The Circle Line would be constructed entirely underground utilizing a Tunnel Boring Machine ("TBM").
TBMs are used to excavate large tunnels with a circular cross section. They can bore through anything from hard rock to sand.
The Circle Line would be Boston's first "deep tube" rapid transit line. The depth of the tunnel would be anywhere from 100 - 250 feet below the surface.
The TBM would limit disturbances to the surrounding ground, making it highly suitable for the heavily urbanized area around downtown Boston.
Watch this 5 minute YouTube video on how a Tunnel Boring Machine works.
Tunneling with a TBM would be much more efficient than traditional drilling and blasting methods, resulting in shortened completion times.
With the majority of construction happening deep underground, activity could continue as usual on Boston's streets.
There would be none of the traffic headaches that reigned supreme during the Big Dig era.