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Dredge a trench when the tunnel will go. Lay a series of steel tubes along the length of the trench. Encase the tube with enough backfill to withstand the water pressure above it. Connect the tubes by removing the intervening bulkheads.
This method is similar to the cut-and-cover method, but is used for digging underwater tunnels. The trench needs to run the length the tunnel will run underwater. Each tube is sealed at either end with a bulkhead. If the tube is for a car tunnel, as in the case of the Ted Williams Tunnel in Boston, the tubes include pre-built road sections. For the Ted Williams Tunnel, this was a 5-foot-thick (1.5 m-thick) layer of rock. Any road or rail segments built into the tubes would be connected at this time as well.