The Blue Plains Tunnel project was the design-build of a 24,000-foot-long, 26-foot-diameter CSO tunnel constructed in soft ground under and adjacent to the Potomac and Anacostia Rivers. In addition to the tunnel, the project included a 132-foot-diameter dewatering shaft and a 76-foot-diameter screening shaft at Blue Plains Advanced Wastewater Treatment Plant to serve as main access point for the tunnel construction; a 55-foot-diameter drop shaft at the main pumping station to serve as terminus point; a 50-foot-diameter drop/overflow shaft at Joint Base Anacostia Bolling; and a 55-foot-diameter junction/drop shaft at Poplar Point Pumping Station. The dewatering shaft and screening shaft were between 180 and 198 feet below ground. The team used an extremely specialized subaqueous tremie method to pour concrete in the three drop shafts, each 120 to 145 feet deep.
The project included an innovative mucking system with a custom designed 110-ton gantry crane and muck box carousel to meet the aggressive design advance rate.
Lady Bird, the TBM, mined and constructed the four-and-a-half-mile-long tunnel in 23 months. The drive was 100 to 140 feet deep with a 26-foot excavated diameter. Tunneling from south to north along the Potomac and Anacostia Rivers, half of the drive was predominantly in Potomac Clay, which can be stiff and sticky, but well suited for earth pressure balance technology. The team utilized Boraid, Traylor’s proprietary surfactant (foaming agent), mixed on site by the company’s custom foam plant.
Lady Bird passed through two shafts during mining, each time pausing to perform cutterhead maintenance. In both instances, as well as when holing through, the shafts were flooded with water to equalize pressure, preventing ground loss. Peak earth pressure was 3.6 bar, and the approximately .25% ground loss was minimal and considered an overwhelming success for the project team. A one-pass precast segmental concrete lining system was used as tunneling progressed.
The team delivered more than 1.4 million man-hours with zero lost time injuries.