OVERCOMING CONSTRUCTION CHALLENGES ON THE CHICAGO SUBWAY: Digging Deep and Thinking Fast.
The Chicago/State station originally opened in 1943, and now handles more than 20,000 passengers per day. It is located directly below the intersection of Chicago Avenue and State Street on the Chicago Transit Authority's Red Line, which runs north and south underneath State Street. Construction on the $28-million renovation project began in May 1999, and involved both expanding and refurbishing the station. Access from street level to the mezzanine was improved at each corner of the intersection by widening stairs, adding escalators, and installing an elevator to provide access for persons with disabilities. Over 10,000 sq ft was added to the mezzanine level, doubling the area that handles fare control, concessions, communications, and electrical service. Access to and from the platform level was also improved, with wider stairs, escalators, and new elevators.
In addition to the structural changes, the look of the station and the surrounding area was dramatically refreshed. At street level, new streetscape plantings, sidewalks, and ornamental lighting make the area inviting, and lead to the improved station entrances. The entrances feature specially-designed kiosks with turn-of-the-century design elements that blend with the existing Chicago Avenue streetscaping. Inside the station, the walls were brightened with blue and white tile incorporating a skyline motif of neighborhood landmarks, such as the recognizable John Hancock Building. Three courses of bright red bullnose wall tile indicate that the station is on the system's Red Line. The stairs and flooring were refurbished with attractive and durable granite pavers. At the platform level, arched tile ceilings are illuminated by stainless steel light trays, providing ample indirect lighting and brighter direct lighting at the platform edge. A tactile edge on the platform improves safety, and acoustical panels dec orated with artwork help reduce the noise levels in the tunnels. The renovated station is safe and pleasant, is fully accessible to passengers with disabilities, and is better able to handle the station's busy pedestrian traffic.
Moving the design from paper to steel and granite, however, involved creative approaches to each phase of construction. John Yonan, CDOT's project manager, explains, "The project was much more complex than it appears. Chicago/State is the fourth-busiest station on the entire Chicago subway system, and the intersection of Chicago Avenue and State Street handles a very high volume of vehicle traffic. Pedestrian, train, and street-level traffic had to be maintained 24 hours a day, seven days a week. The coordination alone was a major undertaking." As with nearly all urban construction projects, the excavation revealed a few unmapped surprises along with many other challenges that the contractor and the construction management team were expecting.
The cut-and-cover construction technique for the expanded mezzanine and the widened access to the platforms required the excavation of State Street from right-of-way to right-of-way. The excavation was approximately 132 ft long, 72 ft wide, and 39 ft deep at its deepest point. On the east side of State Street, the right-of-way is adjacent to an elementary school building and an historic cathedral. The kindergarten through eighth grade students at Frances Xavier Warde School could not be disrupted or endangered during construction. Holy Name Cathedral was built in 1874, with elaborate architectural details that are vulnerable to vibration and ground movement. Both the school and cathedral remained open during construction, which required detailed attention to ground movement and vibration monitoring, excavation techniques, and construction phasing.
Because of the project's proximity to the school and cathedral, CDOT anticipated the need for a ground movement and vibration monitoring subconsultant on the construction team, to be identified and overseen by the construction manager. Baker selected Wiss, Janney, Elstner Associates, Inc. (WJE) (Northbrook, Illinois), to design and implement a monitoring program. One element of the design was a unique earth retention system using a secant pile wall to reduce vibration and ground movement while allowing open excavation. Three-ft diameter shafts were drilled 60 ft deep, adjacent to each other along the right-of-way. Every other shaft was rein forced with a steel H-pile, and the shafts were filled with concrete grout to form a wall. As excavation began, whalers were attached between the walls at every H-pile. Six-in. diameter borings for an upper and lower row of 100-ft tiebacks were drilled at a 45-degree angle through the pile wall, approximately 60 ft deep outside the excavation. A seven-strand cable was plac ed in each boring, and grout was injected to secure the cable. After the grout set, the tieback cable was tensioned to reinforce the pile wall, and then the remainder of the bore was filled with grout.
CDOT's Resident Engineer, Cindy Williams, says, "A more typical solution would be to drive sheet piles to create a retaining wall, but drilling shafts created much less vibration than driving piles. Reducing vibration was critical since the earth retention system was within one foot of the school building." The secant pile wall system was relatively low cost and could remain in the ground once construction was complete. It also allowed open access from above for excavation equipment. The system worked well under the site constraints of the Chicago/State project, and could be valuable in similar urban construction situations.
