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Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
• Round shaft offers a higher lateral resistance
with more shaft area exposed to the
surrounding soil. If necessary, hollow round
shafts can also be grout-filled to further
improve the pile stiffness.
Solid square shaft helical piles do offer some
advantages over their round shaft counterparts.
• Square shaft is a more compact section than
comparably-sized round shafts and will therefore
achieve greater soil penetration for a given
amount of torque. This benefit is particularly
important in tieback applications where the piles
must be installed to certain embedment criteria
as well as torque/capacity criteria.
• Square shaft, again due to its more compact
shape, may penetrate through or into dense
soils or soft or weathered bedrock layers
more easily.
• Square shaft has less surface area exposed to
corrosion and corrosion can only occur from the
outside surface inward. Conversely, corrosion
is possible for round shaft on both the outside
and inside surfaces, although actually limited
on the inside surfaces of closed pipe sections
due to lack of oxygen. See Appendix 2E for
additional information on corrosion.
• The degree of shaft twist may be considered
as another rough indication of applied torque
since permanent deformation begins within
a known narrow range for each product.
Contractors know they have past this threshold
when the shaft twist is not recovered when
the installation torque is released.
Although
these observations can be used as a guide
or point of reference during installation,
FSI does not recommend that shaft twist
be used solely as a measure or estimate of
applied torque.
• Square shaft can withstand more deformation/
twist before shaft failure. Square shaft
is therefore much more forgiving during
installation, allowing less experienced
installers to decrease the applied torque
before shaft damage may occur.
2.3.3 Brackets
A load transfer device (bracket) is used as a
mechanism to transfer the structural load to the
pile shaft. In
new construction
applications,
a bracket; i.e., cap plate or T-cap, is welded or
bolted to the top of the pile and then cast into
the structural concrete, into the grade beam or
pile cap. New construction brackets often consist
of round shaft sleeve material with a flat plate
welded to the top
(Figures 2.3.3.a1 and 2.3.3.a2)
.
Steel reinforcing bars may also be welded to the
sleeve or plate to further engage the concrete.
In compression load applications, the new
construction bracket could theoretically be set
on top of the pile without welding or bolting.
Figure 2.3.3.a1
Rendering of new construction
helical piles cast into a structural grade beam
Figure 2.3.3.a2
Schematic of new construction bracket