"Applications of GT STRUDL on Boston's Central Artery / Tunnel Project
Congress Street to North Street"
by
Paul F. Harrington, P.E.
&
James R. Branch, P.E.
Fay, Spofford & Thorndike, Inc.
Burlington, MA
Abstract
A major component of the Central Artery/Tunnel
Project is the depression of the existing 6-lane viaduct (Interstate I-93)
up to 90 feet below grade through the heart of downtown Boston. Since
the new tunnel is located directly below the existing viaduct, a steel
underpinning system was designed to support the structure and allow removal
of the existing foundations located within the new tunnel alignment. GTSTRUDL
was used to design the underpinning and to analyze the effects of transferring
the 50 year-old structure from the existing "rigid" foundations onto the
flexible underpinning system while the roadway remained open. Slurry
walls are used for support of excavation during construction of the cut-and-cover
tunnel, support of the existing viaduct underpinning and also serve as
the permanent tunnel walls. This required that the analysis and design
of the walls account for all of the excavation, bracing and tunnel construction
stages in addition to loads applied to the completed tunnel. GT STRUDL
was used to perform an incremental non-linear analysis that included the
entire history of internal stresses in the wall due to all stages of construction.
Underpinning Analysis
The primary objective of the underpinning
system was to maintain the structural integrity of the existing steel viaduct
while clearing the corridor for construction of the new tunnel below.
The underpinning system replaces the existing, "rigid" pile foundations
with "flexible" grade beams, which span the excavation. The inherent
non-linear behavior associated with changing an active structures' boundary
condition created the need for a staged analysis.GT STRUDL was used to
perform a linear displacement analysis of the underpinning system.
The model accounted for the jacking
and load transfer operations, such that the stress redistribution within
the existing rigid frame was accurately assessed. The MEMBER DISTORTION
command was used to accurately model the jacking displacement applied to
the underpinning system. In addition, the results of the model were
vital during the actual jacking and load transfer (column cutting) operations
that were performed under live traffic. The predicted load/displacement
correlation allowed Engineers to closely monitor the jacking increments
and make timely adjustments, if necessary.
Incremental Non-linear Analysis of Slurry Walls
Since the slurry walls used for excavation
support also serve as the permanent tunnel walls, a staged construction
analysis of the deep braced excavation was required in order to account
for residual stresses which are "locked" into the walls at the end of the
final construction stage. Using the Beam on Elastic Foundation method
combined with GT STRUDL's NONLINEAR capabilities, structural models were
developed which simulated the changing loads and boundary conditions that
occur during the excavation and construction stages.
A single input file was created that
included all of the structural elements that would be used in the analysis,
and by using the CHANGES and DELETE commands, the boundary conditions and
loads were adjusted for each stage of construction. Then, the NONLINEAR
ANALYSIS CONTINUE command was used to produce member force results for
each stage.
Biographical Data
Paul F. Harrington, P.E. is
a Vice President at Fay, Spofford and Thorndike, Inc. Paul has been
with FST for 19 years and has worked on a variety of heavy civil transportation
projects. Most recently, he has been heavily involved with the ongoing
construction over $1B of tunnel construction as part of Boston's Central
Artery Project.
Education
B.S.C.E. University
of Massachusetts
Biographical Data
James R. Branch, P.E. is a Senior
Principal Engineer with Fay, Spofford & Thorndike, Inc. located in
Burlington, Massachusetts. He is currently Structural Task Manager
for the FST/HNTB Central Artery joint venture team and has over 15 years
experience on heavy construction projects including tunnels, wastewater
treatment facilities and waterfront structures.
Education
B.S.C.E. State University
New York (SUNY) at Buffalo