Engineering Marvel: The Monitor-Merrimac Memorial Bridge-Tunnel (MMMBT)
The Monitor-Merrimac Memorial Bridge-Tunnel (MMMBT), a 4.6-mile long combination bridge-tunnel system (named for the two ironclad ships that fought in the Hampton Roads harbor during the Civil War) connecting two Virginia communities across the mouth of the James River.
The tunnel is 4,800 feet long from portal to portal, and it was built by the immersed sunken tube method, comprised of 15 prefabricated segments each 300 feet long and with two 2-lane bores, placed by lay-barges and joined together in a trench dredged in the bottom of the harbor, and backfilled over with earth. Four percent (4%) maximum grades are utilized in the tunnel, and a 60 mph design speed. The traffic lanes in the tunnel are 13 feet wide, with 2.5-foot-wide ledges on either side of the roadway, and with 16.5 feet of vertical clearance from the roadway to the ceiling. The current shipping channel above the deepest part of the tunnel has 800 feet of horizontal width and 45 feet of vertical depth below the average low-tide water level, and the tunnel was designed and built deep enough to allow for a future enlargement of the shipping channel to 1,000 feet of horizontal width and 55 feet of vertical depth below the average low-tide water level.
The MMMBT opened in April 1992 after seven years of construction. The MMMBT cost $400 million to build, and it includes a four-lane tunnel that is 4,800 feet (1,463 m) long, two man-made portal islands, and 3.2 miles (5.1 km) of twin trestle. (Fabricating the tunnel portion of the bridge and lowering it into place cost $126 million alone.)
Source: Snopes
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