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p 23
Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
loads with an adequate factor of safety.
Helical test probes may also be considered to
back-calculate the soil shear strength from
the pile installation torque determined from
calibrated equipment. Helical test probes
should be extended to depths at least 10 feet
below the anticipated depths of the helical
production piles. Refer to “Geotechnical
InvestigationGuidelines for Helical Pile, Helical
Anchor and Push Pier Design” in Appendix 2G
for additional information.
The helix plate spacing along the pile shaft can
control whether a helical pile acts in individual
bearing or cylindrical shear. Closely spaced
helix plates will exhibit cylindrical shear behavior
while well-spaced helix plates will typically fail
the soil in individual bearing. Research has
shown that the transition between cylindrical
shear and individual bearing generally occurs
at helix spacings of 2.5D to 3.5D, where D is
the diameter of the lead helix plate. Within
that range, either method may be considered
applicable. Foundation Supportworks’ helical
piles, tiebacks and soil nails are generally
manufactured with helix plate spacings of 3.0D.
The individual bearing method essentially
utilizes the traditional bearing capacity equation
introduced by Carl Terzaghi in 1943 to determine
the bearing capacity of shallow spread footings.
This method is also used to determine the end
bearing capacity of deep foundations. The other
two capacity prediction methods (cylindrical
shear and torque correlation) were developed
specifically for helical piles used in tension load
applications. These methods were then later
considered to predict compression capacity as
well. The use of the cylindrical shear method
and torque correlation method for compression
capacity determination may then be considered
conservative since at least one helix plate
(bottom plate) is bearing against undisturbed
soil, while in tension applications, all helix plates
are bearing against partially disturbed soil.
A factor of safety of 2.0 is typically used to
calculate the allowable soil bearing capacity of
a helical pile if torque is monitored during the
helical pile installation. Higher or lower factors of
safety may also be considered at the discretion
of the helical pile designer or as dictated by
local code requirements. Lower factors of safety
may be considered for non-critical structures
or temporary applications. Higher factors of
safety may be considered for critical structures,
structures sensitive to movement, or where soil
conditions suggest that creep movement may be
a concern. 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.
Like other deep foundation alternatives,
there are many factors to be considered
in designing a helical pile foundation.
Foundation Supportworks recommends
that helical pile design be completed by an
experienced geotechnical engineer or other
qualified design professional.
