700-Foot-Long Tunnel Boring Machine Mary Arrives to Dig DC Tunnel (Video)
TBM Mary will travel 2.4 miles for the $819 million Potomac River Tunnel Project and later be joined by TBM Emily.
machineryasia 
A 700-foot-long tunnel boring machine is being assembled in Washington, D.C., to dig a 5.5-mile stormwater and sewage overflow system.
The front shield and 105-ton cutterhead for TBM Mary were lowered January 12 and 13, with the middle shield to follow for the $819 million Potomac River Tunnel Project.
To watch the lowering of the TBM parts, see the DC Water videos at the end of this article.
Mary will be assembled in a starter tunnel about 100 feet underground before she begins her 2.4-mile journey in spring, according to the District of Columbia Water and Sewer Authority.
The TBM will dig an 18-foot-diameter tunnel from West Potomac Park to the Georgetown University entrance site. While sending excavated soil to the surface, it will simultaneously install the tunnel’s concrete walls.
Crews have begun staging equipment at the end site where a drop shaft and diversion facility will be built to connect to the future tunnel.
Mary will travel north. Then later in summer, TBM Emily is scheduled to arrive from Germany to dig 3.1 miles south to connect to the Anacostia River Tunnel. That tunnel, at 2.4 miles long, was completed in 2016 to prevent sewage overflow from entering the river and was excavated by TBMs Lady Bird and Nannie.
The TBMs are named after sisters Mary and Emily Edmonson who were freed from slavery and abolitionists.
The Potomac River Tunnel is part of DC Water’s Clean Rivers Project, a long-term program designed to improve water quality and protect public health and the environment.
“The tunnel system will store combined stormwater and wastewater overflows during heavy rains, preventing millions of gallons of untreated sewage from reaching the Potomac,” DC Water says.
It is expected to begin operations in 2030 and “reduce the volume of sewage and stormwater overflows into the river by 93% and lower the number of times this occurs from 74 to just four times in a typical year.”
Watch TBM Mary's front shield be lowered January 12:
Watch TBM Mary's cutterhead be lowered January 13:
