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The Museum of Science and Industry is located in 57th and Lake Shore Drive. An
underground 3-level parking was constructed adjacent to the northern side of
the existing Museum Building as to satisfy parking needs. Slurry walls
2.5'-thick and 45'-deep braced with three levels of permanent tiebacks were
used to construct the underground parking. Tiebacks were air-drilled and extend
into the hardpan soils with the grouted body being about 60' away from the
slurry wall face. The excavation was 34'-deep and covered an area of 633'x 278'
for the parking and a smaller lobby section on the south.
The site of the Museum is located on reclaimed land from lake Michigan. Soils
in the site consist of 20' of sand fill, over 20'-thick medium to stiff clay,
and very stiff clay for the next 20' feet The water table at the site is
controlled by the lake water elevation, which is at El. +3 ft CCD. Borings did
not penetrate more than 60' of soil but it is known that "hardpan"
soils exist below that depth. Clays at the site have larger unconfined
strengths than downtown Chicago clays. The medium clay at depths of 20' to 40'
has typical unconfined undrained strengths slightly more than 1.0 ksf (sU UU
value) whereas for downtown clays unconfined shear strengths range from 0.3 ksf
to 1 ksf at the same depths. Clays at depths of more than 40' typically had
unconfined undrained sU strengths in the range of 2.0 ksf to 5.0 ksf.
Maximum slurry wall deflections were kept to small values throughout the
excavation period. Most monitored slurry wall panels deflected up to 0.5"
with some panels deflecting up to 0.87" (I-32). Maximum wall movements
were observed above or close to the second levels of tiebacks. Slurry walls
generally rotated about their base with slight bending observed above the
second level of tiebacks. In some locations, approximately half of the total
measured deflections might have been caused by the extensive grouting efforts
that were undertaken to seal water leaks (i.e. 0.4").
Injection grouting was used to repair water leaks through slurry walls during
excavation. It was mentioned that in many locations this grouting caused the
wall to move laterally up to 0.25". The exact locations where these grout
induced wall movements were observed are not known but inclinometer deflections
increased considerably after the 3rd level of ties was installed. It was also
reported that the southeastern re-entrant corner from I-17 to I-8 had seriously
cracked during the excavation and that repeated grouting had attenuated this
cracking.
During the installation of the second level of tiebacks in the southeast corner
of the construction site, cracks were observed in the asphalt concrete pavement
and the concrete slab supporting the observation tower of the Museum. These
cracks were situated approximately 20' to 35' further south from the slurry
wall in which the tiebacks were being installed. Tieback installation in this
location started on Oct 21st and progressed easterly to panel 18 by Oct 23rd.
During this period cracking was observed as the tiebacks were installed.
According to engineers present at the site, the platform of the tower raised in
elevation by approximately 0.5" and then later in the day it settled back
about 0.25". Inclinometer I-17 in that location indicated that movement at
the top of the wall towards the north was in the order of 0.38". It was
believed that drilling with air through the stiff to very stiff clayey soils
resulted in incomplete removal of the clay cuttings between the drill pipe and
the side of the drill hole thus not allowing air pressure to escape (field
report).
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