July/August 2023 - 82

loads from the existing building foundations
on the proposed addition's
foundation walls.
Considering the wall was to be
buried, the contractor elected to use a
single shotcrete facing, and install a
steel end cap on each nail head with
corrosion inhibiting grease. All soil nails
installed were manufactured with the
Post-Tensioning Institute's Class 1
Double Corrosion Protection with all bar
hardware, bearing plates and steel end
caps being galvanized to resist potential
corrosion. The soil nail wall was also
designed for seismic conditions.
Shotcrete placement
passive systems that require wall
deflection to activate and generate
resistance along the soil nails. Although
there were no performance specifications
on the project, there was a
strong concern from the project team
about the movement of the existing
foundations and how it could adversely
affect the existing building structure.
Therefore, a conservative design approach
was taken.
To evaluate the surcharges, the
project owner provided historical
building plans from the 1992 renovations.
These plans provided bearing
pressures of each footing location along
the area to be underpinned. Lateral
surcharge pressures were calculated
using the Snail soil nail wall analysis
and design software using Boussinesq's
equation, modeling the foundations as
strip surcharges at distances behind the
soil nail wall. After considering various
options for the facing, an increased
section was selected with additional
reinforcement to reduce flexural
deflections. Introducing an active
condition was performed by prestressing
each soil nail. To properly apply a
prestress load, each nail would require
a free length within the soil failure
plane wedge, which in turn ensured
that the bond zone was being activated
82 * DEEP FOUNDATIONS * JULY/AUG 2023
beyond the failure plane. To accommodate
the free stressing length and
prestress loads, the facing was designed
for a higher nail head loading, using the
maximum stress found in the soil nails
during analysis.
Nail spacing was also considered in
the design due to the proximity of the
existing building and foundations to the
soil nail wall. Soil nails were spaced on
a 4 by 4 ft (1.2 by 1.2 m) grid. Therefore,
shallower lifts could be made during
construction, reducing the probability
of a lift sloughing.
Another aspect of design was the
requirement for the wall to be permanent,
which eliminated the surcharge
Soil nail lift sequencing
Based on the aforementioned design
requirements, 4.5 in (114 mm) diameter
soil nails ranging from 22-27 ft (7-8 m)
in length utilizing a #10, Grade 75
threaded bar for reinforcement were
selected for the permanent design. The
soil nail design loads ranged from
36-40.5 kips (160-180 kN) depending
on their location and up to four rows
were considered for the cut heights
anticipated. A 6 in (152 mm) thick
shotcrete facing was designed using
two layers of welded wire mesh for the
support of the soil around the nails.
Soil Nail Construction
Typical construction of a soil nail wall
consists of excavating in lifts, usually
around 5 ft (1.5 m), along the whole
length of the proposed wall, installing
soil nails, and installing the reinforcing
and shotcrete facing after all nails are
July/August 2023

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