost helix blade shall be installed to a depth of at least twelve (12) diameters below the ground surface (ICC-ES AC358).

Supportworks Model 287 Helical Pile System

Technical Specifications

  • Outside Diameter (O.D.) = 2.875"
  • Wall Thickness = 0.203"
  • Pile Shaft Yield Strength = 60 ksi (min.)
  • Coupling Hardware: (2) ¾" Grade 8 Bolts with Nuts
  • Available Helix Blade Diameters = 8", 10", 12", and 14"
  • Helix Blade Thickness = 0.375"
  • New Construction Bracket: ¾" x 6" Square A36 Plate (for allowable compression capacities up to 60.0 kips)
  • New Construction Bracket Hardware: (2) ¾" Grade 8 Bolts with Nut

Bracket Specifications

  • Bracket: Weldment manufactured from 0.25", 0.375", and 0.50"-thick steel plate.
    Yield strength = 36 ksi (min.), tensile strength = 58 ksi (min.).
  • External Sleeve: 3.50" OD x 0.216" wall x 30" long with sleeve collar welded to one end.
    Yield strength = 50 ksi (min.), tensile strength = 62 ksi (min.).
  • Bracket Cap: 5.0" wide x 9.0" long x 1" thick plate with confining ring welded to one side.
    Yield strength = 50 ksi (min.), tensile strength = 65 ksi (min.).
  • All-Thread Rod: 0.75" diameter x 16" long, zinc plated. Grade B7, tensile strength = 125 ksi (min.).

Supportworks Model 288 Helical Pile System

Technical Specifications

  • Outside Diameter (O.D.) = 2.875"
  • Wall Thickness = 0.276"
  • Pile Shaft Yield Strength = 60 ksi (min.)
  • Coupling Hardware: (2) ¾" Grade 8 Bolts with Nuts
  • Available Helix Blade Diameters = 8", 10", 12", and 14"
  • Helix Blade Thickness = 0.375"
  • New Construction Bracket: ¾" x 6" Square A36 Plate (for allowable compression capacities up to 60.0 kips)
  • New Construction Bracket Hardware: (2) ¾" Grade 8 Bolts with Nut

Bracket Specifications

  • Bracket: Weldment manufactured from 0.25", 0.375", and 0.50"-thick steel plate.
    Yield strength = 36 ksi (min.), tensile strength = 58 ksi (min.).
  • External Sleeve: 3.50" OD x 0.216" wall x 30" long with sleeve collar welded to one end.
    Yield strength = 50 ksi (min.), tensile strength = 62 ksi (min.).
  • Bracket Cap: 5.0" wide x 9.0" long x 1" thick plate with confining ring welded to one side.
    Yield strength = 50 ksi (min.), tensile strength = 65 ksi (min.).
  • All-Thread Rod: 0.75" diameter x 16" long, zinc plated. Grade B7, tensile strength = 125 ksi (min.).

Supportworks Model 350 Helical Pile System

Technical Specifications

  • Outside Diameter (O.D.) = 3.5"
  • Wall Thickness = 0.313"
  • Pile Shaft Yield Strength = 60 ksi (min.)
  • Coupling Hardware: (4) 1" Grade 8 Bolts with Nuts
  • Available Helix Blade Diameters = 8", 10", 12", and 14"
  • Helix Blade Thickness = 0.375"
  • New Construction Bracket: ¾" x 6" Square A36 Plate (for allowable compression capacities up to 60.0 kips)

Bracket Specifications

  • Bracket: Weldment manufactured from 0.25", 0.375", and 0.50"-thick steel plate.
    Yield strength = 36 ksi (min.), tensile strength = 58 ksi (min.).
  • External Sleeve: 3.50" OD x 0.216" wall x 30" long with sleeve collar welded to one end.
    Yield strength = 50 ksi (min.), tensile strength = 62 ksi (min.).
  • Bracket Cap: 5.0" wide x 9.0" long x 1" thick plate with confining ring welded to one side.
    Yield strength = 50 ksi (min.), tensile strength = 65 ksi (min.).
  • All-Thread Rod: 0.75" diameter x 16" long, zinc plated. Grade B7, tensile strength = 125 ksi (min.).

Determination of Capacity

The ultimate capacity of a helical pile may be calculated using the traditional bearing capacity equation:

Qu = ? [Ah (cNc + qNq)]

Where:
Qu = Ultimate Pile 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 pile installation.

Like other deep foundation alternatives, there are many factors to be considered in designing a helical pile foundation. Supportworks recommends that helical pile design is completed by an experienced geotechnical engineer or other qualified professional.

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

Where:
Qu = Ultimate Pile 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 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.

Free, No-Obligation Inspection

NSquare offers expert foundation inspection services conducted by trained, highly experienced technicians. There is no cost or obligation for this service, and by the time the inspection is over, you will know exactly what is causing the problem, exactly what it will take to fix it permanently and exactly how much it will cost. Call (239) 206-1480 to schedule your site inspection or visit our contact page, and someone will be in touch with you shortly.