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CHAPTER 7
SUMMARY AND CONCLUSIONS
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During the Chicago and State Subway Renovation
Project, the existing subway station and tunnels were exposed
using
the cut and cover excavation technique. The excavation extended
down into the soft Chicago clay to a depth of 12.2 m. Space
limitations presented significant challenges to the project
in that the Frances Xavier Warde School is founded on shallow
foundations located within 1.3 m of the face of the excavation.
Options for providing lateral support of the excavation support
system were further restricted by the presence of the subway
station and twin subway tubes. To minimize the excavation-related
damage to the adjacent school, a stiff excavation support
system consisting of a secant pile wall, cross-lot struts,
and tiebacks was chosen as the support system. The excavation
support system performance data and the building response
data obtained from this project were used to evaluate the
conditions that led to ground movement-induced damage to
the Warde School.
The objectives of this work were to (1)
relate excavation and construction activities to the onset
of cracking in a
structure adjacent to deep excavation in soft Chicago clay,
and from this relation, suggest limiting criteria to preclude
excavation-related damage to similar buildings, and (2)
develop inverse analyses techniques to allow objective updating
of
finite element predictions of deformations associated with
braced excavations.
Based on the results of the monitoring
effort and subsequent evaluation of the field performance
of the excavation support
system, the following conclusions are drawn:
1. The secant pile wall with its combined
bracing system provided adequate support for the adjacent,
shallow foundation-supported
Warde School. As planned in the design, minor cracking
occurred to nonload-bearing portions of the school. The
maximum settlement observed during the project was 40 mm.
Of the maximum settlement, 10 mm occurred during wall installation,
18 mm developed as the soil was excavated and 12 mm occurred
during tunnel demolition and station renovation as a result
of creep and reduction of wall stiffness. Similar percentages
of movement components were observed at other monitored
sections. Only about half the movements developed during
excavation, indicating when the major design criterion
is deformation control, movements associated with wall
installation and long-term effects can become a significant
portion of the total movements for similar stiff support
systems.
2. Settlements extended beyond the secant
pile wall a distance approximately equal to the depth of
the
secant
pile wall. The effect of excavation was to cause larger
settlements within the affected zone, but not to expand
its width. This behavior suggests, for secant pile walls
where significant movements occur during its installation,
settlements should be normalized by wall depth, rather
than be the depth of excavation.
3. Significant excavation-related
movements did not occur until the excavation was lowered
through the stiff clay
crust layer at 5.5 m below grade.
4. In cases where little
or no volume change is expected during excavation and
when the deformations are small such
that localized strains do not develop, excavation-related
settlement behind a support wall can be estimated reasonably
well from inclinometer data. This is significant because
comprehensive survey data are typically not available
during construction.
5. When excavating through saturated
clays, distortions obtained from inclinometer data can
be used to estimate
distortions in an adjacent structure when settlement
and tiltmeter data are not available. Hence, inclinometer
data
can be used to monitor and update estimates of the likelihood
and the severity of potential cracking to adjacent structures.
Based on the results of the observed responses of the Warde
school to the adjacent excavation activities, the following
conclusions are drawn:
1. The damage to the school consisted mainly of hairline
cracks 300 mm to 500 mm long in non-load bearing walls.
Only a few cracks had widths greater than 6 mm. This damage
is classified as "slight" according to the Burland
et al. (1977) classification.
2. Cracks first were observed
at a distortion of approximately 1/920 in a region where
the building deformed in a sagging
mode. Larger distortions occurred in sagging region than
in the hogging zone. Consequently damage to the school,
a 3-story reinforced concrete frame with load bearing
exterior walls and a stiff floor system, would have been
negligible
if distortions in the sagging zone were limited to 1/1000.
3.
Cracks in the west end of the north foundation wall and
north exterior masonry wall were first observed at
a distortion of approximately 1/1500 where the school
deformed in a hogging mode. While these distortions were
smaller
than those in the sagging region, they occurred after
the distortions reached 1/500 in the sagging zone.
4. Most
of the cracking occurred along the west side of the school
in the area where distortions were largest,
and when the maximum distortions increased from 1/920
to 1/400. This response corresponds to the excavation being
advanced from the top of the soft clay layer (at a depth
of approximately 5.5 m) to the final depth (at a depth
of approximately 12.2 m). Further increases to 1/320
during
station renovation and backfill caused little new cracking.
Based on the results of the finite element studies presented
herein, the following conclusions are drawn:
1. The inverse modeling procedure in UCODE,
when coupled with the commercial finite element code, PLAXIS,
is able
to provide optimized parameters which when used in finite
element simulations of the Chicago-State excavation resulted
in quantitatively better computations of deformations.
2.
When monitoring data obtained during the installation of
the secant pile wall were used as the basis of optimization,
the recalibrated model was able to adequately "predict" the
magnitude and distribution of lateral wall movements for
all subsequent construction stages. These results are significant
in that a successful recalibration of the model at an early
construction stage positively affected the movement predictions
throughout construction.
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