A SURPRISE TUNNEL
One secret of the project site revealed itself during the drilling of the secant pile wall along the west side of the site. Drillers encountered an unmapped, water-filled drift tunnel 35 ft below the surface. The tunnel ran perpendicular to the subway tube, and as water drained from the tunnel, a sinkhole appeared 200 ft west of the excavation in the cathedral parking lot. Baker worked with CDOT and the contractor, Walsh/Weis, Joint Venture, to determine what exactly the drillers had encountered, how it was related to the sinkhole, and the best way to fix it.
The team began pumping the water out of the shaft to better assess the situation, but stopped when it became clear that further pumping would simply enlarge the sinkhole. Using a special video camera and a series of test bores, the team determined that they had drilled through a 12-ft diameter tunnel that appeared to run out through the parking lot. Surface excavation near the sinkhole revealed a 25-ft diameter shaft leading down to the tunnel surrounded by metal casing. Presumably the tunnel was used to transport construction materials to and from the station site when the subway was originally built, and then it was sealed and abandoned. The team confirmed that the tunnel did not currently serve any purpose, was not a conduit for any utilities, and did not present any environmental hazards.
To seal the tunnel, the team drilled four three-ft diameter shafts through the tunnel just behind the planned subway wall and filled the shafts with concrete grout to create a bulkhead. The sinkhole in the parking lot was backfilled, and contractors installed a reinforced concrete slab to strengthen the area. The parking lot was restored, and then work continued as planned on the earth retention system.
Further challenges arose as construction began, which underscored the foresight of one of CDOT's project strategies. CDOT had suggested that the Chicago architectural firm on the design team, Ross Barney + Jankowski, Inc., be added to the construction management team as well. The purpose was to have the architects who designed the project be available to clarify the drawings and make any necessary adjustments in the field to preserve the design intent. After the retention system was in place, certain field conditions dictated a major redesign of many of the station elements, and having the architect on the CM team was essential. Necessary structural changes reduced the available space inside the station, and architectural details had to be modified.
Redesign takes time, but the team was under pressure to keep construction moving ahead so that State Street could be reopened. Encountering the drift tunnel had also caused delays, and even the exterior of the existing subway tunnel presented problems. The sides of the train tube had to be demolished to be widened, but as the excavation progressed, the contractor encountered a metal liner with steel ribs that encased the station tunnel. The liner was filled with concrete grout that had to be chipped out to cut through the metal liner and demolish the sides of the tunnel. The extra work added up and threatened the project schedule.
To keep the schedule from slipping further, CDOT, Baker, and the contractor completely revised the construction staging while maintaining pedestrian and train traffic. Originally the project was divided into four phases in which roughly a quarter of the station area would be completely finished, and then pedestrian traffic would be rerouted within the station to allow the next section to be completed. To keep the schedule on track, the team implemented a different strategy. Baker's Resident Engineer, Bob Staiton, explains, "All structural work was completed first, so that the excavation could be closed and State Street could be re-paved and reopened while design details were being revised. After vehicular traffic was moving normally through the area again, contractors went back to complete the station's architectural finishes section by section, routing pedestrian traffic appropriately through the platform and mezzanine levels." Baker managed the construction sequencing, creating nine phases instead of the o riginal four, and coordinated the process with CDOT and the Chicago Transit Authority, the operating agency.
The structural work completed first included demolishing the sides of the train tube; installing structural and concrete reinforcing steel; forming and pouring the new concrete station walls; and placing the structural roof slab. Contractors also installed the new escalators and stairs, placed and compacted engineered fill, and then reconstructed the portions of Chicago Avenue and State Street that had been excavated. Sidewalks were placed, new street lights were installed, and streetscape plantings were completed.
By the time the street level was restored, the architectural redesign was complete. Contractors worked section by section on the finish work inside the station, which included installing the granite payers, terrazzo curbs, granite stairs, structural glazed facing tile walls, vaulted tile ceiling panels, stainless steel light trays, and kiosks. The revised staging kept construction progressing despite inevitable delays during certain segments of construction.
Baker also supervised coordination with utilities to ensure that service was not interrupted. Lines for traffic signals, 911 emergency cable, electric, cable television, telephone, gas, and even fiber optic cable run through the site, and had to be supported and maintained throughout excavation and construction. The sewer line had to be rerouted without interruption. A 36-in. water main ran over the top of the station, and had to be protected and maintained during construction. Old streetcar tracks were buried about six in. beneath the surface of State Street, and Baker had to confirm that they were not currently used as an electrical ground before the tracks, along with their granite and concrete base, were excavated.
