March/April 2024 - 83
Monitoring installation torque
Due to the small size of this project
and the verification of capacity, both by
monitoring installation torque and
adding additional depth to develop
capacity from skin friction, load tests
were not performed.
On this site, a large tree
stump was exactly where a
pile needed to be installed.
Placing the grillage
plain extensions (in this case, 4.5 in [114
m] pipe with no helices) were installed
through the grillage end assemblies also
known as " cans. " The grillage ensured
that the piles were in the correct
position, both with respect to location
and angle. After 50 ft (15 m) of installation,
the torque required to
advance the helical pile was no longer
increasing and the minimum specified
torque had not been obtained. Because
the end-bearing capacity of a helical pile
is directly related to the torque required
to install it, the torque not increasing
indicated a homogeneous soil -
probably clay - that did not increase in
strength with depth as expected and
DEEP FOUNDATIONS * MAR/APR 2024 * 83
therefore would not provide the
required axial capacity. A quick
calculation was done to determine how
much additional capacity was required
from shaft friction resulting from soil
adhesion to the pipe shaft in addition to
the end-bearing capacity of the helical
pile. It was determined that a 70 ft (21 m)
pile would provide adequate capacity,
combining both side resistance from
skin friction from the pipe shaft and
end-bearing from the helical plates.
The tower site
Tying the Helical Piles
Together
With the pile installation completed,
the above ground grillage was used to
mark the piles at the correct elevation to
mount the end assembly cans. The
grillage was again removed so the piles
could be cut 16 in (406 mm) above the
marks locating the cans. With the
grillage out of the way, the piles were
cut to their final length, and a threaded
spud was inserted into the pile that
extended below the can. Finally, the
clamp was installed with the help of
specialty hydraulic tooling that would
deform or crimp the pipe wall enough so
that the spud would be captured in the
pipe below the crimp. Axial load could
now be applied to the pile - tension on
the spud and compression on the clamp.
Bearing plates were placed over the pile
that would sit on the clamps. The
grillage was then placed back over the
piles, supported by the bearing plates,
resting on the clamps. Additional
bearing plates were placed over the
threaded spud, resting on top of the
March/April 2024
Table of Contents for the Digital Edition of March/April 2024
TOC
March/April 2024 - Intro
March/April 2024 - 1
March/April 2024 - 2
March/April 2024 - TOC
March/April 2024 - 4
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