July/August 2024 - 94

groundwater in the lower sand layer
was under artesian pressure, and
estimates found that the factor of safety
against bottom blow out of the less
permeable layers overlying the lower
sand was less than about 1.1. It was
concluded the lower sands needed to be
depressurized prior to excavation and
recommended that the pressure head
in these sand layers be lowered to at
least 3 ft (1 m) below bottom of
excavation, similar to the lowered
groundwater level in the upper sands.
This provided a factor of safety of
greater than 1.3, which is the typical
minimum factor of safety for bottom
blow out.
Considering all these factors, it was
concluded by the design team that a
conventional soldier pile and lagging
sys t em wi th t i ebacks was no t
acceptable as a shoring system. This
shoring system was excluded because
(a) it would not have been stiff enough
to limit deflections to a tolerable
amount per the City of San Jose, and (b)
significant settlement outside the
excavation resulting from the global
lowering of groundwater levels, both
inside and outside the excavation
footprint, would have been expected
with a permeable shoring system.
As a result, the design team concluded
the most practical shoring type
for these conditions would be a soilcement-mixed
wall, which would act as
a cutoff system for the groundwater.
Soil-cement-mixed walls can be
installed by advancing hollow-stem
augers and pumping cement slurry
through the tips of the augers during
auger penetration. Alternatively, cutter
soil mixing (CSM), which uses a tool
with vertically oriented cutting wheels
that forms rectangular treated soil
panels can be used to form a continuous
soil-cement-mixed wall. In either
method, the soil is mixed with the
cement slurry in situ, forming continuous
overlapping soil-cement
columns or panels. Steel beams are
placed in the soil-cement columns or
panels at predetermined spacing to
provide rigidity. Typical soil-cement
walls are considered temporary, and a
permanent reinforced concrete basement
wall is constructed in front of the
soil-cement wall. For this project, the
steel beam reinforcing used in the
shoring wall exceeded the size required
for structural capacity; the steel was
oversized to increase the flexural
stiffness of the shoring walls.
One advantage of a soil-cementmixed
wall is that it is stiffer than a
conventional soldier pile and lagging
system and reduces lateral shoring wall
movement and associated vertical
settlement of the ground surrounding
the excavation. Another advantage is
that a soil-cement-mixed wall is
relatively impervious and is able to cut
off the groundwater, thereby limiting
dewatering to within the excavation
footprint and minimizing drawdown
outside the excavation. By minimizing
the drawdown, the associated settlement
outside the excavation would
be reduced. Limitations of a soilcement-mixed
wal l include the
practical penetration depth of the wall
panels to cut off groundwater.
Artesian condition in the
lower sand and gravel layer
94 * DEEP FOUNDATIONS * JULY/AUG 2024
As noted, the shoring system for the
excavation also required multiple levels
of temporary tieback anchors. Internally
braced shoring configurations were also
evaluated during the design process, as it
was unclear whether all required
easements would be able to be obtained
to install the tieback anchors around the
full perimeter of the site. In the end, all
necessary easements were obtained and
a shoring system relying solely on
tieback anchors for shoring wall
restraint was used. The use of tieback
anchors substantially reduced the cost
of the shoring system compared to a
braced approach; however, specialized
construction methods were required to
install tiebacks below the retained
groundwater table.
Design of Shoring System
The selected shoring system consisted
of a tied-back CSM cutoff wall for the
deep excavation. The shoring designer
for the project was Brierley Associates
and the shoring contractor was Malcolm
Drilling Company. Middour
Consulting, a hydrogeological consulting
firm, was also involved during
the design of the dewatering system.
The CSM wall was approximately 39 in
(990 mm) thick and was reinforced
with W27 beams spaced 4 ft (1.2 m) oncenter.
The beams varied in weight
from 146-281 lb/ft (217-418 kg/m) and
were about 92-98 ft (28-30 m) long.
Four levels of tiebacks were designed
to help resist the earth pressures along
each wall of the basement. The design
loads ranged between 70-326 kip
(311-1,450 kN).
The penetration of the CSM panels
needed to be deep enough to cut off
groundwater in the lower sand and
gravel layers, extending to a depth of
about 135-140 ft (41-43 m). These
depths were approaching the limit of
the CSM equipment.
Due to the presence of significant
sand and gravel below the bottom of
the excavation, which varied in thickness
and extent across the site, it was
unclear if it would be possible to
effectively cut off the flow of groundwater
into the excavation. The hydrogeological
consultant designed a
July/August 2024

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