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p 28
Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
affect K
t
determined from field load testing. The
helical pile industry has long used the differential
pressure across the gear motor for correlation
to installation torque. The installation torque
is then correlated to pile capacity. In other
words, the differential pressure across the gear
motor is commonly used to determine the pile
capacity. The current state-of-practice involves
using a gear motor multiplier (GMM) to convert
from differential pressure to torque. The GMMs
are provided by the gear motor manufacturers
based on theoretical equations and will vary with
the planetary gear ratio, motor displacement
and motor efficiency. Gear motor manufacturers
typically show a linear fit between the differential
pressure and output torque with no scaling
effect. Research has shown that the gear motor
differential pressure to torque relationship is
generally linear, however, there is a scaling
adjustment needed (Deardorff 2007). This results
in a range of GMMs from low to high differential
pressure. The discrepancy between actual
installation torque and torque determined by
correlation to differential pressure is highest at
low differential pressures. This difference often
decreases steadily as the differential pressure
increases up to the point of maximum motor
efficiency. Therefore, it is highly recommended
that gear motors be certified on an annual
basis, or whenever changes occur to alter
their performance, in order to establish their
true differential pressure to torque relationship.
Calibrated in-line torque monitoring devices
may also be used as an alternative to having the
drive motors certified.
Finally, the installation practices of the specialty
contractor and the quality control of the helical
pile manufacturer will affect K
t
. Helical piles
should ideally be installed at a rate equal
to the pitch of the helix plate (3 inches per
revolution) with no more than 25 revolutions
per minute (rpm). The installation rate should
be reduced to about 10 rpm during final seating
of the helical plates. The rate of advancement
can be controlled by the installing contractor
by adjusting the speed and downward force
(crowd) as different soil layers are encountered
and penetrated. The helical pile manufacturer
should provide a helix plate geometry that is
a true ramped-spiral with uniform pitch. The
geometry of the helix plate is instrumental in
providing the downward thrust or pull into the
ground and should be controlled to increase the
installation efficiency and subsequent K
t
. Refer
to Section 2.3.1 for an in-depth discussion
about helix plate geometry. Proper installation
procedures and well-formed helix plates are
critical to minimize soil disturbance.
