Project Description

The initial stage of the Dallas Area Rapid Transit (DART) Light Rail System consisted of two 13,000-foot-long tunnels located along the North Central Parkway Corridor and one underground station at City Place. The project was located in the Austin Chalk Formation, a soft rock, with interbedded montmorillonite. The two 13,000-foot-long tunnels were a minimum of 18 feet in diameter and were excavated by tunnel boring machines. Permanent tunnel support consisted of rock bolts with a nominal shotcrete layer for protection.

Scope of Services

Lachel & Associates, Inc. was retained to design tunnels, specify geotechnical explorations, and provide on-site geotechnical interpretation and inspection. Specifically, this included the initial design of the conventionally mined running tunnels, tunnel portal design, constructibility evaluations; cost estimates; development of specifications for tunneling; construction sequence analysis and specification; and evaluation of contractor claims.

The geotechnical exploration work included program development, specification and supervision instrumentation and monitoring to include groundwater studies. Lachel's initial interpretation of the geotechnical data included a study and analysis of the ground response to construction; design and specification of ground support; and analysis of the groundwater regime. During construction, Lachel provided on-site geotechnical interpretations and inspection of the tunneled sections of the project, for which Lachel provided the initial design and scope.

The Lachel-designed subway mezzanine had a clear span of approximately 63 feet, at the time, the largest-ever developed underground opening in the Austin Chalk Formation. This challenging project was located only 35 feet below one of the busiest highways in the Dallas area. Lachel developed an innovative design and specified the use of a sequential tunnel excavation with immediate support using rock bolts and dowels, combined with a sophisticated instrumentation system. Progressive analysis allowed us to determine deflections as individual modules of rock were removed. This work plan minimized movements in this critical area and permitted successful project completion.