 A number of methods will be used to tunnel for 13 @.@ 7 kilometers ( 8 @.@ 5 mi ) underneath Manhattan , which is densely populated . About 90 % of the tunneling is to be performed by a tunnel boring machine . The rest will be done using the cut and cover method and mined drill and blast , for sections , generally the 16 stations , that average 275 meters ( 902 ft ) in length . The stations at 86th and 72nd Streets were mined . This was challenging , given the number of expensive high rise properties in their vicinities . The 96th Street cut @-@ and @-@ cover station was about 15 meters ( 49 ft ) deep , making it one of the shallowest stations being built on the line ; the shallowness was so that the new line could align with the preexisting piece of subway tunnel built in the 1970s between 99th and 105th Streets . Stations at the two mined stations are between 25 @.@ 9 and 27 @.@ 4 meters ( 85 and 90 ft ) deep in rock . The construction method that was used was supposed to ease concerns for buildings above the station sites , because only two shafts were required for excavation . 
