The double-deck structure of Wacker Drive has been a distinctive feature of downtown Chicago since its completion in 1926, but now it has reached the end of its useful service life. In several areas, shoring towers supplement the deteriorated structure where it was in danger of collapsing. Protecting against the damaging effects of water is an important aspect of many features of the design of the new drive.
The drive, a main thoroughfare through Chicago?s business district, runs along the southern shore of the Chicago River from Lake Michigan 1.2 miles west to Franklin Street, then south 0.8 miles along the western edge of the downtown business area. All traffic into Chicago?s "Loop" from the north has to cross the river over bridges that connect to the section of Wacker Drive that is currently under reconstruction. The drive is also a vital artery for utility and communications services. Water mains, sewer mains, natural gas mains, fiber-optic cabling?they all run under Wacker Drive.
All of this activity along the drive posed a logistical challenge for the reconstruction designers.
"First and foremost was impact to the traveling public," Patrick Cassity told Roads & Bridges. Cassity is a principal bridge engineer at J. Muller International (JMI), headquartered in New York. JMI is the viaduct program manager for the reconstruction and plays other roles as well.
"Whatever we did, we needed to make sure that it could be staged in such a way as to minimize the impact to the traveling public," Cassity continued, "and also be able to maintain access to each of the buildings during construction. Minimizing the construction duration was also desirable."
About 35,000 vehicles travel along upper and 30,000 along lower Wacker Drive every day, not to mention the number of pedestrians who use it. The drive intersects 20 streets in the Loop and 19 bridges and provides access to 57 high-rise buildings on the upper and lower levels.
"What makes Wacker so busy is it has eight intersections that cross the river, so when you total all the cars that are crossing Wacker, coming in and out of the Loop, it?s closer to 200,000," said Denise Casalino, the project manager on the Wacker Drive reconstruction for the Chicago Department of Transportation.
"Some of the buildings there get up to 80 deliveries a day," added Kevin Fitzpatrick, a construction manager at Alfred Benesch & Co. at the Wacker Drive Central Office in Chicago. Fitzpatrick serves as the resident engineer for project management oversight of phase three of the Wacker construction.
"You?ve got to meet the grades at nine bascule bridges [in the current contract], every building entrance on both levels, provide access to all these buildings throughout the course of the construction at both the upper level and the lower level," commented John Hillman, a senior structural engineer at Teng & Associates Inc., Chicago. Teng is the prime consultant for the design of section three of the project.
The work was split up into sections so that the intersections could be done individually and staged so that traffic across the drive would never be completely blocked. The contractors put in pedestrian bridges to maintain access to the front of the buildings on the upper level, and a one-lane path is kept clear through the construction zone on the lower level to permit delivery trucks access to the loading docks.
"We coordinate over weekends or nights when we?re doing demolition," said Fitzpatrick. "We have building management meetings weekly with all the building managers in the area to go over the schedule for the next week and coordinate the demolition and temporary loading docks or if we?ve reduced their clearance."
"Walsh has a person whose primary responsibility is building coordination," added Casalino, "so whenever they?re going to do something near the building, he?s responsible for coordinating it. Through a lot of communication, everything is working pretty well down there." Walsh Construction is the prime contractor on contract A.
Planning for the reconstruction began years ago with a letter from Mayor Richard M. Daley to the building owners encouraging them to support the project.
Planning for the original Wacker Drive started just about 100 years ago. Wacker Drive was built in two years, from 1924 to 1926, according to a comprehensive plan for city development that was created by Daniel H. Burnham. Burnham, a prominent architect in the city, was selected to draft a plan for the future development of the city by the Chicago Plan Commission, a group of prominent citizens drawn from the Commercial Club and the Merchants? Club.
The commission unveiled the Chicago Plan on November 1, 1909. Burnham had conceived a plan of such sweeping scope that some people considered it a pipe dream that would never be realized. Wacker Drive was intended to be a broad thoroughfare that would displace the crowded clutter of the produce market then located at the east end of South Water Street on the south bank of the Chicago River.
Burnham and his assistant Edward H. Bennett conceived of the drive as a double-deck viaduct. Pedestrians and the still-young phenomenon of automobiles would travel on the upper level. Traffic congestion would be relieved by letting trucks make pickups and deliveries and carry away trash on the lower level.
With Charles H. Wacker, then chairman of the Chicago Plan Commission, as the prime motive force, Chicago?s officials oversaw the completion of the double-deck structure two years faster than expected, and the drive has stood for 75 years.
A new generation
The designers of the new Wacker Drive hope to create a structure that will last for at least another 100 years. The reconstruction project is divided into three design contracts and three construction contracts. The section now under construction, contract A, runs from just east of Franklin Street to just east of State Street, including five intersections and five midblocks, making a total of 10 sections. Walsh Construction is the prime contractor for this $75 million contract, which began construction activities in February.
