How are transit tunnels made, like the one on I-95 North between Washington and Baltimore? Are they dropped into the water the tunnel passes through or do engineer’s dig under the water?
ANSWER PERSON RESPONDS: There are several different tunneling methods. The 1985 Fort McHenry tunnel (the I-95 tunnel under Baltimore Harbor) and the 1957 Harbor Tunnel (I-895) use the “immersed tube” method, in which a tube is floated to location and sunk to the bottom of the body of water, into a trench that has been dredged in preparation. It’s then covered by various fill and protective materials. Here’s a website about the 1985 tunnel: http://www.roadstothefuture.com/Fort_McHenry_Tunnel.html
Shallow tunnels on land commonly use the comparable cut & cover (or open cut, or covered excavation) method of tunneling, in which an open cut is made and then covered over. This is common on subway lines that are just below street level. Deeper tunnels require excavation with a ferocious-looking tunnel boring machine (TBM), usually with some sort of shield to keep the hole from caving in after being drilled, until the tunnel lining can be installed. Older nineteenth century railroad tunnels used more manual excavation techniques, not up to current OSHA standards.
Here’s one of many online article about tunnels, with links to tunneling shields, boring machines, and what-not: http://en.wikipedia.org/wiki/Tunnel
AP has always regretted not pursuing an engineering career!