When the 2,000-ton cutterhead of a tunnel-boring machine named Harriet made a U-turn last summer in Miami, the event marked a major turning point for the Port of Miami tunnel project.

After Harriet broke through the end of one 4,200-ft-long, 42.3-ft-dia tunnel under Biscayne Bay last July, crews with the design-build team, led by Bouygues Civil Works Florida, used a giant Teflon turntable to rotate the TBM's cutterhead and shield in preparation for boring a twin tunnel in the opposite direction.