March/April 2022 - 93

When a structural slab was poured
against the SOE dur ing stat ion
construction at backfill stages, it was
allowed to reduce moment based on the
simply supported beam model by 33%
2
(wL /12 instead of wL /8) to account for
fixity provided by the structural slab.
The limit equilibrium method was
2
adopted to design the SOE system. Since
the SOE lateral bracings needed to be
removed and/or detensioned during
ma in s t a t i on cons t ruc t i on, a l l
construction stages needed to be
modeled to capture the highest possible
load exerted to the SOE. The DeepEX
software program (Deep Excavation,
2019) was extensively used to model all
construction stages and to design the
SOE system. Lateral bracing (i.e., strut
and tieback) loads were extracted at
each construction stage and used as
benchmarks to make sure actual loads
did not exceed calculated loads. This
helped to verify the assumptions that
were made during the design process for
the SOE in terms of lateral pressures
being valid. Also, the results were used
to develop contingency plans in case an
excessive load was observed during
construction.
In one of the models with internal
bracing, it was observed that the strut
load at the second level would experience
a 26% higher load (42.76 kip/ft vs.
33.78 kip/ft, or 58 kN/m vs. 45.8 kN/m)
once the third-level bracing was
removed for roof-level construction.
This proves how important it is to model
all intermediate construction stages to
capture the maximum possible load and
to design the SOE accordingly.
Spiles (breasting boards) under Times Building footing
Building Underpinning
The Times Building is a 10-story
building that sits next to the station,
with part of the building at the basement
level protruding into the footprint of the
station. Therefore, an underpinning
system was integrated into the SOE
system to support the vertical load from
that building's basement wall and to
prevent lateral movement and
settlement due to surcharge from the
The first two rows were followed by
a hand-dug pit where reinforced
concrete was constructed before
excavation, and then ground anchors
were installed once the excavation
reached almost 3 ft (0.9 m) below each
ground anchor elevation.
In addition, the first row of the
panels formed an inverted L shape to
support the bottom of the basement
showed a maximum lateral surcharge of
2
4.8 kip/ft (230 kN/m ) at 3 ft (90 cm)
below the top of the underpinning.
Although the lateral surcharge can
dissipate rapidly by depth based on the
Boussinesq Equation, the minimum
2
lateral surcharge was limited to
2
1.9 kip/ft (91 kN/m ) per Metro criteria
for the building to achieve a conservative
design.
2
DEEP FOUNDATIONS * MAR/APR 2022 * 93
building's foundations behind the
underpinning system.
The challenging underpinning
system consisted of ground anchors
wi th precas t concre t e pane l s /
diaphragms that were designed to
support the Times Building's vertical
and lateral surcharges. The first two
rows typically consist of 5 ft by 6 ft (1.5
m by 1.8 m) precast concrete panels to
expedite the construction procedure.
Since a higher lateral surcharge from
the Times Building was expected based
on the Boussinesq Equation, ground
anchors were installed on every panel
at the first two rows. The excavation
was conducted locally under the
basement wall to install only one panel
at a time, which assured minimum
settlement occurs during underpinning
construction.
wall footing and to transfer the vertical
load into the ground anchors. Each
concrete panel was also connected with
a series of plates and bolts to make sure
the loads from the Times Building were
distributed evenly and that the whole
system works together as a single unit.
Spiles (breasting boards) were
installed before excavation for the
concrete panels to prevent soil behind
the wall footing from collapsing into the
excavated area for the panels. Spiles are
closely spaced steel bars that are
mainly used in tunneling to provide
temporary protective support for soil.
A limit equilibrium model with
DeepEX was generated for the underpinning
system based on the criteria
provided by Metro. The lateral surcharge
from the building was calculated
with the Boussinesq Equation, which
March/April 2022

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