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The Northwestern University Parking Garage is a twelve level
structure bounded by Superior, St. Clair, and Huron Streets. Olsen
Pavilion of Northwestern Memorial Hospital occupies the eastern
portion of this block. The garage includes a relatively deep (23'),
single basement level for loading dock facilities. Slurry walls
were used to construct the basement walls of the new garage structure.
Caissons were constructed in between slurry wall panels since
the slurry wall bearing capacity was would not have been sufficient
to carry the superstructure loads. The soil profile at the site
is typical of downtown Chicago. Street level at the site is approximately
at elevation El. +13 CCD (Chicago City Datum).
In order to construct the slurry wall and the foundation caissons on the same
alignment the caissons were constructed first to the top level of the slurry
wall and then the slurry wall panels were excavated in between caissons. The
caissons were provided with a steel shell with welded stubs of sheetpiles or a
pipe for connection with the slurry wall. The stubs were to extend into the
slurry wall by 4" to 6" to reduce seepage along the contact of steel
shell and the slurry wall concrete.
Most slurry wall panels were close to 18' long and 39' deep, all being 2.0'
thick. Bracing was provided by temporary tiebacks installed at El. +3.0 CCD at
14° inclination from the horizontal. Two tiebacks were placed near the
caissons with a typical 4'3" horizontal spacing. Measured data was
available until the site was thereafter excavated to El. -4.5ft CCD, after
which there were no more available data regarding the performance of the
retention system. It is known that the final excavation reached El. -7.0ft CCD.
The slurry wall excavation system performed well with small horizontal soil
movements occurring during the excavation. Caisson construction caused small
near surface soil movements in the order of 0.05", while slurry wall
construction caused horizontal soil to movements up to 0.1" towards the
site.
Excavation to 11' below street caused soil to move towards the excavation by as
much as 0.35", while tieback installation reversed deflections by as much
as 0.15". Thereafter soil movements slightly increased as the excavation
progressed to 17.5 ft below street level. The upper 2' of all inclinometer
casings deflected more than the remainder portion of the casing, whereas 4'
below the deflection was much smaller (Fig. 5.24). Most likely this additional
deflection was caused by backfilling against the casing.
One inclinometer was placed 20' from the western slurry wall (I-2) while other
two others were located 5' from the slurry wall (I-1, I-3). All these
inclinometers did not show any significant differences in the magnitude of the
soil movements except for the upper portion of the casing but movements for the
inclinometer 20' from the excavation were slightly smaller below the tieback
elevation (C9). The final deflection shape of the inclinometer 20' away was
mostly cantilevering whereas other inclinometers displayed some translative
motion at the toe of the wall as well (0.2"). Measurable deep-seated soil
movements were observed in all inclinometers down to 40' below the slurry wall
base. These movements were related to the clay that is present below the
excavation, and clearly show that deep-seated soil movements occur in soft
clays even for relatively shallow excavations.
Despite the fact that there was mention of settlement markers there was no
available settlement data found for this project. However, settlements must
have been small if we consider the small magnitude of soil movements and the
fact that the excavation was not very deep.
There were numerous locations where seepage was observed at the joints between
caissons and slurry wall panels, despite the care had been taken to construct
watertight joints. These leaks were to be repaired with grouting or another
sealing method.
In several areas wet bentonite was observed along the wall which was later
removed. In addition several voids were observed at the slurry wall face that
had to be patched with concrete. Although all of the above needed repair work
was probably minor, it does clearly demonstrate that an increasing the number
of construction joints increases the chance of water leaks through the slurry
wall, and thus should be avoided.
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