Further coordination and creative thinking was required during construction when Commonwealth Edison determined that the station needed its own 33-ft by 19-ft electrical service and transformer vault. Baker's Assistant Resident Engineer, Damon Rhodes, says, "Space was extremely tight at the site, but Baker identified the one possible location for the vault and coordinated its construction independently outside of the station walls."
Baker was also responsible for overseeing the noise, vibration, and ground movement monitoring program, and adjusting construction techniques as appropriate to protect the school, historic cathedral, and surrounding structures. Any vibration or shift of the earth during construction had the potential to damage architectural details, such as decorative plasterwork in the cathedral. Drilling rather than driving the piles for the earth retention system was just one element of a detailed plan developed by WJE. Vibrating wire strain gauges were placed at various locations on the braces between the two retaining walls. Load cells were used to measure the tieback loads. Data were read daily during excavation, and weekly during the remainder of the construction to measure movement of the earth retention system. Slope inclinometers were installed to measure horizontal ground movement near the retaining wall. Surface monuments, tilt meters, and string potentiometers were also used to measure ground movement at various locations inside and outside the school and cathedral, and vibrations were measured inside each building.
Dr. Richard Finno, a geotechnical engineering professor at Northwestern University, served as a consultant to the design and construction teams and developed a program to monitor ground movement. Dr. Finno ran a computer model once a week based on data gathered by instruments at the project site, which allowed him to forecast how much more ground movement could be expected. His initial forecast estimated ground movement to be 1.5 in. both horizontally and vertically, which ultimately was accurate.
His weekly ground movement forecasts often indicated that higher levels of ground movement were likely to occur, and Baker worked with the contractor to alter excavation techniques based on Dr. Finno's models. As a result of careful monitoring, there was no damage to the cathedral. As anticipated, there was minor cosmetic damage to the school, primarily surface cracks in some of the plaster walls adjacent to the excavation. Baker assessed the damages and coordinated repair work around the school holiday schedule.
Baker's work yielded benefits that extend beyond this individual project. CDOT directed Baker to develop a quality assurance plan that could be used as a model for other projects throughout the city. Baker developed and tested the plan, and it has since been implemented by CDOT on other city projects. Baker also implemented and recommended enhancements to CDOT's database management system, developed in 1995, to track more than 700 submittals and RFIs that were required during construction. Williams notes, "The database tracking system has been continually refined, and benefits will be gained as the system continues to evolve and take advantage of updated technology."
The Chicago/State station is the fourth project in CDOT's State Street Subway Renovation Program, which began in 1995. Thomas Ambry, CDOT's transit design manager, says, "Stations along the system's Red Line are being rehabilitated and updated to include similar design elements, such as the skyline wall tile and red bullnose tile. Projects are planned at the Grand, Clark/Division, and State Street Continuous Platform stations in the near future."
As Chicago's infrastructure grows and improves, construction managers will continue to dig deep and think fast as they resolve the maze of challenges that arise during urban construction.
Mr. Hayes is the Project Manager for the Construction Manager, Baker Engineering, Inc., Chicago, Illinois.
Chicago Transit Facts
Chicago's transit system is the second largest in the U.S., with 151 stations and 108 miles of track.
Most of Chicago's metro transit network is on elevated track known as the "L."
The first loop of elevated track opened around the city center in 1897, and the heart of Chicago's downtown is still known as The Loop.
Work began on Chicago's first underground subway in 1938 at the Chicago Avenue/State Street station.
The revolutionary West Side Subway, which opened in 1958, was the first major project in the U.S. providing rail rapid transit in the right-of-way of a multi-lane highway.
In the downtown area, the State Street line has a record-breaking 3,300-ft continuous platform that stretches between three stations.
In 2000, the Chicago Transit Authority launched the SmartCard pilot program, testing plastic rechargeable farecards with embedded computer chips. Passengers place the cards on touch pads on the front of bus fareboxes or rail turnstiles, and their fare is automatically deducted.
Mayor: Richard M. Daley
Owner: City of Chicago Department of Transportation, Bureau of Bridges and Transit
CDOT Commissioner: Miguel d'Escoto
CDOT Deputy Commissioner/Chief Bridge Engineer: Stan Kaderbek, S.E., P.E.
Operator: Chicago TransitAuthority, Frank Kruesi, President
Funding: Federal TransitAdministration and Illinois Department of Transportation
Prime Design Consultant: DeLeuw, Cather & Company (a Parsons Transportation Group Company)
Construction Manager: Baker Engineering, Inc. (a unit of Michael Baker Corporation)
Prime Construction Contractor: Walsh/Weis, Joint Venture
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|Author:||Hayes, Kevin P.|
|Article Type:||Statistical Data Included|
|Date:||Sep 1, 2001|
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