September/October 2023 - 17

the existing pylons. Also, as the wall geometry became more
complex at the four snorkels, the slurry walls became a less
attractive option. In the end, it was determined this approach
just wouldn't work. Building the slurry walls would have
occupied the entire site with equipment, laydown for
reinforcement cage construction and the slurry plant itself.
This would have stalled other construction operations during
this period, which would cause considerable schedule delays.
Soldier Pile Solution
The project's geotechnical engineer, Hart Crowser, suggested
using a permanent soldier pile and lagging wall. The project's
engineer had used a similar wall on a limited scale at the
Minnesota Vikings' U.S. Bank Stadium in Minneapolis.
However, there were few, if any, projects to use such a system
for this large or deep of an application. The W18 soldier piles
would be installed in drilled holes, and the installation could
proceed while other work occurred around the site. The use of
the tied-back soldier piles also enabled excavation to begin in
some areas while the wall was being installed elsewhere on
the site. Unlike the slurry walls, the soldier piles allowed for a
drainage board to be placed behind the permanent walls.
Eliminating the hydrostatic pressures on the walls turned out
to be a significant advantage with regard to the wall design
and required tieback capacities.
The soldier piles were fabricated with tieback seats at
approximately every 11 ft (3.5 m) within the web of the pile.
Initial construction included lagging between the piles using
wood and installing the tiebacks as the site was excavated. All
the tiebacks included class 1 corrosion protection, with the
HDPE sleeve and grout fill within the sleeve providing two
layers of corrosion protection around the wire tendons. A
significant challenge was finding a way to tie back the wall
around the buttress foundations
that were to remain in
place. These were 25 ft (8 m)
square battered foundations
that would remain in the
middle of three sides of the
new foundation wall. Because
of the width of the buttress
legs, adjacent soldier piles on
each side would be more than
20 ft (6 m) apart. To resist the
loads from the longer span, The engineering team designed a
double soldier pile at each end of the span with tiebacks that
splayed out at various angles to achieve adequate spacing
between the bond zones. Horizontal steel walers were then
used between the double soldier piles, and the wood lagging
was installed vertically between the walers, rather than
horizontally between the soldier piles.
The splay in the tiebacks at the double soldier piles caused
additional complications. At the foundation levels, all the
splays were outward with respect to the buttress foundation,
which then started interfering with the tiebacks on adjacent
conventional soldier piles. Consequently, these also had to be
splayed to maintain a minimum distance of about 4 ft (1.2 m)
where tiebacks crossed within the bond zone of the adjacent
tiebacks. The tiebacks on as many as five soldier piles on each
side of the buttress had to be specially oriented to maintain
that gap. In some cases, the lengths had to be adjusted to help
stagger the bond zones. The process of looking for clashes
between the densely spaced tiebacks around the buttresses
quickly went beyond trying to review the orientation of the
tiebacks on a two-dimensional drawing. At first, the team
used an Excel spreadsheet that calculated the threedimensional
vectors of each tieback to look for the potential
clashes. However, the process was cumbersome and relied on
extensive data entry. Eventually, the engineering team
employed a combination of a model generated from Revit and
virtual reality imaging to identify clashes. The use of virtual
reality headsets allowed the design team to " walk " through
the maze of tiebacks to get a full understanding of the location
and relative proximity of each one.
The temporary wood lagging provided support while the
At the foundation levels, all the splays
were outward with respect to the
buttress foundation, which then started
interfering with the tiebacks on
adjacent conventional soldier piles.
site was excavated and other structural work was completed.
Near the end of the construction, a drainage board,
waterproofing membrane and protection board were placed
over the lagging, and a 12 in (305 mm) thick layer of reinforced
shotcrete was sprayed over the wall to serve as the
permanent lagging and foundation wall for the below grade
space. The shotcrete is supported by studs welded to the
soldier piles, and the heads of the tiebacks are encased in the
shotcrete layer, which provides additional corrosion
protect ion. Being able to
separate the temporary and
permanent walls means the
permanent wall construction
was kept off the critical path.
Whi le not par t of the
original evaluation, the soldier
pi les also provided more
flexibility for design and
construction changes than any
other wall type. This proved to
be an advantage since the design continued to evolve even
after the soldier piles were in place. Many of the changes
involved minor adjustments in access locations or surface
features, which required adjustments to the cap beam grade.
With the steel soldier piles, adjustments to length could
easily be made with splices. Other temporary structures -
such as the underpinning for the roof or utility installation -
DEEP FOUNDATIONS * SEPT/OCT 2023 * 17
September/October 2023

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