The 108 North State Street (Block 37) Project is bounded by State, Dearborn, Randolph and Washington streets in the heart of downtown Chicago. Sitting vacant for numerous years, this city block represented one of the few undeveloped parcels in downtown Chicago. In 1989 the site was cleared to allow for a City of Chicago-backed development. Ultimately the development was never realized and the city-proclaimed “eyesore” sat vacant for 17 years. In the summer of 2005, a new developer decided it was time and the project was once again resurrected.

One of the driving forces of this development has always been the Chicago Transit Authorities’ (CTA) vision for a below grade station connecting the existing subways lines below State and Dearborn Streets. This connection ultimately provides the CTA with express route public transit options connecting both O’Hare International and Midway Airports with the Loop. With the CTA’s portion funded, it was only natural to build above grade as well. The above grade portion of the project includes two structures. A 17-story office structure located at the southwest corner of the site will house the local CBS station’s broadcasting center as well as MorningStar. The second is a multi-use structure featuring a 5-level retail space podium with a 39-story luxury residential and hotel tower rising over the northern portion of the development.

The site location as well as its history played a very important part in the design and construction of this project. The site is bound on the east and west sides by existing subway tubes extending 40 ft. below grade. The north and south side of the site are bound by numerous utility ducts and banks as well as historic buildings lining the opposing street fronts. Existing structures located on the site dated back to the late 1800’s. One building in particular, the “Stop and Shop” building built in the 1920’s fronted on the south central portion of the site caused concern. The building was supported on hand dug rock caissons with 4 basement levels. During site clearing, the building was demolished above grade and the basement levels were filled with construction debris.

In fall of 2005, Case Foundation Company was selected to be the deep foundation contractor for this project. The deep foundation elements consisted of a structural diaphragm (slurry) wall, rock and belled drilled shafts, placement of cast in place concrete columns as well as structural steel columns and a secant pile wall retention system.

As is the case with any urban construction, limiting ground movements is of the utmost importance. The CTA portion of the work required a 40 ft. excavation running diagonal across the site from southeast to northwest. In addition to the CTA station, it was decided to place 4 levels of below grade parking in the remaining space. Conventionally bracing a site of this size or placing tieback anchors between subway tubes and utilities were not feasible options so a top down construction method was selected. A 36” thick structural diaphragm wall was selected to be installed around the perimeter of the CTA/Parking Area. The structural diaphragm (slurry) wall would act as the earth retention support system with the floor slabs as the bracing element. A total of 938 lineal feet of slurry wall was installed totaling 65,140 square feet of excavation. The wall was divided into approximately 39 panels ranging in depths from 66 ft. to 79 ft. requiring 600 tons of panel reinforcing steel. Slurry wall panels where the Subway connections are made require fiberglass rebar to allow for subway tube penetration work to be performed.

For top/down construction to work, columns must be in place to facilitate the above grade construction as the mining and excavation proceeds below. A combination of belled caissons and slurry supported rock caissons were installed to support the column loads for this project. In traditional top down column construction, drilled shafts are usually extended to grade using a strippable smooth wall liner in the exposed column areas. Floor slab connections are doweled into the columns and slurry walls, and construction continues down until the final basement level is reached.

Due to the high axial loads defined for this project, the columns were growing too large for the traditional approach to work in the CTA station. The CTA station did not have the space for such large concrete columns so a smaller section had to be used. The decision was to install steel columns at a select number of locations where space was at a premium. The difficulty of this method was that the drilled shaft cut-offs were up to 50 ft. below grade. To further complicate the process, a number of the drilled shafts were polymer slurry-supported, which required removal of up to 6 inches of laitance to assure sound concrete prior to column placement. Oversize casings were used to protect manned entry into the shaft, laitance removed as required, anchor bolts surveyed, anchor bolts drilled and grouted, column leveling plate installed and grouted and column steel installed. This work took place in a 6 ft. to 10 ft. diameter casing, 20 ft. to 50 ft. below grade with a final installation tolerance of ¼” vertical and 1” from center required. Once the columns were placed, sand backfill was used to fill the casing prior to casing removal. Of the 144 drilled shafts installed on the project, 36 steel columns were set. The belled caissons ranged in shaft diameter of 30” to 102” with bell diameters of 5 ft. to 24 ft. The belled caissons were approximately 80 ft. long. The rock caissons ranged in diameter from 4 ft. to 10 ft. with an approximate drill depth of 100 ft. The installation of the steel columns provided a unique twist on top/down construction which proved to be a successful necessity to the project.

With two separate buildings being constructed on the same site another unique situation occurred. The 17-story office building contained only 1 level of basement space. The CTA/Retail Tower building had 4 levels of basement space. The CTA/Retail Tower was top/down construction while the Office Building was conventional. To make matters worse, the buildings met at the location where the old “Stop and Shop” building once stood with 4 levels of construction debris still in place. Like the rest of the perimeter, the structural diaphragm wall was required to separate the buildings and act as the watertight earth retention system for the top/down construction.

Constructability issues with the slurry wall installation arose with the fear that bentonite loss during slurry panel excavation through the rubble fill would occur. The design required a stiff system due to a large unbraced midspan length of 30 ft. in this area as a result of the CTA station directly adjacent. Case proposed a secant pile wall be installed replacing the slurry wall for this portion of the project. The system was approved. Case Foundation Company installed 142 secant shafts, each 880mm diameter x 70 ft. long. All secant shafts were installed with 5,000 psi concrete with a W24x117 steel wide flange beam placed in each shaft.