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Micropiles, Screw Piles and Helical Piers

Helical piles 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 or flights, welded to the lead 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 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 pile loaded in axial compression, while the term ‘anchor’ more often refers to a helical pile loaded in axial tension.

Design Considerations

Helical piles are designed such that most of the axial capacity of the pile is generated through 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.

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 at the ground surface but installed at a batter away from each other in order 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).

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 be 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.

 

CTA: To find out more, contact Thrasher Commercial at 1 (800) 827-0702.

Helicast™ Grouted Helical Pile System

Traditional helical piles are used for deep foundation systems in areas with unstable surface soils. This foundation system has been proven to stabilize existing foundations and new deep foundations. However, when slender helical piles are advanced through loose, soft, or fluid soils, buckling analyses are considered, which may result in either down-rating the pile capacity and requiring more piles, or upsizing to a larger shaft diameter that can support the design working loads.

Helicast™ grouted helical piles offer several advantages in areas with weak soil profiles. As the pile is advanced, a column of grout is formed around the pile shaft. The grout increases the pile shaft’s resistance to buckling and provides corrosion protection. The grouted shaft also increases the bearing capacity of the foundation.

Thrasher Commercial Group is trained to effectively implement this proven system. To schedule an on-site consultation, contact us today.

How grouted piles work

Helicast™ piles consist of standard solid square shaft lead and extension sections with a lead displacement plate generally at the first coupler location, and extension displacement plates at each coupler location thereafter.

The pile is advanced through a grout reservoir at the surface. The lead displacement plate pushes soil outward and away from the central shaft and allows specialty micropile grout from the reservoir to flow by gravity into the void being created. Extension displacement plates help to maintain the size and shape of the grout column as the pile is advanced.

Advantages of grouted helical piles

  • Helicast™ piles can be advanced with smaller equipment in limited access areas. Installation is vibration free and does not generate spoils.
  • The grout column surrounds the pile shaft to create a pile that is more resistant to buckling.
  • The grout column offers additional corrosion protection in aggressive soil profiles.
  • A grouted helical pile may develop capacity in marginal soil conditions where a standard end-bearing helical pile may not. There is additional “frictional” capacity at the grout/soil interface.
  • Frictional piles typically deflect less than end-bearing piles for a given load.
  • Where favorable soil conditions exist, a Helicast™ pile may achieve allowable capacities exceeding 100 kips, which may then be considered as an economical alternative to large diameter helical shaft sizes or other deep foundation options.

Contact Thrasher Commercial Group

If you’re interested in grouted piles for your commercial property or need any other foundation repair services, Thrasher Commercial Group is here help. When you work with our experts, you can count on guaranteed results. To find out more, contact Thrasher Commercial Group at 1-800-827-0702.