July/August 2023 - 81

that eliminated the need to consider
variable soil conditions, a soil profile
consisting of only sand was conservatively
assumed in the design (no
rock was used in the design).
Soil Nail Wall Application and
Feasibility
Based on a review of the subsurface
conditions and the results of additional
test pits next to the adjacent existing
foundations, soil nailing offered a
feasible option for the support of the
earth and surcharge pressures. Based
on the test pits performed, the material
beneath the existing footings was
confirmed to be a dense well-graded
granular subgrade, similar to roadway
base course. Below the dense fill
material was dense to very dense silty
sands, which had varying amounts of
gravel. This soil was considered to have
some apparent cohesion. These soils
were a factor in the proposal of a soil
nail wall and were well suited for the
application. Conservatively, no cohesion
was used in the design of the
active soil nail system.
In addition to the favorable soils,
several other advantages were considered
with the use of an active soil nail
wall, such as improving the soil
stability. By transitioning the passive
nature of a soil nail wall to an active
state prior to excavation, the deflections
typically seen with a soil nail wall due to
reinforcement elongation and activating
loads within the nails were mitigated.
This was more significant where the
location of the proposed active soil nail
wall was in close proximity to the
existing footings. Other advantages also
include safety and minimal soil disturbance.
Considering the variable
nature of the soil within the building's
footprint and the irregularity of the
existing footings, there would have been
increased risk while excavating beneath
the existing footings.
Due to the nature of the construction,
the soil nails would be drilled
in the shotcrete facing, which was
placed in shallow, sequentially excaSoil
nail wall facing above excavated rock face
vated faces in front of the existing
footings. Drilling would create low
vibrations on the existing structure.
During sequencing, the soil nail
concrete facing was placed, and soil
nails were drilled though using casing,
which further mitigated soil disturbance
and soil loss resulting in limiting
settlement of the existing foundations.
The depth to rock was variable
across the site, and in turn, variable
depths of soil nail wall were needed
below the existing foundations to reach
the top of rock. A soil nail wall could
effectively be terminated at the top of
rock, while the nail spacing was
maintained. Although the soil nail wall
would be terminated at the top of the
rock, the excavation was to continue to
around 16 ft (5 m) below the bottom of
existing footings. A rock shelf in front of
the soil nail facing was proposed to
support the soil nail facing, while the
excavation continued below to the
proposed footing depths. To facilitate
the excavation of rock, rock below grade
was line drilled by the contractor in
several areas to the bottom of footing
elevation just outside the proposed
footing footprint. This allowed the rock
to be excavated with limited disturbance
to the rock beneath the
existing footings and soil nail wall.
Active Soil Nail Wall Design
The need to minimize vertical and
horizontal movement of the existing
foundations was the primary consideration
for utilization of an active
soil nail wall design. These are typically
DEEP FOUNDATIONS * JULY/AUG 2023 * 81
July/August 2023

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