Second Avenue Subway - Construction Methods

Construction Methods

Planned construction methods vary depending on the section of the line, due to varying underground conditions. The methods planned for each section are as follows:

A number of different methods will be used to tunnel for 13.7 kilometers (8.5 mi) underneath Manhattan, which is densely populated. 90% of the tunneling will be performed by a tunnel boring machine. The rest of it 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 will be mined. This will be challenging, given the number of high value, high rise properties in their vicinities. The 96th Street cut and cover station will be at about 15 meters (49 ft) deep, making it one of the shallowest stations. Stations at the two mined stations will be between 25.9 meters (85 ft) and 27.4 meters (90 ft) deep in rock. The construction method that will be used should ease concerns for the above buildings, because only two shafts will be required for excavation.

In Phase 1 there will be tunneling between East 63rd and 92nd Streets and a 248-meter-long (814 ft) by 23-meter-wide (75 ft) TBM launch box will be built. That will ultimately become part of the 15-meter-deep (49 ft) 96th Street Station. Two access shafts will be constructed for the East 72nd Street Station. Slurry or diaphragm walls, 1.1 meters (3.6 ft) wide and 6.1 meters (20 ft) long and approximately 35 meters (115 ft) deep, will be built alongside the sections between East 93rd and 95th Streets. Inasmuch as the rock is shallower between East 91st and 93rd Streets, 1.1-meter-diameter (3.6 ft) secant piles will do the same work at shallower depths.

Earth excavation will be conducted between walls, once they are installed, and a box structure will be built using a bottom-up construction method. Temporary decking will constitute the top of the box, and the decking will both brace the excavation and support the walls and Second Avenue traffic.

The tunnels and stations will be up to approximately 30 meters (98 ft) below street level. Of the below-ground obstacles, Arup director of construction David Caiden says: "It’s a spaghetti of tunnels, utilities, pipes and cables – I’ve never seen anything like it." Additionally, the project must go over, or under, subway lines, Amtrak railway lines, and the Queens-Midtown Tunnel linking Manhattan and Queens.

There are geological anomalies along the way. Manhattan's geology changes along the subway’s length, passing through rock and soft ground, consisting of sands, silts, and clays over Manhattan schist, and there are faults and shear zones as well as fractured rock. Hard-rock Tunnel Boring Machines 6.7 meters (22 ft) in diameter will tunnel during the first phase, progressing at anticipated rate of about 20 meters (66 ft) per day.