Constructed below one of the busiest highways in the Dallas area with only 30 feet of rock cover, the tri-level Cityplace/Uptown station is the only underground station on the Dallas Area Rapid Transit (DART) Light Rail system. Completed in 2000, this challenging project for Section NC-1B included designing and building the station and two parallel 18-ft diameter tunnels in the Austin Chalk Formation.

Schnabel provided tunnel design and specialty geotechnical engineering services for the HDR-led team. Our work included initial support design for the conventionally mined running tunnels and subway mezzanine at Cityplace, tunnel portal design, construction sequence analysis, development of tunneling specifications, constructability evaluation, and cost estimating. During construction, we provided on-site geotechnical characterization and construction inspection, and assisted the owner in evaluating contractor claims.

The tunnels were excavated in the formation – a soft limestone with interbedded bentonitic seams – using tunnel-boring machines along the North Central Expressway corridor. Permanent tunnel support consisted of rock bolts with a nominal shotcrete layer for protection.

The Schnabel-designed subway mezzanine at Cityplace had a clear span of approximately 63 ft, which, at the time, was the largest-ever permanent underground opening in the Austin Chalk. We developed an innovative design and specified the use of sequential excavation methods combined with field observation and a sophisticated instrumentation system for quick response analysis of excavation-induced deformations.