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Northwestern University, Chicago, IL

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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