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Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
bending forces. The external sleeve extends
through and below the foundation bracket to
essentially create a bracket that is 30 inches
tall. Since the external sleeve and the pier shaft
are confined by the earth, the bending moment
dissipates quickly into the surrounding soils and
generally within the first few feet. The depth
at which the bending moment dissipates is a
function of the soil strength and is greater in
soft soils and less in stiff soils. With the external
sleeve present to resist most of the bending
forces, the capacity of the pier section is
preserved to resist the axial compressive forces.
The second way to address retrofit helical
pier eccentricities is to increase rigidity of the
bracket connection to the foundation. With an
adequately designed rigid connection, much
of the eccentricity is transferred back to the
foundation and less to the pier section. This
connection detail typically consists of several
strategically-located, deeply embedded adhesive
anchors. For example, the FSI HP350B helical
bracket is attached to a foundation with six (6)
5/8
-inch adhesive anchors embedded 7.5 inches.
2.7 Helical Bearing Capacity
Design Overview
There are three common methods for predicting
helical pile capacity; the
individual bearing
method
, the
cylindrical shear method
and
the
torque correlation method
. The first two
methods are rooted in traditional geotechnical
methodology, slightly modified with empirical
data. The individual bearing and cylindrical
shear methods are generally used to calculate
or estimate the pile capacity during the design
phase. The individual bearing method relies on
each helix plate to act independently in bearing
with no overlap of significant stress influence
between adjacent helices. The cylindrical shear
method is applicable for multi-helix piles and
assumes that the top or bottom helix plate acts
in bearing (depending upon direction of loading)
and a cylindrical shear surface develops
between the top and bottom helix. The helical
pile designer must have adequate subsurface
information or a thorough knowledge of the local
soil conditions in order to select the geotechnical
parameters for use in these design equations.
The torque correlation method is fully empirical
and generally used to confirm or verify capacity
during field installation. The torque correlation
method uses the linear relationship between
installation torque and capacity; i.e., the
capacity is calculated as the product of the
installation torque and an empirical torque
factor established through decades of full scale
load testing. The torque correlation method has
even been used on projects with insufficient
soil information as the sole determination of
pile capacity. However, there are increased
risks with relying on this method alone due to
potential weak soil layers that may be present
below the bottom of pile elevation.
Foundation
Supportworks
recommends
that subsurface information be determined
to a depth of at least 5 to 10 feet below the
anticipated helical pile depth. Soil borings
should be extended into competent bearing
soils capable of supporting the design working