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CHAPTER 1
INTRODUCTION
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Infrastructure rehabilitation has become commonplace in
many urban areas across the country. Space for construction
activities is usually limited because of the proximity of
adjacent structures; this limited space necessitates the
use of excavation support systems. A major concern for projects
involving deep
excavation is the impact of construction-related ground movements
on adjacent buildings and utilities. The degree of
stiffness required for an excavation support system is determined
from the movements associated with its construction and their
effects on the adjacent structures. It is necessary then
to quantify the tolerable deformations for each affected
structure. However, determining tolerable deformation by
purely analytical means is extremely difficult. The amount
and severity of excavation-related damage depends on the
building type and configuration, support system type and
performance, ground conditions, and construction activities.
Consequently, a semi-empirical approach based on tolerable
deformations is usually employed to design excavation support
systems. The damage to a structure is related empirically
to the magnitude and distribution of the deformations expected
during the project. The stiffness of the excavation support
system is chosen based on the limiting criterion developed
from this relation. Limiting criteria have been developed
from observing the excavation-related ground deformations
and building responses associated with different types of
support systems in various soil conditions.
A necessary component
of this semi-empirical approach is complete and comprehensive
documentation of the response
of buildings adjacent to excavation support systems, including
a complete record of the ground conditions and building
responses before, during, and after the construction activity.
This
information contributes to the understanding of the interactions
between the soil, excavation support system, and adjacent
structures.
Predicting the magnitude and distribution of
these movements is critically important to the design process,
as these predictions
are used to estimate the tolerance of the structures to the
induced deformations. However,
there are a number of uncertainties associated with these
predictions, such as soil properties variations, support
system details, construction sequence, and surcharge loads.
In practice, the observational method is commonly used to
compare
predicted performance with observed responses. This
method requires much engineering judgment, and it is very
difficult to quantitatively judge how well the work is proceeding
in terms of the predicted responses, especially given the
time constraints associated with construction. Ad hoc numerical
tools are needed to compare observations and predictions.
This
report presents the observations made during the excavation
and construction the Chicago Avenue and State Street Subway
renovation project in Chicago, IL. It
also summarizes a numerical procedure that "objectively" updates
design predictions of deformations for supported excavations
in clay. The monitoring data from the Chicago-State
excavation are used as observations in an inverse analysis
that calibrates the numerical model of the excavation. The
report shows how the updated "predictions" made at an early
stage of construction can be used to accurately "predict" the
responses at the later stages of the project.
The Chicago
Department of Transportation (CDOT) began constructing
the subway improvements in June 1999. The renovation project
includes excavating 12.2 m of soft to medium clay to expose
the existing subway station and tunnels and to allow for
capital improvements. The improvements included adding
a
mezzanine level for office and vendor spaces and making
the station handicap accessible.
Space limitations presented
significant challenges to the project. The project was
located in close proximity to the
Frances Xavier Warde School and the Holy Name Cathedral.
The Warde School was founded on shallow foundations that
were located within 1.3 m of the face of the excavation.
The Holy Name Cathedral was located approximately 15 m
from a corner of the excavation. Options for providing lateral
support of the excavation support system was further restricted
due to the presence of the subway station and twin subway
tubes. The temporary support walls were 2.2 m from the
outer
wall of the subway tubes
Baker Engineering, the construction
manager for the project, contracted with Wiss, Janney, Elstner
Associates, Inc. (WJE)
to monitor ground movements and structural responses associated
with the excavation and the subsequent renovations. WJE's
primary responsibility was to assess the potential for structural
damage to the adjacent buildings. Northwestern University
was subcontracted by WJE to perform the portion of the contract
involving monitoring the field performance of the support
system and predicting the ground movements that resulted
from the excavation activities. Data obtained from the monitoring
activities included lateral soil deformations, pore water
pressures, building movements, and support loads. Data were
obtained daily during wall installation and excavation, and
at least on a weekly basis after the excavation had reached
its final depth. Building movements were optically surveyed
weekly during construction.
Chapter 2 of this document presents
technical background concerning excavations in soft clay
that employ stiff excavation
support systems. This chapter discusses the general deflection
behavior of braced walls in soft clays and the deformation
response of the soil resulting from the wall deflections.
It shows the influence of the soil conditions, the system
stiffness, and the installation techniques of the support
system on the deflections of the support wall. It reviews
methods to empirically evaluate the settlement distribution
behind excavation support walls in soft clays and summarizes
the observed responses of buildings situated adjacent to
an excavation. Lastly, this chapter provides a literature
review and discussion of limiting deformation criteria
established to minimize excavation-related damage to structures.
Chapter
3 presents the subsurface conditions of the Chicago Avenue
and State Street project site and describes the excavation
support system and field instrumentation. It provides descriptions
of the structures adjacent to the excavation. It also summarizes
the design methodology used to select the excavation support
system.
Chapter 4 presents the observations and performance
of the field instrumentation during the project. It summarizes
in
detail the project construction sequence and presents the
lateral ground movements and building settlements measured
during the project. This chapter also presents the pore
pressure measurements and the loads in the structural supports
observed
during excavation.
Chapter 5 presents the observed response
of the building adjacent to excavation. These observations
include building
settlements and distortions as well as observations of
excavation-related damage to the structure. This damage is
compared to observed
distortions.
Chapter 6 summarizes the inverse analysis used
to calibrate the finite element model of the excavation
and procedures
needed to update design predictions. This
procedure is validated by applying it to the detailed observations
made at the Chicago-State project. The
soil was represented as by the Hardening Soil model, a
multi-yield surface, elasto-plastic effective stress model.
The
results of the inverse analysis are presented for optimizations
made at an early stage of construction as well as at later
stages. Model statistics that quantify the improvement
of the predictions over the original, non-optimized values
are given.
Chapter 7 summarizes this work and presents
conclusions.
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