Which Helical Pile Type to Choose & Why: Square, Round, Combo or Displacement

elical piles come in a variety of types and sizes, giving engineers and contractors flexibility to find the best solution for a project based on soil type and site conditions. Since the A.B. Chance Company started making helical anchors in 1912, engineers have been innovating and improving the technology to suit a broad range of applications.  

The four main types of helical piles that Chance has developed and still manufactures today are:

Square Shaft (SS) Helical Piles

Development of a high-torque, shaft-driven, multi-helix anchor began in 1963, culminating in the introduction of Chance Type SS 1-1/2” square shaft multi-helix anchors in 1964-65. The SS pile family has since expanded to include 1-1/4”, 1-3/4”, 2”, and 2-1/4” square shafts. Extension shafts with upset sockets lengthen these anchors to penetrate most soils at significant depths for many civil construction applications.

Engineers and contractors are encouraged to use square shaft helical piles as much as possible due to their advantages with torque correlation efficiency and better penetration in dense soil.  Square shaft piles have the lowest cost per capacity at load ranges 50 kip or less. Ultimate capacity ranges from 55 kip to 200 kip depending on shaft size. They transfer load to the soil in end-bearing via the helix plates at the pile tip. Square Shaft is the most effective shaft type to maximize bearing capacity on the helix plates.  It is the best choice for tension only loads.  

It is important to remember that square shaft is solid, not hollow.  It is manufactured from grade 70 for SS5 and grade 90 steel bar for SS150, SS175, SS200, and SS225.  If the geotechnical report declares the corrosion potential is moderate to severe, the solid cross section and low perimeter surface area of a square shaft helical pile is the best choice.

Square shaft has the highest torque correlation factor of any other shaft type.  Torque correlation means that it is generally accepted that the installation torque of helical piles can be used to verify their axial capacity. Read more about torque correlation in this article.

Square Shaft Product Ratings 
(complete ratings available in section 7 of the Chance Technical Design Manual)

Try HeliCAP® helical capacity design software to calculate the capacity of different pile types in your custom-input soil profile. Create your free account online at www.hpsapps.com/helicap.

Combo Pile

Combo Piles are the third type of helical pile shaft and offer “the best of both worlds” from the previous two pile types discussed.  They are a “combination” of the advantages of square shaft and pipe shaft.  A cast transition connector or “coupler” is used to join the square and round shaft sections together.  The most common combination is a square shaft lead section followed by the transition coupler to which one or multiple pipe shaft extensions are added to make up the full length of the helical pile.  Ultimate capacity ranges from 54 kip to 147 kip depending on shaft size.

Combo piles “combine” the advantages of square shaft and pipe shaft piles. Square shaft offers the most efficient penetration into firm, hard, or dense soil. The pipe shaft extensions that follow provide stability against columnar buckling in soft soils or when the shaft is exposed at the surface.  Pipe shaft extensions also provide lateral capacity that square shaft cannot.  Combo Piles also provide higher torque correlation factors compared to straight pipe shaft helical piles.

Combo piles are limited to the torque strength of the lowest rated shaft element, which is usually the square shaft lead section, but not in all cases.  No tooling change is required – just add the tapered coupler between the square and pipe shaft and then proceed installing pipe shaft extensions until installation is completed.

Since pipe shaft piles have to displace a little bit more soil than the square bar does, it takes more energy to overcome that friction, which means the torque-to-capacity relationship is not as efficient as square shaft. When you combine them, you get maximum bearing strength or capacity with the square shaft, plus the larger section of the pipe minimizes any concerns about buckling and provides lateral capacity. 

Selected Combo Pile Product Ratings
(complete ratings available in section 7 of the Chance Technical Design Manual)

Displacement Pile

A displacement pile is a helical pile that has the steel shaft section encased in a grout column.  Displacement piles can be installed with either square or round shaft piles, with the grout column providing greater section properties for shaft stability and lateral resistance in soft soils. Just like with other Chance helical pile products, no soil is removed – it is compacted around the shaft which increases soil strength and decreases installation depth. This saves time on the jobsite with no spoils to haul away and eliminates environmental and safety concerns that arise when there is contaminated soil on the jobsite.

The grout column provides additional capacity from friction along the sides of the shaft in contact with the soil. Grout fills the void created as the pile advances. The grout column provides additional resistance to corrosion. Displacement piles are often used when design loads exceed about 60 kip.

One type of displacement pile is called the Helical Pulldown™ Micropile. This video shows an installation of this type of pile:

The installation process can employ grout only, or grout in combination with steel or PVC casing.  The result is a helical pile with a composite steel/grout section that is similar drilled and grouted Micropiles or auger cast-in-place piles.  Ultimate capacity ranges from 55 kip to 430 kip depending on shaft type and size.  Ultimate lateral capacity with steel casing ranges between 3 kip and 15 kip.

Displacement piles are used to prevent columnar buckling in very soft or loose soil, especially when used with square shaft material.  In addition, the grout column increases axial capacity with side resistance via skin friction on the shaft above the helix plates.  The grout column also increases the “axial stiffness” of the pile, which means the pile will support load with less settlement. Displacement piles with square shaft material has the lowest cost per capacity in most situations at load ranges above 50 kip. 

Disadvantages include possible refusal in dense or obstruction-laden soil and increased cost with the inclusion of labor and material for the grout.

If you have any questions for our engineering team about which shaft type would be best for your application, reach out to us at civilconstruction@hubbell.com.