The Challenge
A new transit concourse threaded directly beneath an active rail line kept in service throughout
The project required excavating and constructing an underground transit concourse directly beneath active rail lines in Chicago that could not be taken out of service. Any settlement of the track bed, even minor, would have triggered service disruptions and potential safety holds the transit authority couldn't accept. The excavation had to proceed in a dense urban environment with existing utilities above and a critical load path between the new construction and the operating railway. The project also had a fixed delivery window tied to a broader station modernization program, meaning delays caused by track issues would have cascaded across the entire effort.
Our approach
We designed a support-of-excavation system that transferred track loads to temporary foundations at the perimeter of the excavation, maintaining a continuous load path above at every stage of the work below. The underpinning was sequenced in short panels so that no significant length of track was ever left without support at any given time. A dense array of survey monuments and settlement gauges monitored track position in real time throughout construction, with automated alerts set to trigger if movement approached the tolerance limit. Every structural operation underground was tied to a specific monitoring protocol, and the transit authority's engineers reviewed the instrumentation data on a weekly basis.
The outcome
The concourse opened on schedule with trains having run uninterrupted above it for the full duration of construction. Track settlement stayed within tolerance throughout, with no monitoring alerts triggered and no service holds issued at any point. The instrumentation record showed the rail bed moved less than two millimeters over the entire construction period, well within the threshold the transit authority had established at project outset. The concourse now serves as a primary transfer point in the station, handling daily passenger volumes the old layout could not accommodate.
0
Service interruptions
38 ft
Depth below grade
24/7
Rail kept in service
"Trains ran above us the entire time. That tells you everything about how carefully this was engineered."
Michael Barnes, Project Director, Chicago Transit Authority
