Dallas Area Rapid Transit
Section NC-1B of the Dallas Area Rapid Transit (DART) Light Rail system consists of parallel 18-ft-diameter tunnels located along the North Central Expressway corridor and an underground station at Cityplace. The project was constructed in the Austin Chalk Formation, a soft limestone with interbedded bentonitic seams, and the two 13,000-ft-long tunnels were excavated using tunnel boring machines. Permanent tunnel support consisted of rock bolts with a nominal shotcrete layer for protection.
Lachel provided tunnel design and specialty geotechnical engineering services for the HDR-led team. This 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.
The Lachel 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. This challenging project was constructed with only 30 ft of rock cover below one of the busiest highways in the Dallas area. Lachel 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.
In addition, during construction Lachel provided on-site geotechnical characterization and construction inspection, and assisted the Owner in evaluation of contractor claims.