Tunneling Below Second Avenue

In Manhattan, where street traffic tends to stall, only one subway runs the length of the East Side. Every weekday, 1.3 million passengers — more than are carried in 24 hours by the transit systems of Boston, Chicago and San Francisco combined — cram onto the Lexington Avenue line. Yet the chaos above and below has inspired afeat: about 475 laborers are now removing 15 million cubic feet of rock and 6 million cubic feet of soil — more than half an Empire State Building by volume — out from under two miles of metropolis.

In December 2016, that tunnel will make its debut as a portion of the Second Avenue subway — the great failed track New York City has been postponing, restarting, debating, financing, definancing and otherwise meaning to get in the ground since 1929.

This past spring, between 69th Street and 72nd Street on Second Avenue, cages descended every eight hours, five days a week, lowering roughly 50 men in neon vests and hard hats into a deep hole. Overhead, fluorescent bulbs provided a noonish light and yellow ventilation tubes undulated. A cool, roaring wind filled the void and carried the intense aroma of Emulex explosives, an ammonialike, Fourth of July smell. Men with tripods surveyed; men with blowtorches welded; men guiding hoses poured concrete . They took brief lunch breaks and relieved themselves hastily where and when they could.

The hurry actually began more than 80 years ago, when city leaders first proposed constructing a new subway parallel to the Lexington line to serve the developing East Side. It would run from 125th Street south to Houston and cost $86 million. Then came the Great Depression. Then World War II. Then existing subways needed repairs. In the early ’70s, short sections of the Second Avenue tunnel were burrowed at the foot of the Manhattan Bridge, between 99th Street and 105th and between 110th and 120th, before the city’s looming bankruptcy in 1975 halted all digging.

The dream of a Second Avenue subway lay dormant until April 12, 2007, when contractors for the Metropolitan Transportation Authority again broke ground — to extend the Q line from 63rd and Lexington over to Second Avenue and up to 96th Street. That alone costs $4.5 billion. Eventually they will lengthen the Q to 125th and dig a new line, the T, from the Financial District straight up Second Avenue to 125th Street. At least that’s the plan.

One evening in March, Amitabha Mukherjee, an engineering manager at Parsons Brinckerhoff, the firm supervising construction at Second Avenue, led a small group through a tunnel headed from 69th Street toward 63rd. The tunnel was dark, but there was, in fact, a light burning at the end. Where the rock was naturally fractured, groundwater squeezed in, darkening the walls with Rorschach figures: here a stegosaurus, there a lady in a gown.

“Geology defines the way you drive the tunnel,” Mukherjee said. The bedrock below Second Avenue and for much of the rest of Manhattan is schist — a hard, gray black rock shot through with sheets of glittery mica. Some 500 million years ago, Manhattan was a continental coastline that collided with a group of volcanic islands known as the Taconic arc. That crash crumpled layers of mud, sand and lava into schist, lending it an inconsistent structure and complicating tunneling: in some places, the schist holds firmly together, creating self-supporting arches; in others, it’s broken and prone to shattering, forcing workers to reinforce the tunnel as they go to keep it from falling.