Gantries will be attached one by one as TBM Mary advances through the tunnel until it’s fully assembled.
The gantries will be progressively lowered and connected in a continuous underground operation. They will support systems for power, ventilation, material transport and worker access.
Once fully assembled, Mary will weigh 1,200t, have a 6.4m diameter and measure 213m long.
Before Mary started her journey, contractors built a 30m deep overflow mining shaft and excavated a starter tunnel.
The front portion of Mary was lowered and assembled inside the shaft and starter tunnel earlier this year.
During the launch, the first concrete tunnel ring was installed and connected to a thrust frame, allowing the TBM’s hydraulic jacks to push the machine forward.
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As the cutterhead continues rotating, it will excavate the ground while crews will simultaneously build the tunnel lining behind it using precast concrete segments.
Mary will bore approximately 3.8km north from West Potomac Park to Georgetown University, excavating through the District’s hard bedrock.
Her sister machine, Emily, is scheduled to arrive from Germany later this spring and will excavate south toward Joint Base Anacostia-Bolling, completing the full 8.8km tunnel system.
