July/August 2024 - 96

While the drawdown in the lower sand layers was larger
than anticipated, the piezometer data were within an
acceptable range from a geotechnical standpoint.
generally less than 0.5 in (13 mm),
which were similar to the estimated
deflection per the shoring design.
Additionally, the lateral deflections
measured along the face of the wall
showed bulging of less than 0.5 in (13 mm)
during excavation work. Settlements
outside the excavation were measured
to be less than 1 in (25 mm).
The groundwater monitoring results
from inside the excavation showed the
dewatering and depressurizing wells
were performing adequately. The
weekly monitoring reports showed the
groundwater levels were continuously
maintained at a depth of at least 3 ft
(1 m) below the bottom of excavation.
The groundwater levels measured in
the piezometers outside of the excavation
showed the cutoff wall was also
performing adequately. The VWPs
installed in the upper sand layers
showed a steady drop of approximately
1-2 ft (0.3-0.6 m) during excavation
work. This is consistent with the
amount of drawdown that was estimated
by the project hydrogeologist.
However, quick rises and lowering of the
groundwater level were also recorded
(i.e., spikes in hydrostatic pressure). The
rise of the groundwater was due to the
close proximity of the VWPs to the
tiebacks that were being installed and
post grouted during those time periods.
The VWPs installed in the lower
sand layers showed a steady drop of
approximately 3-4 ft (1-1.2 m) in
groundwater level during excavation
work and then generally showed a
stabilized groundwater level through
the construction of the basement.
During the last few months of the
basement construction, the VWPs
showed a steady drop of approximately
1.5-2.5 ft (0.4-0.8 m) in groundwater
level, which may be related to the
96 * DEEP FOUNDATIONS * JULY/AUG 2024
seasonal change in the groundwater
levels. While the drawdown in the
lower sand layers was larger than
anticipated, the piezometer data were
within an acceptable range from a
geotechnical standpoint. This conclusion
was further justified by the
minimal settlement calculated from
the survey data.
Conclusions
Multiple rounds of shoring concept,
design development and corresponding
pricing evaluations were performed to
optimize the shoring system's performance
and cost and to minimize
schedule impacts to the project.
Results from the construction
monitoring program showed the
performance of the selected shoring
system was similar to what was
anticipated during design. The tied-back
CSM shoring wall provided adequate
rigidity to limit ground movements,
which successfully prevented damage
to surrounding improvements. The
relative impermeability of the CSM
cutoff walls, and the ability to install
these walls to an adequate depth,
allowed the shoring system to sufficiently
provide the groundwater cutoff
of the lower sand and gravel layer. The
groundwater monitoring data showed
the dewatering and depressurizing
system limited the majority of the
piezometric drop to inside the excavation;
therefore, minimizing the
amount of drawdown outside the
excavation and preventing meaningful
settlement of adjacent improvements.
Additionally, the groundwater monitoring
data confirmed the groundwater
levels in the upper and lower sand and
gravel layers were able to be maintained
at 3 ft (1m) below the bottom of
excavation. As a result, the potential for
bottom blowout due to excessive
pressure in the lower sand and gravel
layer was mitigated. Lastly, the allowable
lateral deflection of the shoring
wall was not exceeded during excavation.
Overall, the tied-back CSM cutoff
shoring wall adequately addressed the
geotechnical challenges that were encountered
during design and construction
of a deep basement in downtown
San Jose, California.
Kelli McCurdy, P.E., is a project engineer at
Langan . She provides geotechnical
engineering services to projects in the San
Francisco Bay Area, including technical
assistance and project management
involving seismic hazards, deep and shallow
foundations, deep excavations, earthwork,
and site-specific ground motions.
John Gouchon, P.E., G.E., is a principal at
Langan and provides geotechnical engineering
services for mid- and high-rise
buildings, landfills, levees, and embankments,
which involve driven and drilled pile
foundations, mat foundations, earthwork
and deep excavations. He also develops
ground motions for performance-based
design.
Serena Jang, P.E. G.E., is a principal at
LANGAN, where she has provided geotechnical
engineering services for more than
26 years in the San Francisco Bay Area. She
has experience in providing geotechnical
engineering services for shallow and deep
foundations, deep excavations, earthwork,
and seismic hazards.
Eric Lindquist, Ph.D., P.E., is Brierley
Associates' nationwide director of engineering.
He specializes in the design of complex
support of excavation and underpinning
systems, deep foundations, and tunnels.
Rob Jameson heads the engineering
group at Malcolm Drilling. Throughout his
more than 30 years in the sector, he has held
leadership and technical oversight roles on
large, complex and innovative geotechnical
construction works across North America.
Ryan Nagle is a project manager for
Malcolm Drilling Company. He specializes
in design-build cutoff walls, temporary
shoring and ground improvement.
July/August 2024

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