March/April 2022 - 92

Support of Excavation (SOE)
Design
Metro required that live traffic be maintained
on 2nd Street during the construction.
To achieve this, a decking
system consisting of W-shaped beams
was designed and incorporated into the
shoring system. The decking system not
only provided support for the traffic but
also acted as the first level of lateral support
(i.e., the strut) for the shoring system.
The soldier beams were required to
Surrounding buildings next to Historic Broadway Station
All construction stages including
internal bracing removal and tieback
detensioning for station construction
were modeled to capture the maximum
possible load during construction.
These loads were then used to design all
structural elements of the shoring
system, such as soldier pile, tiebacks,
struts/walers, and to develop action
plans in case greater loads were
experienced during construction.
A unique underpinning system,
which consisted of precast concrete
blocks with ground anchors, was
designed and incorporated into the
shoring system to support the Times
Building foundation and prevent
settlement. Rigorous comprehensive
finite element analyses were conducted
at different sections to calculate shoring
wall deflection and surface settlement,
and to develop action plans in case of
excessive movement.
Project Description
The Historic Broadway Station has a cutand-cover
excavation, 500 ft (152.4 m)
long by 97 ft (30 m) deep, underneath
2nd Street. The shoring system was
comprised of internal bracings and
tiebacks. The shoring system design was
made challenging by the depth of
excavation and the need to maintain the
traffic on 2nd Street during the
excavation, along with the proximity of
92 * DEEP FOUNDATIONS * MAR/APR 2022
existing buildings, underground
utilities, Metro's requirement for tieback
detensioning, and the protrusion of the
Times Building into the station footprint,
which required underpinning.
The stratigraphy analysis of the
station site revealed 15 ft (5 m) of
artificial fill and alluvium underlain by
the Fernando Formation. The Fernando
Formation is essentially a bedrock that
consists of siltstone and claystone.
Since the depth of excavation for the
project was almost 100 ft (30 m), the
majority of the excavation was within
this Formation.
be modeled as a simply supported beam
between lateral supports (i.e., tieback or
struts). This assumption ignored the
presence of negative moments generated
at the lateral support location,
which reduced the positive moments
occurring between lateral supports. To
account for positive moment reduction
due to negative moment at lateral
support locations, the positive bending
moment from the simply supported
beam model was allowed to be reduced
by 20% at intermediate stages until
excavation reached the bottom of the
2
final excavation. That is, wL /10 was
2
used instead of wL /8 for moment calculation
in the simply supported beam,
where w is the lateral pressure and L is
the distance between two lateral
supports.
Underground utilities below 2nd Street
March/April 2022

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