July/August 2024 - 95

dewatering and depressurization system
and developed a 3D groundwater
model to estimate groundwater drawdown
outside the excavation. This
analysis showed that the groundwater
would be lowered by only 1-2 ft (0.30.6
m) outside the excavation, which
would limit settlements to less than
0.25 in (6 mm) as result of dewatering.
During excavation, all shoring
systems will deform, which will cause
surrounding improvements to settle.
The magnitude of shoring movements
and resulting settlements of the ground
surface behind shoring walls are difficult
to predict with precision because
they depend on many factors, including
the method of installation and the
contractor's skill in the shoring installation,
the rigidity of the system, spacing
of tieback anchors, and variability of the
soil conditions. Previous research concluded
that the settlements can vary
from 0.1-0.3% of the excavation depth
for relatively typical shoring systems for
which deep seated soil global movements
are not significant. Therefore, for
an excavation depth of 54-58 ft (16-18 m)
and using an estimated settlement in the
range of 0.1-0.2% of the excavation depth,
settlement immediately behind the
shoring wall would be about 1 in (25 mm).
The shoring designer supplemented
empirical estimates of shoring wall
movements based on published literature
with an elastic beam analysis
using the flexural stiffness of the
shoring wall in combination with the
shoring design loading. It was estimated
that, during excavation to subgrade,
the shoring system would deform
laterally on the order of 0.5 in (13 mm),
which the City of San Jose deemed
acceptable. As noted above, larger than
structurally required shoring wall piles
were utilized to enhance the systems
deformation behavior.
Construction Monitoring
During below-grade construction
activities, the team monitored the
settlement of the adjacent ground
surface, movement of the top and face
of shoring wall, and groundwater levels
both inside and outside the excavation.
Survey points were installed on the
adjacent improvements and along the
temporary shoring walls to monitor
vertical and lateral displacements.
Measurements were recorded and
reviewed weekly during excavation
work and until the street-level floor
slab was constructed. From a geotechnical
standpoint, an acceptable
range of lateral and vertical movement
would be less than 0.5 in (13 mm) and
1 in (25 mm), respectively.
Two dewatering wells, four combined
dewatering and depressurization
wells, and one monitoring well were
installed inside the excavation. The two
dewatering wells were installed to a
depth of about 68 ft (21 m) below ground
surface and targeted the upper sand
layer. The four dewatering and depressurization
wells were installed to a
depth of about 122 ft (37 m) below
ground surface and targeted both the
upper and lower sand layers. The upper
sand layer was dewatered and the
lower sand layer, which is under artesian
pressure (higher groundwater
readings), was depressurized. The
monitoring well was installed to a depth
of about 67 ft (20 m) below ground
surface. The upper and lower sand
layers were dewatered and depressurized,
respectively, to at least 3 ft (1 m)
below the bottom of the excavation. The
wells inside the excavation were
monitored daily and a dewatering
status report was provided weekly.
Additionally, two piezometers were
installed outside of the east and south
cutoff wall. In each of the piezometers,
two vibrating wire piezometers (VWPs)
were installed in a single borehole. At
each piezometer, a VWP was placed at a
depth of 45 ft (17 m) below ground
surface (upper sand layer) and a second
VWP was placed at a depth of 95 ft
(29 m) below ground surface (lower sand
layer). The piezometers were installed
outside the excavation to monitor the
drawdown during dewatering and to
confirm that the upper and lower sand
layers were cut off. The VWPs provided
continuous data of the piezometric
head of the upper and lower sand layers
throughout construction.
Summary of Results
The survey monitoring data and
measured movements were within an
acceptable range from a geotechnical
standpoint. During excavation work, the
lateral deflections measured at the top of
the wall during excavation were
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.
DEEP FOUNDATIONS * JULY/AUG 2024 * 95
July/August 2024

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