The $2.1-billion Red/Purple Line modernization project aims to change that. Phase 1 of the federal- and city-funded project is scheduled to begin by the end of October—or as quickly as permits are issued this fall. Joint venture design-build contractor Walsh-Fluor is tasked with the erection of a bypass track and bridge for the CTA Brown Line just north of the Red/Purple Line’s Belmont station. Currently, the Brown Line intersects the Red/Purple Line tracks at a 112-year-old serpentine junction, requiring a series of traffic signals to have to manage train traffic on all three lines. The design-build team also includes lead designer Stantec, supported by EXP.

Walsh-Fluor will replace track and erect new supports between the Lawrence and Bryn Mawr stations on the north side during Phase 1. Stantec’s design has a closed deck, concrete supports and a drainage system, and the joint venture contractor is also responsible for corridor signal improvements. The project is expected be completed by 2025, a construction period during which the CTA Red/Purple Line will stay open with only intermittent disruptions when track use will be reduced from four tracks to two to accommodate construction. Phased stops will occur when major structural pieces such as the steel-and-concrete straddle-bent to support the new Brown Line flyover will be installed. Such structural elements will be installed over weekends.

“It is a lot of work over that period, despite the start we’re getting,” says David Shier, Walsh’s program manager. “We have worked to coordinate the project with CTA to make a plan to get it all done, but it is, ultimately, a very big job.”

CTA officials said the design provides an intermediate support for the bypass structure, which will straddle the Red/Purple Line and allow spans of a reasonable length instead of a clear span, limiting superstructure depth and achieving both the clearance necessary over the Red/Purple Line as well as meeting grade limitations. The straddle bent allows the Red/Purple Line to pass beneath without lateral expansion of the tracks to allow for other types of substructure to support the bypass. It also will allow phased removal and reconstruction of the existing four-track, serpentine structure used by the Red/Purple Line while creating separate, independent tracks for the Brown Line.

“Walsh-Fluor had an incredibly well thought-out proposal and a very robust process in the procurement,” Bushell says. He noted that Walsh-Fluor figured out a way to use fewer columns and still maintain the same structural system strength for the overall rail system thanks to proper placement.

“These columns attach to the deep foundation 60 ft to 80 ft down into the ground with a caisson,” Bushell says. “The fewer columns you need to put out there, the fewer caissons you need to drill, the quicker you can build it.”

Bushell says Walsh-Fluor’s approach brought in several efficiencies and innovations, but safely limiting the number of columns was the fundamental efficiency that enabled the joint venture contractor to shave a significant amount of time—over six months—from CTA’s prospective baseline schedule.

CTA broke ground on the project in early October.