March/April 2022 - 94

Finite element PLAXIS model for underpinning
Calculated loads from the DeepEX
model were then used to design
concrete planks/diaphragms and to
determine the number of strands
required at each ground anchor
elevation. Based on the Metro criteria,
the anchors were tested for two times
the design load and locked off at the
design load.
Finite element analysis with
PLAXIS 2D (PLAXIS, 2019) was
conducted to evaluate the deflection of
the proposed underpinning system. All
intermediate construction stages were
modeled to calculate the cumulative
deflection of the underpinning system.
Based on geotechnical information, the
top two rows were located within the
layer of the Fernando Formation that
was moderate to highly weathered,
while the rest of the underpinning was
within the slightly weathered Fernando
Formation. The unbonded and
bonded lengths of anchors were
modeled with node-to-node anchor
and embedded beam row elements.
Hardening soil conditions were used to
model the soil behavior.
The soil deflection profile was
extracted and saved at each stage of the
construction. The soil showed positive
deflection (i.e., backward) at the initial
stages of excavation and, as excavation
proceeded, the deflection became
negative (i.e., inward). This was because
anchor design loads were based on the
maximum possible load during
construction stages, which in the
underpinning system, occurred at the
final excavation stage. Therefore, the
94 * DEEP FOUNDATIONS * MAR/APR 2022
anchor lock-off
load was higher
than the actual load at the initial stages,
which theoretically would push the
wall back into the soil.
The maximum calculated deflection
of 0.41 in (1 cm, and 0.07% of the wall
height) occurred at the final excavation
stage, which was less than the allowable
deflection of 1 in (2.5 cm). The data from
the inclinometer and tilt sensors at the
top of the Times Building showed
minimal settlement and movements
during and after construction.
Conclusion
The support of the excavation (SOE)
system for the Historic Broadway
Station was designed based on Los
Angeles County Metropolitan Transportation
Authority (Metro) criteria
using the limit equilibrium method. It
was shown how extremely important it
is to model all construction stages to
calculate the maximum possible loads
on the SOE system. Calculated lateral
bracing loads at every construction
stage were used as benchmarks to
verify assumptions made during the
design process. They also helped in
developing contingency plans in case
excessive loads were observed.
The challenging underpinning
system used consisted of ground
ancho r s and conc r e t e pane l s /
diaphragms. The system was designed
and constructed to support part of the
Los Angeles Times Building that
protruded into the station footprint, as
well as building foundations that sit
Times Building underpinning
adjacent to the station. The underpinning
elements were designed based
on results from the limit equilibrium
method, and then the deflection was
checked with finite element analysis
(FEA). Data from settlement and tilt
sensors showed minimal movement,
which aligned with FEA model outputs.
Also, data from multiple monitoring
points positioned around the station,
which captured wall deflection and
surface settlement, showed minimal
movement.
Excavation reached the bottom of the
station in January 2019, and structural
walls in front of the shoring system have
been completed. Currently, the mechanical
and architectural elements are
being installed, with the project on track
to open to the public in fall 2022.
Babak Mamaqani, P.E., is the project
manager with PB&A. He is a certified project
management professional with over 15 years
of experience in designing a wide variety of
structures, including retaining wall systems
and deep foundations. Mamaqani has
published several technical papers for deep
foundation conferences and structural
engineering journals.
Pirooz Barar, S.E., is the principal
engineer and founder of PB&A. He has over
40 years of structural engineering analysis
and design experience, with the last 30 years
devoted to the development of advanced
technical methodology for the design of
shoring, underpinning and earth retention.
March/April 2022

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