This 1913 photo of Lexington Avenue, between 105th and 106th Streets in Manhattan shows a construction method called cut and cover. Like the name suggests, this method involved cutting or digging a trench in the street, installing the necessary infrastructure, and then covering with the street on top. During construction business went on as usual around construction. With temporary wooden structures placed over the work below, trolleys and other traffic were able to pass over top of the construction. Many of our first subway tunnels were built using this method beginning in 1900, which made construction faster, cheaper, and easier than trying to dig deep under the ground. It allowed sand hogs (the nickname given to tunnel workers) to avoid the difficult and unstable bedrock underneath most of Manhattan called Manhattan schist.

This photo shows tunneling under Central Park for the Lenox Line. Unlike cut and cover, these tunnels are deep underground and cannot be built by digging through the street. Drillers made holes for dynamite that helped explode the rock, and then excavators removed the spoil and cleared the way for the shield, a device that kept workers safe as they dug. The shield is a temporary support that acts to protect the workers from falling debris as they chip away at rocks ahead of the shield and put in place permanent support walls behind it. Sandhogs used pickaxes, shovels, air powered drills, and sometimes dynamite, to break through the rock.

Underwater Tunneling, the third type of construction, occurs underneath the river. This photo shows construction of the Joralemon Street tunnel. Subway water tunnels are built approximately 30 feet below the riverbed. Because of the immense pressure exerted by 30 feet of rock, sand, and silt, plus 50 feet of water, such tunnels were in danger of collapsing. To counteract this force, tunnels underwater were pumped full of pressurized air, which pressed out against the walls of the tunnel and kept it in place. However, this pressurized air made for difficult working conditions for sandhogs. When working in pressurized air, workers were in danger of developing decompression sickness, similar to what divers might experience. Because of this, workers could only spend three hours under water before needing to spend several hours above ground. Before exiting or entering the tunnel, sandhogs would adjust to the pressurized air in a decompressionchamber, also called an air-lock or a man-lock.

The 72nd st station on the Second Avenue subway opened in 2017. Today when tunnels are dug deep underground workers do not have to dig by hand but instead are assisted by a tunnel boring machine, or TBM for short. This massive machine is is over 300 feet long and weighs around 485 tones! Attached to the front is a blade head 22 feet in diameter which acts as a large drill that breaks apart the rock and then moves debris out the back on a conveyor belt. Tunnel boring machines can move about 50 feet per day. The TBM pictured above, name Adi, was used to dig the tunnels for the second avenue subway.

Cut and cover construction is still used for some tunnels. The street is excavated in sections and support for metal decking is installed as seen in this photo of Second Ave subway construction at 96th st. This allows traffic to flow above during construction which continues on beneath the decking. A cut will remain open to bring in supplies and remove excavation material. This method is often used to construct stations, which we want to be close to street level to easily access, and for certain sections of tunnel as well.