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Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
2.3.2.2 Round vs. Square
Solid square shaft helical piles have been used
successfully for decades in tension applications;
i.e., as anchors, tiebacks and soil nails, and
have proven to be a suitable and reliable support
alternative for such projects. Not surprisingly,
some manufacturers then adapted the use of
square shaft helical products to be installed
vertically for the support of compression loads.
There is much discussion amongst design
professionals and even professionals within
the helical pile industry about appropriate
applications for square and round shaft products.
With just a little understanding of the design and
manufacturing of these two systems, it quickly
becomes apparent for what applications the
products are better suited.
Square shaft helical piles have traditionally
been used in tension applications whereas
hollow round shaft piles have been used in
both tension and compression. In general,
FSI believes that hollow round shafts are
better suited for compression whereas solid
square shaft may provide some advantages
in certain tension applications. That said,
project parameters and site-specific soil
conditions vary, which may push the merits
and advantages of one system over the other,
and the design professional should select the
product best suited for the project. Please
contact the FSI Engineering Department with
any questions regarding product selection.
Hollow round shaft helical piles are particularly
suited to compression loading applications and
offer the following advantages over comparably-
sized square shaft piles.
• Round shaft helical piles, excluding those
with upset couplers, generally have more rigid
coupling connections. Square shaft helical
piles typically have a socket and pin coupling
which increases variances from straightness,
introduces eccentricity to the system, and
increases buckling potential. Refer back to
Figure 2.3.2.1.j
. Square shaft piles may be
considered for some light compression load
applications in soil profiles that offer sufficient
lateral support for these loads; e.g., Standard
Penetration Test (SPT) blow count values ≥
10 blows/foot (ASTM D1586).
• As stated in the Coupler Detail section, The
FSI round shaft helical piles are designed so
the pile shafts are in direct contact within the
coupling connections
(Figure 2.3.2.1.g)
. The
load path for round shaft piles in compression
is then directly through the shafts without
having to pass through the welds or bolts
at each coupling. Shaft to shaft contact
is more difficult to achieve within forged,
upset couplers. For square shaft piles, both
compression and tension loads are then
transferred through the one or two coupling
bolts in double shear.
• The area of steel for a round shaft is located
outward from the centroid, thereby providing
a greater structural section modulus and a
higher moment of inertia. In layman’s terms,
a round shaft pile is more resistant to bending
(Figure 2.3.2.2.a)
. This is an important
consideration for piles with unsupported
lengths, piles penetrating loose or soft soils,
or for piles that are eccentrically loaded such
as in a retrofit application.
• Round shaft typically has a higher installation
torque rating than a comparably-sized square
shaft. For certain product comparisons, this
results in higher pile capacities.
Figure 2.3.2.2.a
Section comparison between 2.875”
diameter, 0.276” wall round shaft and 1.5” square shaft