Congress Line

The Chicago Transit Authority (CTA) was reconstructing transit lines between the south portal of the Dearborn Subway and the west end of the Illinois Medical District Station at Damen Avenue. Some areas of the project had an Initial Bearing Value (IBV) less than 4%. Those conditions meant that 12” of undercut and fill with new aggregate would be required before the sub-ballast and ballast could be placed. This time-consuming solution would have conflicted with the scheduling constraints in place around working on CTA lines.

Tensar engineers used Tensar+ design software to recommend a sub-ballast stabilisation solution consisting of 8” of sub-ballast and 12” of ballast on top of Tensar NXR2 geogrid and a layer of TrackTex™ anti-pumping geocomposite to preserve the ballast from the fine-grained subgrade. This solution prevented CTA from having to undercut and fill 12” of poor subgrade material, saving CTA an estimated $500,000 and replaced 1,200 truckloads of poor subgrade material and new aggregate with 4 truckloads of geosynthetics. All this means less wear and tear on Chicago streets, less CO2 in the air, and a much tighter construction timeline achieved.