September/October 2023 - 112

uplift, torsional and lateral moment
capacity for eight steel foundations to
support for camera poles up to 110 ft
(34 m) in height and digital message
signs along a highway. The initial test
borings provided for the project indicated
that the soils were relatively weak
with SPT blow count (N) of 2-3
throughout the length of the borings.
Additional boring data classified the
soils as medium stiff to stiff clay,
however, it could not be determined
where the transition was without an
extensive boring program. Additionally,
due to the anticipated disturbance to the
soil during installation and uncertainty
regarding when the foundations would
go into service, the foundations were
conservatively designed using the
parameters that modeled the weaker
soils in the area and implemented
reduced soil to steel frictional resistance.
The geometry of the foundations
was primarily controlled by the bolt
circle geometry of the poles to be attach
to the foundations. This geometry
facilitated the use of the anticipated
plate for the fabrication of the foursided
winged steel foundation. Once the
geometry was established, the FE
analyses were performed to establish
the minimum required embedment
such that the foundations would
provide satisfactory bearing, overturning
and torsional resistances and
structural integrity.
A second series of analysis was
performed using parameters that
reflected the medium stiff and stiff clay
material, to predict the bearing capacity
of the foundations in the stiff clay
material. These analyses were used to
provide a recommendation regarding
appropriate equipment to be used to
install the foundations to their design
elevation should stronger soils be
encountered. The project was successfully
constructed in 2019. The
average installation time was about 15
minutes per foundation.
A utility project included the
instal lat ion of 52-winged steel
foundations to support transmission
line structures for a capacity upgrade in
an urban setting along a general utility
right-of-way. As such, the foundations
were installed within as little as 6 in
(150 mm) from gas, water and sanitary
underground utilities. Overhead was the
existing power line that had to remain
energized because of an area outage due
to inclement weather that had moved
into the project area. The steel foundations
were offered as an engineering,
value-added alternative to the use of
drilled piers. Basically, at some locations
there was not enough room to install the
drilled piers, which would have resulted
in a greater impact to the surrounding
area, including working in close
proximity to schools and commercial
and residential occupancies. The
flexibility of the analysis and design
criteria for the foundations provide a
spectrum of axial and lateral load
capacities that facilitated the substitution
of the steel foundations for other
types of conventional foundations.
A Mobilram with an oscillating
dynamic hammer was selected for the
installation. It has the capability of
lifting the foundations and setting
them in place, and it was the only high
mast piece of equipment that needed to
enter the power line corridor. As a
result, the power company was able to
swing the lines away from the mobile
ram mast, thus allowing the foundation
installation. The ability to minimize
ancillary work usually associated with
this type of construction facilitated the
installation of four to five foundations
per day, even with noise constraints
limiting the workday to a start time of
10:00 a.m.
The Mobilram has the facility to
impart a variable crowd force and
oscillating dynamic force ranging from
zero to a machine rated maximum
centrifugal force depending on the
dynamic hammer, and one with a
112 * DEEP FOUNDATIONS * SEPT/OCT 2023
potential maximum centrifugal force of
1,500 kN (153 T force) was used. The
arrangement of the machine's mast
with the hammer affixed can produce a
variable crowd force from 0-37 kip
(0-165 kN) and a variable dynamic load
from zero to a maximum of 1,500 kN
(153 T force). After some trials, it was
determined that by applying a crowd
force of 73% or 27 kip (120 kN) of the
maximum and a 10% dynamic force on
the order of 150 kN (17 T force), a
satisfactory rate of installation on the
order of 1 ft (0.3 m) per minute was
achieved. This applied a dynamic force
that produced oscillation in comparison
to a typical vibratory hammer
that imparts significant vibration to the
foundation and surrounding area. As
such, the oscillations were on the order
that minimized ground disturbance
and the potential to liquify the saturated
subgrade soils while provided a
reasonable rate of installation.
The Mobilram was selected over the
use of a hanging vibratory hammer
because typical installations using
hanging vibrators al low lateral
movements that would have likely
resulted in damage to the adjacent
utilities and possibly the buildings
along the utility right-of-way. This
machine mounts the hammer on trails
that control the vertical alignment and
lateral movement of the foundation
during installation by locking the
foundation to the trails by way of the
hammer's locking mechanism.
Ground survey control was used to
locate the foundations and to verify
machine settings for elevation and
compliance with foundation location
and orientation. The monitoring process
was conducted continuously
throughout the installation process. A
vibration meter was used to monitor
induced ground response at 15 ft (4.5 m)
from the driven foundation. The soils at
the site ranged from a loose to dense
layered, saturated granular substrate.
The measurements provided that the
September/October 2023

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