March/April 2024 - 81

Timber matting protects
environmentally sensitive
wetlands
FEATURE ARTICLE
Helical Piles Support Transmission
Line in Sensitive Wetland Environment
A short section of 230 kV transmission
line was built with four 95 ft (29 m) steel
monopoles to connect a solar farm to a
substation through an environmentally
sensitive wetland in the southeastern
U.S.
The foundation required to support a
monopole is primarily a moment resisting
foundation, supporting an eccentric
lateral load that creates an overturning
moment at the base. The moment created
tends to be quite large, which often
results in a large and expensive foundation.
In this case, there were two different
load conditions to support. The first
load condition, a 1,612 ft kip (2,185 kNm)
moment combined with 16.4 kip (73 kN)
axial compression and 21.7 kip (97 kN)
shear, was supported by a 12 pile above
ground steel grillage. The second load
condition, a 1,323 ft kip (1,794 kNm) moment,
14.8 kip (66 kN) axial compression
and 20.0 kip (89 kN) shear, was supported
by an eight pile above ground grillage.
Typical deep foundation choices for
these types of loads would include:
* Large diameter drilled shaft -
replacement foundation, soil is
removed and replaced with concrete.
* Driven pi les - displacement
foundation with a concrete pile-cap
at the surface to tie the piles together.
* Micropiles - replacement foundation
consisting of small diameter,
drilled and grout-reinforced piles
tied together at the surface with
either a concrete cap or a steel
grillage.
* Helical piles - displacement
foundation with an innovative steel
grillage system made from mostly
recycled steel to tie piles together at
the surface.
Prior to construction, 10 soil borings
were completed to a depth of 12 ft (3.6 m)
on the agricultural land intended for a
AUTHORS Shawn Downey, Hubbell Power Systems
DEEP FOUNDATIONS * MAR/APR 2024 * 81
Installing a 4.5 in (114 mm) diameter pipe shaft
helical extension
photovoltaic power plant. No subsurface
investigation was done specifically
for the transmission line that was going
through a wetland area. Hubbell Power
Systems recommends soil borings be
taken to depths deep enough to qualify
the soil profile adequately for the
project. In the case of transmission line
structures, soil borings are recommended
at high angle and dead-end
structures and also at intervals of every
three to five tangent structures along
the line. The soil borings indicated the
top 0.5 ft (0.15 m) were topsoil, underlain
by 3-8 ft (1-2.4 m) of very loose silty sand.
Below the silty sand was a sandy fat clay
with a medium stiff consistency. Water
was not encountered in the bore holes at
the 12 ft (3.6 m) depth but, water was
above the surface 6-12 in (152-305 mm)
in most places where the tower line was
to be built.
The successful bid included aboveground
steel grillages to support the
monopoles that, in turn, were supported
by helical piles. The helical pile quoted
was a combination pile (also known as a
March/April 2024

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