Contract B runs from just north of Randolph to the western limit of contract A; contract C runs from the eastern limit of contract A to Michigan Ave. These two contracts were advertised for bids on Aug. 10 and are planned to be constructed simultaneously next year at a price of about $50 million each.
The three design sections of the project are slightly different from the areas covered by the three construction contracts. Overall program management and oversight of the three design sections are supplied by Earth Tech, which also designed most of the contract A work. Bridge program management and oversight are supplied by JMI, which also did the structural design of the viaduct, superstructure and columns. The other prime consultants include Teng and McDonough Associates Inc.
Benesch is the overall construction manager. Earth Tech did the foundations and the civil, electrical and mechanical work. JMI developed the concept for the bridge and worked with the Illinois DOT and the Chicago DOT to get the concept and the type, size and location approved by IDOT.
Contract A was 30% complete in August. The project was on budget and on schedule, with the expectation that nine of the 10 sections of the upper deck would be completed by the Nov. 20 deadline for opening as much as possible of the roadway to holiday traffic. The contract required seven of the 10 sections to be completed by the November moratorium.
Construction will resume Jan. 2, and the last section of contract A will be completed then. The contractors also will finish installing the new utility lines next year and pave the lower level of the drive.
Construction of contracts B and C is scheduled to begin in October 2001 and end by Nov. 27, 2002. Only the facade work on contract C will extend into 2003.
The good kind of tension
The main factors in the long life of the new structure are post-tensioning of the concrete upper deck and the use of high-performance concrete. The decks are cast-in-place concrete slabs with a wet cure period of seven days.
"High-performance concrete gives us extremely low [moisture] permeability and really good resistance to salt scaling and freeze-thaw cycles," according to JMI?s Cassity.
The main ingredients in the high-performance concrete mix are high-quality sand and rock that will not absorb moisture and stringent restrictions on the amount of silica in the aggregate and alkali in the portland cement and fly ash.
The high-performance concrete mix was designed by Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.
The high-performance concrete is one factor in making it possible to reduce the thickness of the upper deck of Wacker Drive from 24 in. to about 13 in.?one of the reasons the new lower Wacker will have more vertical clearance.
One of the primary design goals for the new drive is to increase the vertical clearance of the lower level. The original Wacker Drive had a minimum vertical clearance of 12 ft 6 in. When the original structure was built, automobiles were still relatively new, and there was very little vehicular traffic.
Besides high-performance concrete, the other factor involved in increasing the vertical clearance is post-tensioning of the upper deck of Wacker Drive.
"The use of a post-tensioned superstructure gives us the benefit of excellent durability, and also it allows us to achieve a thinner superstructure than a traditional reinforced concrete superstructure such as what is there today," said Cassity. "That allowed us to cut about 9 in. out of the superstructure thickness and be able to improve the vertical clearance. Then through a combination of that thinner deck and lowering the lower-level pavement wherever possible, we were able to achieve that 13 ft 9 in. vertical clearance."
Post-tensioning is a method in which steel tendons are strung through the concrete slab and pulled taught in order to cancel out the deflections caused by the weight of the slab itself, otherwise known as the dead load.
In the case of Wacker Drive, the tendons are encased in polyethylene ducts. Each strand is 0.6 in. thick. The ducts are laid in the concrete forms with a curvature the same as the dead-load curvature of the deck. In other words, the deck will naturally bend down in the middle of a span and up at a support column, so the tendons are curved the same way.
Grout forms a physical bond between the strand and the duct. The grout and the polyethylene duct also form a redundant barrier to moisture that might corrode the metal strand.
The engineers have not forgotten about the aesthetics of the new Wacker Drive, viewing the structure as part of a mission to beautify Chicago and make better use of the river front. The plans call eventually for a wide river walk with plants and trees where pedestrians can walk along the river all the way from Lakeshore Drive on the east end to Lake Street on the west.
The original Wacker Drive also had a river walk, but one that did not allow pedestrians to walk under the bridges. A person had to climb the stairs up to street level at each street, cross the street and then walk back down to the river level. The new Wacker Drive includes plans to widen the river walk by 25 ft between the bridges to allow a walkway under the bridges.
Wherever possible, the new facade toward the river will use the same Indiana limestone blocks that were placed on the original structure. Before taking them off, the contractors labeled and catalogued the location of each original limestone piece so it could be reinstalled in the same location on the bridge towers after the reconstruction is completed.
Reusing the limestone blocks, along with adding sidewalk planters and historical light fixtures, will provide continuity with the Wacker Drive of the past and take the Chicago landmark into the future.